Fermentation: Desirable Effects of Microbes .. ... ., Cheese and bread are just two of the many food products that are produced by fermentation. Objectives After studying this chapter, you will be able to describe the types of microbes that impact the food supply. list factors that impact the growth of single- celled organisms. differentiate among yeast, bacterial, and mold fermentation. identify food products that are a result of fermentation. 380 Chapter Key Terms microbiology microorganism microbe Monera Fungi bacteria micrometer cytoplasm bacilli cocci spirilla Gram's stain aerobic anaerobic facultative fungus hyphae 17 mycelium spore mold yeast pure culture starter proteolytic lipolytic halophilic genus species pasteurization fermentation by-product brine curd
Transcript
Fermentation:Desirable Effects ofMicrobes
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...
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Cheese and bread are just two of the many foodproducts that are produced by fermentation.
Objectives
After studying this chapter,you will be able to
describe the types of microbes that impactthe food supply.
list factors that impact the growth of singlecelled organisms.differentiate among yeast, bacterial, andmold fermentation.
identify food products that are a result offermentation.
380
Chapter
Key Terms
microbiology
microorgan ism
microbe
Monera
Fungibacteria
micrometer
cytoplasm
bacilli
cocci
spirilla
Gram's stain
aerobic
anaerobic
facultative
fungus
hyphae
17
mycelium
spore
mold
yeast
pure culture
starter
proteolytic
lipolytic
halophilic
genusspecies
pasteurization
fermentation
by-product
brine
curd
Chapter 17 Fermentation: Desirable Effects of Microbes
Microbiology is the study of living organisms too small to be seen by the unaidedhuman eye. Living organisms that are onlyvisible through a microscope are calledmicroorganisms or microbes. Microbes are allaround you. They are in the soil under yourfeet and on the desk in front of you. They arein the air you breathe, the water you drink,and the food you eat. They multiply rapidly,transfer from one surface to another easily,and blow around in the wind.
You may be unaware that microbes areunavoidable. Perhaps you would rather notthink about the fact that microbes live on andin. you. However, you can think of manymicrobes as friends and allies. There are thousands of different microorganisms. Severalhundred of these are associated in one way oranother with the production of food products.Without microbes, many foods you enjoywould not be possible. Foods produced withthe help of microbes include chocolate, coffee,tea, cheese, soy sauce, pickles, sauerkraut, andyeast breads.
This chapter will look at the main kinds ofmicrobes. You will study how they multiplyand how they change food into new products.You will also read about how the nutritionalvalue of foods is affected by microbes.
The Types of S{ngle-CelledOrganisms
Three categories of microbes can have positive uses in foods. They are bacteria, yeasts,and molds. These microbes have some similarities. Individually, they cannot be seen bythe human eye. They reproduce very rapidlywhen given the right environment. They alsodepend on outside sources of food to growand multiply.
Research in the twentieth century changedscientific understanding of microscopic organisms. This led to disagreement on how to classify microbes. Most of the microbes that affectthe food supply belong to one of two kingdoms of organisms. Most biologists classifybacteria as members of the Monera kingdom.Yeasts and molds are members of the Fungikingdom.
381
BacteriaBacteria are extremely small single-celled
organisms that multiply through cell division.The head of a pin can hold thousands of bacteria. These microbes must be magnified 1,000times with a microscope to make them visible.Bacteria are usually one to three micrometers(Jllll) in length. A micrometer is one-thousandth (0.001) of a millimeter. Suppose youwere to lay bacteria that are 1 11m in lengthend to end. It would take 1,000 of them toequal 1 millimeter. It would take one millionof these bacteria to equal 1 meter.
Bacteria cells have rigid walls and nonucleus. The cells are filled with a gelatinousliquid called cytoplasm. The processes ofmetabolism and reproduction take place within this liquid. Bacteria are classified accordingto their shape, their cell wall structure, andtheir oxygen needs.
Bacteria have three basic shapes: rod,spherical, and spiral. Rod-shaped bacteria arecalled bacilli. Spherical bacteria are calledcocci. Spiral bacteria are called spirilla.See 17-1.
Bacteria have two basic types of cell wallstructures. The two types of cell walls areidentified by their ability to be stained by acrystal violet dye. One of the first steps inidentifying a type of bacteria is a stainingprocess called Gram's stain. Hans ChristianGram developed Gram's stain in 1884. Hedeveloped the process because bacteria cellsare nearly colorless. This makes them difficultto see, even with a microscope. Gram-positivecell walls will tum blue-violet during thestaining process. Gram-negative cells will tumred. See 17-2.
Doctors need to be able to tell the twotypes of bacteria cell wall structures apart.This allows doctors to decide what medicineto prescribe for a bacterial infection. Manyantibiotics will kill either gram-negative orgram-positive bacteria but not both. Plant cellwalls and animal cell membranes are notmade of the same material as bacteria cellwalls. This is why antibiotics can kill bacteriawithout damaging body tissue.
The oxygen needs of bacteria are important to food scientists who work with foodrelated bacteria. Some bacteria need oxygen to
382 Unit V Food Microbiology: Living Organisms in Food
17-1 The three basic shapes of bacteria are bacilli (rods), cocci (spheres), and spirilla (spirals).
17-2 The blue-violet color of the bacteria on thetop indicates they are Gram-positive. The red bacteria on the bottom are Gram-negative.
Gram-positive bacteria
function in either environment are calledfaCIlitative.
You can get an idea of how aerobic andanaerobic bacteria work from the example ofcabbage. Cabbage spoils when aerobic bacteria are present and given time to multiply.Aerobic bacteria must have oxygen presentfor respiration to occur. In simple terms, respiratioll is the transfer of electrons to releaseenergy for cellular function. In aerobes, oxygen is the electron receptor. In contrast, theenzymes in anaerobes will not function in thepresence of oxygen. They use carbon dioxideor sulfur- or nitrogen-based compounds aselectron receptors. If cabbage is submerged inwater, where oxygen levels are low, anaerobicbacteria will begin to multiply. Sauerkraut ismade by submerging cabbage in salt water. Inthis environment, the aerobes cannot spoil thecabbage because oxygen is unavailable.However, the anaerobes can grow, and theyare responsible for developing the characteristic flavor and texture of sauerkraut.
Bacteria's rate of growth depends on theirenvironment. When temperature, air, pH, andfood supply are right, bacteria can reproducein as little as 20 minutes. Bacteria reproduceby increasing their cell size. The cytoplasmmaterial divides equally in half, and the cellssplit into two daughter cells. Every time thecells divide, the number of cells doubles.
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multiply. Others will multiply only in an oxygen-free environment. Bacteria that must haveoxygen to function are called aerobic. Bacteriathat ftmction best in an oxygen-free environment are called anaerobic. Bacteria that can
FungiAfllllglls is a plant that lacks chlorophyll.
It also lacks definite roots, stems, and leaves.Unlike bacteria, fungi are not always singlecelled, and their cells contain a nucleus. Fungiinclude mildews, molds, mushrooms, rusts,
...
17-3 Fungi absorb nutrients through their myceliumstructure and reproduce through spores.
Mycelia tend to grow in a circular pattem.This pattem is created as the hyphae extendoutward from a spore or a single cell. When amycelium becomes large enough, it is visiblewithou t magnifica tion.
Of the various types of ftmgi, molds andyeasts play key roles in food processing. Mostyeast and mold cells are three to five timeslarger than the cells of bacteria. Molds andyeasts vary in length, width, and structure.
Chapter 17 Fermentation: Desirable Effects of Microbes
smuts, and yeasts. They are widely distributed in nature and playa major role in helpingorganic matter decay. They break down thecomplex macromolecules of organic matterinto usable nutrients, which they absorb.
Fungi are classified by their structure andreproduction methods. The basic structure ofmost fungi is made of filaments or tubes calledhyphae. Hyphae are elongated cells or chainsof cells that absorb nutrients from the environment. As the hyphae lengthen, they intertwine and form a branched network called amycelium. Part of the mycelium grows downinto an energy source to absorb nutrients. Theother part remains in the air above the energysource and reproduces through spores.
Spores could be called the seeds of fungi.Spores usually develop in a sac- or balloonlikestructure. This structure explodes when full,sending spores out into the surrounding air.Spores are microscopic, resistant to harshenvironments, and easily carried to other surfaces. See 17-3.
mycelium spores
383
Historical HighlightFood Poisoning inSalem, MA?
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In Salem, Massachusetts, in1692, over 100 people wereaccused of witchcraft. Twelvewere found guilty and hanged.The teenage girls who made theaccusations suffered from a number of symptoms. These symptoms were similar to those ofpeople who take hallucinogenicdrugs like LSD.
A fungus that grows on ryewhen the weather is especiallycool and wet is called Glaviceps
purpurea. (Its common name isergot.) One of the by-products ofthis fungus is LSD, a powerfulhallucinogen. The fungus cancause a foodborne illness knownas convulsive ergotism. Thesymptoms range from mild tosevere and can even causedeath, especially among theyoung. The disease causes prickling feeling to severe pain in thejoints. There may also be blindness, deafness, hallucinations,
fits, and laughing and cryingspells.
Outbreaks of ergotism occurthroughout the world when ryecrops are weakened by cold anddamp weather. Historical recordsindicate these conditions existedin New England from 1690 to1692. This is one of many possible explanations proposed for theevents in Salem.
384 Unit V Food Microbiology: Living Organisms in Food
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MoldsMolds are fungi that form a mycelium
structure with a fuzzy appearance, Visiblemolds have a wide range of colors. These colors include yellow, rust, red, green, and black.Many molds give off an antibiotic that killsbacteria likely to be growing in the same area.
Like other fungi, molds reproducethrough spores. During their reproductivestage, some molds produce a visible sporecase. This spore case is called a bnsidiocnrp. Thebasidiocarp has a stem, a cap, and gills. Thegills, which contain the spores, are locatedunder the cap. The stem lifts the cap and gillsinto the air so the wind can disperse thespores. Mushrooms, toadstools, and puffballsare examples of basidiocarps produced bymolds, 17-4.
YeastsYeasts are fungi with a single-celled struc
ture. They reproduce by budding. This meansthey form buds that swell and separate into aduplicate cell or form a chain of cells.Reactions involving yeast result in the production of alcohol.
A unique feature of some fungi, i.ncludingsome yeasts, is the ability to form both mycelium and single-celled structures. When a sporefrom one of these fungi lands on soil or plants,it produces a mycelium structure. When thesefungi are fOlmd in people or animals, theyreproduce by budding. See 17-5.
17-4 Mushrooms are the visible spore-carryingstructure of the basidiomycetes group of fungi.The stem holds the mushroom above the groundso the breeze can catch and disperse the sporesthat form along the gills under the cap,
Common Characteristics of MicrobesBoth bacteria and fungi grow very rapidly
and can be good sources of edible protein.Microbial sources of protein are used in animal feed. Research is underway to developmicrobial proteins that are safe for humanconsumption. Microbes enhance or add to thenutritional value of foods. An example is yeastbread. When yeast dough is held at room temperature for one hour, it will double in volume. TI1e ino-ease in volume is caused by carbon dioxide, which is released as the yeastfeed, grow, and multiply. Yeast cells in bakedbread add small amounts of protein, vitamins,and minerals. Baking kills the yeast but doesnot destroy the nuh'itive value of the yeast.
Bacteria and fungi can also enter a dormant, or inactive, state. This last characteristicallows these microbes to protect themselves.When temperature, pH, moistme level, or thesurrow1ding air becomes harsh, the cells willdehydrate themselves. The dehydrated cellscan remain dormant for long periods. As soonas the envirolU11ent matches a cell's growthneeds, the cell will rehydrate and begin toreproduce again.
The ability of microbes to go into a dormant state helps manufacturers mass-producethem. Manufacturers isolate the desiredmicrobe and create ideal conditions for it togrow. The result is a pure ClIltllre, which is alarge volun1e of one type of microbe that haspurposely been grown in a nutrient medium.After mass-producing a desired microbe,manufacturers create an environment thatencourages the microbes to dehydrate themselves. Manufacturers can ship cultures or
17-5 Yeasts form fuzzy mycelium structures whenthey grow in plants and soil. In animal sources,yeasts have smooth cells and reproduce bybudding.
Chapter 17 Fermentation: Desirable Effects of Microbes
dehydrated microbes to food processingplants. At processing plants, large volumes ofmicrobes are used in the development offoods such as cheeses, pickles, and beverages.When a pure culture is mixed with a foodsource, a starter is made. A starter is a substance containing microorganisms that isadded to food to bring about desired flavor,texture, and/or color changes. Once startersare mixed, the microbes come out of the dormant state. Then they can begin to reproduceand change a food product.
Microbial EnzymesChanges in food products are usually a
result of enzymes produced by the microbesin the food. Microbes use large organic molecules as a food source. However, most organicmolecules are too large to be transportedthrough a microbe's cell wall. To access theenergy in these compounds, microbes excretedigestive enzymes. These enzymes breakdown the large molecules. Then the microbescan absorb the resulting fragments and usethem as fuel.
Another way to classify microbes is by thetype of organic molecules they use as food
385
sources. The classifications are based on thetypes of enzymes produced by the microbes.Most microbes rely on sugars and starches fortheir energy source. These microbes produce avariety of carbohydrases. See 17-6.
A few microbes rely on protein as theirfood source. Microbes that make enzymes todigest protein are called proteolytic.Proteolytic microbes change proteins to aminoacids. Some uses of proteolytic bacteria infood production are tenderizing meat andclotting milk. Proteolytic bacteria also helpremove the outside pulp and develop thechocolate flavor of cacao beans.
Lipolytic microbes produce enzymesthat digest fats. Some uses include flavorproduction in cheese and removing egg yolkfrom egg white. A nonfood use of lipolyticbacteria is to help clean up industrial oilspills. The bacteria digest the oil and changeit into a form that is easily removed from theenvironment.
A few microbes thrive in salty environments. They are found in nature in saltdeposits that result from the evaporation ofseawater. Microbes that require high salt concentrations to function are called halophilic.
Uses for Sugar- and Starch-Digesting Microbes
Enzymes Produced, Microbial Sources
Amylases AspergillusRhizopusBacillus
Cellulases Aspergillus
Uses
Convert starch to sugar for baking, brewing, and syrupproduction
Change cellulose to fermentable productsClarify fruit juices
Sucrases
Invertases
Pectinases
SaccharomycesStreptomycesAspergillusPenicillium
S. cerevisiaeCandida utilis
AspergillusRhizopus
Convert maltose to glucose in breWingConvert glucose to fructose in corn syrupConvert glucose to gluconic acid in liquid eggs
Convert sucrose to glucose and fructosePrevent crystallization in soft-centered candiesAid in production of artificial honey
Clarify wine and fruit juiceRelease juice from fruit for increased yieldsRemove pectin for the production of concentratedfruit juices
17-6 Microbes that produce carbohydrate-digesting enzymes are used for a variety of food processingfunctions.
386 Unit V Food Microbiology: Living Organisms in Food
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Such microbes are used in Asia to make several fish- and soybean-based products, including Chinese cheese and bean cake.
Bacteria have varying tolerances for salt.The addition of salt to cabbage helps slow thegrowth of spoilage bacteria, which have a lowtolerance to salt. The bacteria that develop thecharacteristic flavor and texture of sauerkrauttolerate higher levels of salt.
Scientific Names for MicrobesMicrobes are classified by two Latin
names. The first is the name of their genus. Agenus is a group of living organisms that havesimilar characteristics. You could say that thegenus is a family name like your last name.The name of the genus is always capitalized.
The second Latin name used to classifymicrobes is the name of the species. Species isthe basic category of the classification of livingorganisms. It identifies the type of microbewithin the family and is never capitalized.
An example of a microbe name isLactobacillus acidophilus. This is the name of abacterium used in the processing of fruits,vegetables, meats, and dairy products. Lactorefers to milk. It is the same root used in theterm lactose, which is milk sugar. The -bacillusending on the first name indicates the bacteriacells have a rod shape. The endings -bacter, monas, and -ella also indicate rod-shaped bacteria. The -coccus ending is used if the genus ofbacteria has a spherical shape. (None of thespiral-shaped bacteria are used in food production.) The species name acidophilus indicates the bacteria give off an acid. Therefore,the name Lactobacillus acidophilus tells you thisbacterium probably lives in or feeds on milk.From the name, you can also tell the bacterium has a rod shape and produces an acid.
Scientists often abbreviate these long Latinnames when referring to particular microbes.Scientists typically use only the initial of thegenus name. For example, Lactobacillus acidophilus is shortened to 1. acidophilus. In thisbook, the first time a bacterium is discussed,the full name will be written. After that, thename will be abbreviated.
Factors Affecting MicrobeGrowth
A number of factors are known to affectthe growth of microbes. These are food supply, water availability, pH, and temperature.However, the specific conditions that bestsupport growth differ. for each type ofmicrobe. Each microbe has a preferred rangewithin each factor. Therefore, there is no oneset of guidelines that can be followed forevery bacterium, mold, and yeast.
Food SupplyMicrobes are composed of complex mole
cules made from carbon, oxygen, nitrogen,and hydrogen. Therefore, microbes need afood supply that provides these four chemicalbuilding blocks. Most microbes use organiccompounds (carbohydrates, lipids, or proteins) as a main source for these chemicals.
Some microbes need protein for their foodsupply. Other microbes need lipids, and others need starches. Some microbes can feed offseveral types of macromolecules. The foodsupply needed by a microbe depends on theenzyme systems the organism can make. If abacterium can only produce proteolyticenzymes, then it will only be able to digestproteins. A proteolytic bacterium could besurrounded by sugar and never reproduce.This is not because there is no source ofenergy. It is because the energy is in a form thebacterium cannot use.
Microbes need small amounts of mineralsto enhance enzyme activity. Microbes needvitamins, too. Many vitamins act as coenzymes. Therefore, a microbe's need for vitamins varies according to the enzyme reactionsthe microbe performs. The microbe mustabsorb directly from its food supply any vitamin it cannot produce.
WaterLike all living organisms, microbes need
water to function. Some microbes are able toremain alive in a dried condition. Thesemicrobes survive but cannot grow or reproduce
Chapter 17 Fermentation: Desirable Effects of Microbes 387
Recent ResearchBacteria Turns Pollutioninto Protein
Microbiologists have found agroup of bacteria called carboxydobacteria. These bacteria usecarbon monoxide from automobile exhaust as their energysource. They reproduce rapidly inan environment of 50% air and50% exhaust. The bacteria cells
contain 65% crude protein. Thereare about 350 million cars in theworld. Experts estimate thesebacteria could produce 500,000tons of protein per year from allthe exhaust.
In the future, scientists coulddeveiop a method for producing
this protein from carboxydobacteria. Then they can mass-producecarboxydobacteria to turn poisonous car exhaust into nutritiousprotein. Food scientists may havea new source of protein to use inproducing tasty foods.
Preferred 8", Ranges for Microbes
17-7 Like all living things, microbes need acertain amount of water to grow and reproduce.
pHEach type of microbe has a different pre
ferred pH range. Many fruits have a pH of less
A safe aw for most food storage is considered to be 0.70 or lower. There are three mainways to lower water activity level. These areremoving water, adding solutes to the solution, and freezing. Removing water from theenvironment causes microbes to dehydrate.Adding solutes, such as sugar or salt, createsan imbalance that pulls water out of themicrobe cells. Freezing Jocks water moleculesinto a crystalline structure and limits access tothe water.
Preferred aw
0.83-0.960.90-0.91
0.87-0.94
0.70-0.80
Bacteriamost spoilage bacteria
Most yeastsMost molds
Microbe
without water. On the other hand, microbescannot grow in pure water. This is becausepure water does not provide many of thenutrients microbes need. Each microorganismhas a range of water content that it prefers.Generally, bacteria need more water thanyeasts, and yeasts need more water thanmolds for growth.
Food scientists measure water needs interms of water activity, aw • (See Chapter 7.)Pure water has an aw of 1.0. A saturated saltsolution has an a", of 0.75. See 17-7.
Determ ining wa ter needs of microbes canbe difficult. This is because factors such astemperature and pH also impact whether amicrobe will grow. Freezing temperaturesmake water unavailable to microbes. Highwater temperatures can kill microbes. Forexample, yeast dies when the water used inbread dough is too hot. The pH level alsoaffects which type of microbe can grow at agiven a", level. For example, the a" of mostfruits is around 0.97. Bacteria should multiplyrapidly in such an environment. However, thereason bacteria do not spoil many fruits has todo with the fruits' acidic pH. The low pH killsbacteria. This allows molds that usually prefera Jo,ver ai(' to gro\v.
388 Unit V Food Microbiology: Living Organisms in Food
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than 4. Most bacteria are killed if the pH isbelow 4.6. Molds, on the other hand, can survive with a pH as low as 1.5. This explainswhy molds are more likely than bacteria tospoil fruits. However, as molds grow on fruit,they release substances that can cause the pHof the fruit to rise. The pH of the fruit canbecome high enough for bacteria to begin togrow. This is especially true of bacteria thatgive off lactic or acetic acid. These bacteriawill usually tolerate a lower pH than proteolytic bacteria, which produce ammonia.See 17-8.
TemperatureThe last factor that affects microbe growth
is temperature. You have already studied howfreezing can slow enzyme activity and cellreproduction. Extreme cold generally does notkill microbes. It slows them down or causesthem to enter a dormant or resting state. Onthe other hand, heating can kill microbes. Thetemperatures that support microbe growth arebetween the freezing and boiling points ofwater. The temperature at which microorganisms die is usually 5°C to 12°C (9°F to 22°F)above the temperature at which maximumgrowth occurs.
Food processors often use high temperatures to kill harmful microbes. Processors
pH Ranges for Microbe Growth
must think about how heating foods willaffect quality and production costs as well asmicrobes. For example, pasteurization is aprocess in which a liquid is heated untilpathogens and some spoilage bacteria havebeen destroyed. This process is used to helpprevent disease that can be caused by harmfulbacteria. It also helps lengthen the shelf life ofmilk. However, it can affect the flavor of milk.At a low pasteurization temperature of 63°C(145°F), it takes 30 minutes to kill the bacteria.This affects milk flavor to a more noticeabledegree. Using a high pasteurization temperature of 72°C (161°F) for 15 seconds reduces thedamage to the milk flavor. See 17-9.
Food scientists can successfully grow cultures of microbes for use in food processing. Aculture can be a specific microbe or a mixtureof select microbes. To produce the desired culture, scientists must create the right growingconditions. They need to know the microbe'spreferred food supply, water activity level,pH, and temperature. Trained technicians areneeded to keep track of these factors at eachstep in the growth process.
Ideal conditions for growing microbes arecreated in large vats or tanks. Producers mixthe necessary food supply with a smallamount of the desired microbe. The vats holding the food-microbe mix are kept within the
Microbe
Bacteria (most)
Yeasts
Molds
Minimum
2.9-6.0
1.5-3.5
1.5-3.5
Preferred
6.5-7.5
4.0-6.5
4.0-6.8
Maximum
8.0-10.0
8.0-8.5
8.5-10.5
17-8 The pH level of a food product affects what types of microbes can live in the product.
Pasteurization Times and Temperatures
Process
Vat processing
High temperature-short time pasteurization
Ultrapasteurization
Temperature
67.2°C (153.0°F)71.7°C (161.1 OF)
138.0°C (280.4OF)
Time
30 min.
15 sec.
2 sec.
17-9 By increasing pasteurization temperatures, food processors can reduce the amount of time a productmust be exposed to heat.
Chapter 17 Fermentation: Desirable Effects of Microbes 389
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17-10 Yeast used for breads (Saccharomycescerevisiae) is dehydrated and then packaged incakes, packets, or jars.
Yeast FermentationFoods fermented by yeast have been used
since the dawn of recorded history.Babylonians used yeast to make beer as earlyas 6000 B.C. Egyptian tomb reliefs show theapparent commercial production of leavenedbread, wine, and beer around 2400 B.C.
All yeast breads, alcoholic beverages, andvinegar require yeast in the productionprocess. The most commonly used yeast isSaccharomyces cerevisiae. S. cerevisiae grows bestin a warm, moist environment where su"ars
°and/or starches are available. See 17-10. It isthe main yeast in brewer's yeast and is alsoused for breads.
Saccharomyces means sugar fun!rus. TheS. cerevisiae family of yeasts relies onOsugar asits main energy source. It can also feed onhoney (high in fructose), molasses, or comsyrup. However, high levels of sugar in breaddough will slow yeast growth. This is because111gh sugar levels lower the aw' When the awdrops too low, mIcrobes carmot reproduce.
Although some types of yeast canhydrolyze starch, S. cerevisiae carmot. Somebreads, such as Italian and French breads, donot contain an added sweetener. Therefore,these breads rely on added enzymes or amylases naturally fOlUld in the flour to breakdown the starch.
Some breads are prepared with quickrising yeast. This is a commercially producedhybrid product made from two yeast strains.
Fermentation
preferred temperature and pH levels to maximize microbe growth. The cultures are thenshipped to manufacturers for use in variousfood products.
Food manufacturers often add microbesto foods to bring about fermentation.Fenllelltatioll is an enzymatically controlledchange in a food product brought on by theaction of microorganisms. Some desiredchanges in food products occur as microbesrelease digestive enzymes. These enzymesbreak down components in the food product.Other changes are caused by the release of byproducts. A by-product is a substance that isproduced in addition to the main product of areaction. The primary product of a microbialreaction is energy. In prod ucing energy,microbes also produce by-products such ascarbon dioxide, acetic and lactic acids, andethanol. Such by-products can change thecolor, texture, flavor, aroma, and pH of a food.
Fermentation is an anaerobic process. Itrelies on microorganisms that use organiccompounds for their food supply. Thesemicrobes release enzymes to break down proteins, carbohydrates, and lipids that are nearby. After the enzymes break down these largemolecules, the microbes absorb the smallermolecules through their cell walls. Themicrobes use these nutrients for growth andenergy. Fermentation is the part of the processin which the nutrients are converted to energy.
Glucose is the energy source for mostliving organisms. It is converted into twopyruvate molecules. The next step is wherefermentation occurs. Pyruvate is brokendown into either an acid or alcohol and carbondioxide. Many organisms can also break protein and lipids down into pyruvate for energy.Energy is released as by-products are formedfrom pyruvate.
Fermentation varies in terms of the byproducts that are created. Microbes aregrouped as to the by-products they give off asa result of fermentation. You will study thefermentation process, the types of by-products, and foods produced by yeasts, bacteria,and molds.
390 Unit V Food Microbiology: liVing Organisms in Food~",
The result is a yeast that releases both amylases and sucrases. This speeds the productionof carbon dioxide as a by-product.
Before the commercial production ofyeast, starters were the main source of yeastfor baking. A yeast starter is a mixture of equalparts of flour and water that has natural yeastgrowing in it. Natural yeast is present in theair in small amounts. This yeast would settleonto the flour and water mixture. The yeastwould use the mixture as a food source togrow and multiply. Bakers learned thatadding the mixture to bread dough created alight, airy product when baked. The bakersdiscovered how to add some of the starter tothe bread and save some for later. The savedportion would have equal amounts of flourand water added. It would be stirred and covered daily until the yeast had multiplied and itwas time to bake bread again.
Cooking TipYou can make a yeast starter by combining250 mL (1 cup) water, 250 mL (1 cup) flour,and 1 package of yeast. The container usedto store the starter should be no more thanhalf full. This allows room for the starter toexpand as the yeast feeds on the flour.
BreadYeast fermentation changes heavy, dense
yeast dough into light, porous bread. Yeast isaffected by several important steps in thebread-making process. The first step is mixing. Most yeast bread recipes call for warmingliquids, which activates the yeast. Mixing theflour, liquid, and other ingredients distributesthe yeast evenly throughout the dough. Thedough is kneaded, or worked with the hands,to develop gluten. Gluten is an elastic proteinsubstance formed when flour is combinedwith liquid and manipulated.
The second step in the bread-makingprocess that affects yeast is proofing. Proofingmeans allowing the dough to sit in a warmenvironment. During the proofing time, theyeast releases enzymes so it can feed, grow,and multiply. These enzymes break downsugars in the dough, releasing alcohol(ethanol) and carbon dioxide as by-products.The carbon dioxide becomes trapped in small
pockets throughout the dense dough. Thepressure of the increasing volume of thistrapped gas causes the gluten to stretch. Thisis what makes the dough rise.
Baking is the final step in bread makingthat affects yeast. During baking, the alcoholproduced during proofing quickly evaporatesand the yeast cells are killed. The remains ofthe yeast stay in the dough, providing someflavor and nutritional value. The yeast breadwill continue to rise in the oven until a crustforms and the protein structure is set. This isbecause the carbon dioxide expands as it isheated. The result is a moist, light product.
If guidelines are not followed when making bread, the product may be unsatisfactory.For instance, yeast is killed by relatively lowtemperatures. The ideal temperature for yeastfermentation is 30°C to 35°C (86°F to 95°F). Ifthe S. cerevisiae is hotter than 40°C (104°F), itwill begin to die. If the yeast is killed beforethe dough is proofed, the bread will be heavyand flat. Bread recipes recommend heating theliquids to 41°C to 46°C (105°F to 115°F). This isbecause the addition of yeast cools the mixture to the ideal fermentation temperaturerange.
Two things happen if the dough is proofedtoo long. Keep in mind that the longer thedough sets, the more by-products are produced. When too much carbon dioxide is produced, the dough will be stretched too far.This will cause it either to collapse or developa coarse, dry texture during baking. When toomuch alcohol is produced, the bread will havean undesirable flavor.
All bread products have the followingingredients in common: flour, yeast, salt, andwater. Additional ingredients that are oftenused in yeast breads are eggs, milk, sugar,honey, molasses, spices, and seeds. Theseingredients change the flavor, nutritive value,and/or texture of the finished bread.
WineWine is the fermented juice of plant prod
ucts. Honey, dandelions, and various fruits areused to produce wine. However, classicalwines are made from fermented grape juice.
The quality of wine depends partly onwhich microbes are present. Therefore, manywine makers treat the crushed grapes to kill
Chapter 17 Fermentation: Desirable Effects of Microbes 391
all wild yeasts, bacteria, and fungi that may bepresent. Sulfur dioxide (502) or potassiummetabisulfite is added to inhibit the growthof unwanted organisms. These sulfites alsostabilize the wine color. The crushed grapesthen have commercially produced yeasts(5. cerevisiae and 5. ellipsoideus) added.
The yeast feeds on sugars naturally foundin the fruit juice, releasing alcohol and carbondioxide as by-products. The fermentationrequires anaerobic conditions for one to fourweeks. The fermenting juices are held in smalloak barrels or large stainless steel tanks, 17-11.The fermentation process is complete whenbubbling from carbon dioxide productionstops. The wine is then put in barrels or vatsfor aging. Chemical interactions within thenew wine slowly develop the characteristicflavors.
The original sweetness of wine is determined by the degree of fermentation andsugar content of the fruit. In sweeter wines,the fermentation process is stopped before theyeast breaks down all the sugar. These wineshave an alcohol content of 8% to 9%. Lesssweet, or dry, wines are allowed to fermentuntil the yeast has broken down all the sugar.
These wines contain 12% to 14% alcohol.However, the sweetness and alcohol contentof finished wines is sometimes adjusted by theaddition of unfermented juice, sugar, and/oralcohol.
During wine production, the carbon dioxide produced as a by-product may be allowedto escape. It may also be collected, compressed, and sold for commercial uses.Sparkling wines, such as champagne, aremade by retaining some of the carbon dioxideas a gas solute.
Other Alcoholic BeveragesIn processes similar to wine making, yeast
is used in the fermentation of other alcoholicbeverages. The flavors and names of thesebeverages are determined by the food sourceused to feed the yeast. Beers are usually madefrom fermented, malted (germinated) barley.Sake is a Japanese beverage made from fermented rice.
After the fermenting process, whiskey ismade by distilling the fermented mixture toconcentrate the flavor and alcohol content.Irish whiskey is made from a variety of grains.Scotch whiskey is made mainly from barley.
17-11 Oak barrels are used to give a characteristic flavor to wine as it ferments.
392 Unit V Food Microbiology: Living Organisms in Food
The amount of contact grapejuice has with the grape skinsdetermines the color and flavorof wine. If the skins are removedone to two days after fermentation begins, the wine will be arose wine. This is a wine with apink color. If the skins stay in thefermentation tanks for 5 to 10days, the wine will be red. Whitegrapes produce white wine.
However, red grapes can also beturned into white wine. Thisrequires the skins to be removedbefore adding the yeast. Noticethe next time you eat a red grapethat only the skin has color.
(Wine gets its color from the skins ofthe grapes used to make it.
Bourbon is a whiskey that originated 111
Kentucky. It is made from corn.Fermented ingredients are distilled to
make a number of other alcoholic beverages.Rum is made from sugar cane or molasses.Brandy is distilled wine or fermented fruitjuice. Liqueurs and cordials usually have abrandy base with sugar and flavorings added.Popular liqueurs are creme de mentlie, which isflavored with mint and curn,no, which is madefrom bitter oranges.
Bacterial FermentationFoods are fermented by microbes other
than yeast. A number of types of bacteria areused to ferment food products. There are threemain classes of bacterial fermentation. Theseare lactic acid, proteolytic, and acetic acid fermentation. Some foods require two separatefennenting agents.
Bacterial fermentation often causes texturechanges and a unique sour flavor in foods. Forinstance, the thick texture and sour flavor ofyogurt result when bacteria ferment milk. Thesour taste is caused by acids that are released
as by-products. The acids also act as preserving agents.
Lactic Acid FermentationMany of the foods produced through bac
terial fermentation are fermented by bacteriawhose major by-product is lactic acid. Somelactic acid bacteria also produce other byproducts. These include acetic acid, formicacid, and carbon dioxide. Lactic acid bacteriaare fOlmd in the genera (plural of genus) ofStreptococcus, Pediococcus, Leuconostoc, andLnctobncil/us. Lactic acid bacteria are used toferment vegetables, meats, and dairy products. See 17-12.
Sal/erlo-autSauerkraut means acid cabbage.
Sauerkraut is the result of lactic acid fermentation of cabbage submerged in a vat of brine.Brine is a mixture of salt and water. In thiscase, a 2% to 3% salt solution is used. The salthelps discourage the growth of unwanted bacteria and fungi by controlling water activitylevel. The salt also pulls water with dissolved
Chapter 17 Fermentation: Desirable Effects of Microbes 393
17-12 Lactic acid fermentation changes cucumbers into pickles and cabbage into sauerkraut.
sugar and nutrients to the smface of the cabbage. This provides the water and food sourceneeded for the bacteria to grow. If there is toolittle salt, the sauerkraut will be soft with apoor flavor. If there is too much salt, the lacticacid bacteria are slowed. The sauerkraut willbe darkened, and yeast may begin to grow.
Three types of lactic acid bacteria that prefer the slightly salty environment work in succession over a three-week period. The bacteriafeed on the sugar present in the cabbage. Thecabbage is shredded to expose more surfacearea on which the bacteria can feed. The bacteria release mainly carbon dioxide and lacticacid into the brine. The result is a creamywhite, shredded product with a soft but firmtexture.
Air must be kept from the fermentationprocess to control the growth of yeasts andmolds. The cabbage is weighted down belowthe surface of the brine. Sheets of plastic arelaid over the vat to keep out dirt and air. Thiscreates the necessary salty, anaerobic environment needed to make sauerkraut.
PicklesCucumbers can be turned into pickles by
three basic methods. They may be heated in aspiced vinegar solution. They can be refrigerated in an acid brine. However, the oldestmethod is by fermentation with lactic acidbacteria. These processes are used to picklefoods other than cucumbers. Such foodsinclude watermelon rinds, beets, cauliflower,okra, and onions.
When cucumbers are pickled, they arepacked in a sal t brine for the same reasons ascabbage. It is important that all carbohydrates used during the pickling process be
fermentable. This is because bacteria, molds,and yeasts are fowld on cucumbers. If extrasalt is not added during fermentation andsimple carbohydrates remain, yeasts canbegin to multiply. The yeast can feed off thelactic acid produced by the bacteria. This raises the pH and allows spoilage bacteria to contaminate the pickles.
Commercial pickling starts by washingthe cucumbers in a chlorine solution. Thisremoves wlwanted yeasts and molds. If thesemicroorganisms are not removed, they cancause softening and bloating. (Bloating is theformation of a large air pocket in the center ofthe pickle.)
After the cucumbers are washed, they areplaced in brine and a pure culture ofLactobacillus is added. The bacteria feed onnatural sugars from the cucumbers andrelease lactic acid into the brine. This lowersthe pH and gives pickles their crisp textureand sour taste.
Some picklers choose to make pickles by ana tmal process. For this process, cucumbersare placed in a brine of the same salt concentration used in the commercial process. Thensalt is gradually added to the brine dming thefermentation period. The salt level is morethan doubled by the end of the process. Theadditional salt is not needed in the commercial process. This is because the chlorine washing process has already removed the undesirable microbes the salt is used to control.
The conunercial process has two advantages. First, it requires less salt, which isimportant for low-salt diets. Second, it helpspicklers meet Environmental ProtectionAgency (EPA) standards regarding the dumping of brine into streams.
OlivesOlives are fermented by the same types of
lactic acid bacteria as cucumbers. Howevel~
fermenting olives calls for an added preparation step that is not required in making pickles.Olives me washed in a lye solution to removebitter flavor compowlds. The lye can removeneeded microbes, and rinsing rel110ves nutrients as well as the lye. To overcome theseproblems, lactic acid is added to neutralizeany lye remaining after the olives are rinsed.This reduces the amolU1t of washing needed.
394 Unit V Food Microbiology: living Organisms in Food
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Sugar is added, and lactic acid bacteria staTtthe fermentation process.
Fermenting olives also requires a highersalt concentration than making pickles. Thesalt solution in which olives are fermentedneeds to be kept between 5% and 15%. Thisrange is ideal for lactic acid bacteria to growand multiply but is too salty for most spoilagebacteria. As the olives ferment, sugar is pulledout of the olives and salt is pulled into them.This lowers the salt concentration of the brine.Salt must be added anytime the brine dropsbelow the 5% level. Fermentation of olives cantake two weeks to several months.
Green or Spanish olives are picked whenthey have reached full size but before theyhave ripened. They are then cured and oftenpitted and stuffed, most commonly withpimento. Black or ripe olives are picked at ariper stage. They have a deep green color whenpicked. The lye curing process and oxygenation turns them black. Olives that are treeripened have a dark brown to black color. Mosth·ee-ripened olives are pressed for their oil.
MeatsMeats are fermented with lactic acid bacte
ria to make semidry and dry sausages. Thefermentation process increases the acid level.This tenderizes the meat and adds a tart flavor. Fermentation, along with smoking anddrying, lowers the aw level to prevent spoilage.
Fermented sausages are made by mixingchopped meat with sugar, spices, and salt. Thesugar provides food for the lactic acid bacteria. The spices and salt add desired flavor.Sodium nitrate, sodium nitrite, or a combination of these is mixed into the meat. Theseadditives prevent the growth of spoilage bacteria. Lactic acid bacteria are added to ensureproper color and flavor of the sausage.
CII/tured Dairy Prodl/ctsCultured dairy products include sour
cream, yogurt, and buttermilk, 17-13. Theseproducts are made with the help of lactic acidbacteria. The strains of bacteria used varydepending on the desired end prod uct.Several kinds of Streptococci are used becausethey are the fastest lactic acid producers.They enable the milk base to acidify quickly.This reduces preparation time and the risk of
17-13 Cultured dairy products are fermented withbacteria that feed on lactose.
contamination. Lel/COnDstoc and Lactobacillistrains are added to produce the desired flavors.
Fresh milk contains microorganisms thatwould cause it to spoil. Therefore, milk beingused for cultured dairy products is pasteurized. Pasteurization ensures that unwantedmicrobes are destroyed. This helps produce aconsistent, high-quality product.
After pasteurization, starter cultures ofbacteria are added to the milk. The bacteriafeed off the lactose in the milk. They releasecarbon dioxide, lactic acid, and a number offlavor compow1ds. The acid denatures theproteins and causes them to coagula teo Thedegree of coagulation is determined by thecombina tion of bacteria. Temperature, pH, fermentation time, and added enzymes alsoaffect the texture of the product.
Most cultured dairy products are made bysimilar processes. The products are heated,cooled, mixed with a culture, and fermented.The fermentation process is stopped by cooling. The products are then ready for packaging.
Cultured dairy products are often suggested for people with lactose intolerance. This isbecause the bacteria use the lactose as theirenergy supply. The result is low-lactose products that are easier to digest.
People with lactose intolerance can alsouse lactose-free milk prod ucts. These products are made by adding 1. acidophi/lls. The
..
Chapter 17 Fermentation: Desirable Effects of Microbes 395
L. acidophilus releases an enzyme that breakslactose down into glucose and galactose. Theproducts that result are sweeter than milkbecause glucose tastes sweeter than lactose.People with lactose intolerance can add commercially produced lactase drops or tablets todairy products, too.
Health TipAntibiotics will often destroy helpfulmicrobes in the intestines as well as disease-causing bacteria. Eating yogurt thatcontains "active cultures" can help replenish intestinal microbes after the use ofantibiotics.
CheesesLike making cultured dairy products,
making cheese starts with pasteurizing themilk. The proteolytic enzyme rennin and aculture of lactic acid bacteria are then added tohelp form the curds. CII1'ds are clumps ofcoagulated protein. In this case, the protein iscasein. The lactic acid bacteria lower the pI-I sothe proteolytic enzymes in reruun will coagulate the casein more effectively.
The curds are cut into small cubes. Themixture is then heated to 38°C to 40°C (lOO°Fto 104°F) for about 45 minutes. The cuttingand cooking process helps the whey separatefrom the curds. Whe1J is a liquid high in soluble whey proteins. TIle whey is drained offand collected to be used as an additive inprocessed foods sum as baked goods, mixes,and margarine. The curds are then salted toadd flavor and reduce the risk of spoilage.
At this point, the meese making processvaries according to the type of cheese beingmade. Curds being used to make cottagemeese are mixed with cream that has had cultured skim milk added. Curds for agedcheeses, sum as Cheddar, Edam, Swiss, brick,and blue, are put in presses. The pressessqueeze out excess moisture.
Most cheeses need a ripening period.Bacteria remaining in the curds will ripensome cheeses. For other cheeses, microorganisms such as mold are added to the saltedcurds to do the ripening. During ripel'ling, thecheeses are wrapped or covered with wax andplaced in curing rooms. The curing rooms
have controlled humidity and temperaturelevels. These conditions are designed to matmthe growth needs of the microorganismsdoing the ripening.
Specific types of fermenting bacteriaand/or molds are used to give each type ofc11eese its maracteristic flavor. Propionic acidbacteria are added to Swiss cheese. These bacteria develop the carbon dioxide that formsthe maracteristic eyes, or holes, in Swisscheese, 17-14. Molds are used to make meesessuch as Limburger and blue cheeses.Limburger uses a mold at the beginning of theripening process. TI'lis mold lowers the acidityso proteolytic bacteria can develop thecheese's characteristic flavor, texture, andaroma. Blue cheeses, such as Roquefort, havea blue-green mold added. This mold needsoxygen to grow. Therefore, the surfaces ofblue cheeses are pierced with needles to allowoxygen to ream the mold witl'lin the cheese.
The sharpness of cheese refers to thestrength of its flavor and aroma. Sharpness iscaused by acids and a variety of aroma compounds. These compowlds are formed asbacteria and enzymes in cheese continue toferment lactose and other orgmuc compoundsduring a curing process. Therefore, the sharpness of cheese depends on the length of timethe cheese is cured. For instance, mildCheddar cheese is cured for four months.Sharp Cheddar is cured for about a year.Extra sharp Cheddar is cured for up to h"oyears. Tasting samples of mild, sharp, andextra sharp Cheddar cheese will illustrate
USDA
17-14 The holes in Swiss cheese are a result ofpropionic acid fermentation.
396 Unit V Food Microbiology: living Organisms in Food
these flavor differences. You will also note texture differences. Cheeses that are cured longertend to have firmer textures, are more crumbly, and melt into sauces more readily.
Mold FermentationSome fermented foods are produced by
the action of molds. Molds create a wide rangeof by-products. These by-products includeantibiotics, flavor compounds, and enzymes.
Soy SauceSoy sauce, an important flavoring ingredi
ent in Asian cooking, is a fermented mix ofsoybeans and wheat. The fermenting agent isa mixture of cooked rice and several strains ofmold from the Aspergillus family. When addedto the soy-wheat mixture, the molds produceenzymes. These enzymes hydrolyze the proteins a.nd carbohydrates in the soybeans andwheat. Once mold covers the soy-wheat mixture, a brine is added. The brine stops thegrowth of lLl1wanted microbes. Lactic acid bacteria can then multiply, causing the pH of themixture to drop. Toward the end of the fermentation period, yeasts are added. TI1ey fermentthe sugars remaining from the hydrolysis of thecarbohydrates. The fermented mixture is filtered, pasteurized, and bottled.
TempehAnother soy product produced through
mold fermentation is tempeh. Telllpeh is anAsian soybean cake. The soybeans are cooked,mashed, and formed into cakes. The cakes areinoculated with Rhizoplls molds. The cakes arethen wrapped in banana leaves and fermentedfor one to two days.
Two-Step FermentationMany fermented foods are made with two
or more fermentation steps. Each step mayrequire a different kind of microbes. Sometwo-step fermentation processes involve lacticacid bacteria plus other microbes. Acetic acidfermentation is an example of a two-stepprocess. It requires yeast as well as bacteria.
Lactic Acid Plus Other MicrobesTwo-step fermentation often involves lac
tic acid bacteria in one step and othermicrobes in a second step. The aged cheeses
you read about earlier are examples of thistype of fermentation. Lactic acid fermentationis used to make the cheese. Then othermicrobes are used to develop the characteristic flavors and textures.
Sourdough bread is another example of afood product made with a two-step fermentation process involving lactic acid bacteria. Thefirst step of the process requires lactic acidbacteria to ferment the yeast starter. TI1is stepis what gives sourdough bread its characteristic sour flavor. See 17-15. The second step ofthe process is the same yeast fermentationused to make other yeast breads.
The strain of bacteria used in sourdoughstarter is wctobncilllls snrifmncisco. This bacteriwnis indigenolls, or native, to the San Franciscobay area. During the Alaskan gold rush,many prospectors would keep jars of yeaststarter in their parkas. This would keep thestarter warm. The prospectors would usepart of the starter to make bread, biscuits, orpancakes. Then the prospectors would addmore flour and water to feed the base starterfor the next day. Many of these prospectorswould sail from San Francisco or return tothe bay area for the winter. The bacteriawould get into their starter when they werein San Francisco.
Acetic Acid FermentationThe first step of acetic acid fermentation is
yeast fermentation. Yeasts are added to a foodprod uct under anaerobic conditions. Theyeasts use sugars in the food product for their
17-15 A yeast starter for sourdough is a mixtureof equal amounts of flour and warm water with apackage of yeast.
Chapter 17 Fermentation: Desirable Effects of Microbes 397
food supply. The yeasts release alcohol as aby-product as they break down the sugars.After the yeast fermentation is complete,the second step of acetic acid fermentationcan begin.
For the second step, Acetobacter bacteriaare added to the food product. These bacteriaare aerobic. They use the alcohol produced bythe yeast as their food supply. They releaseacetic acid as a by-product as they break downthe alcohol. The chemical changes caused byAcetobacter are shown by the following chemical formula:
C,HsOH + 0,~ticilcid
b."leteria• CH,COOH + H,o
vinegar depending on the food on which itfeeds. v\Then acetic acid bacteria are present inred wine, red wine vinegar is produced. Amixture of water and wheat will producewhite vinegar. Apple juice with acetic acidbacteria will yield apple cider vinegar. TheFDA requires that vinegar contain at least4 mL of acetic acid per 100 mL of water. Thismeans vinegar is a 4% solution.
Other foods that require acetic acid fermentation are cacao beans and candied citron.This two-step process helps turn cacao beansinto chocolate. Candied citron is a fermentedproduct of citron lemons used in baked goodssuch as fruitcake.
Vinegar is one of the foods produced as aresult of acetic acid fermentation. Acetic acidis what gives vinegar its sour taste. Aceticacid bacteria will produce different types of
alcohol oxygen acetic add water
Benefits of FermentationFermenting food products has a couple of
key advantages. First, microbes help preservesome foods. Milk will keep for about a weekin the refrigerator. Cheese can keep for
Historical HighlightLouis Pasteur: One of theWorld's Greatest Scientists
During his lifetime, LouisPasteur (1822-1895) made majorcontributions to science, medicine, and the food industry. Hisscience contributions began withhis work in chemistry with thestructure of crystals. He thenturned to studying microbes. Hewas the first to prove the theoryof spontaneous generation wasfalse. This theory stated thatliving things (microbes) couldcome from nonliving material,such as dirt.
Pasteur made several contributions in the areas of human
and animal medicine. He helpedprove that microorganisms arethe source of infectious disease.He worked to prevent the deathof silkworms. He also focused onthe development of immunity todisease through vaccination. Hedeveloped vaccines to protectsheep from anthrax and chickensfrom cholera. He also developedthe vaccine for rabies.
Pasteur made an impact onthe food industry in his day. In1857, he wrote in his journalsthat alcoholic fermentation wascaused by yeast. He noted that
lactic acid fermentation wascaused by round organisms (nowknown as lactic acid bacteria). In1861, he identified rod-shapedorganisms as the source ofbutyric acid fermentation. He alsonoted these bacteria were anaerobic. In 1864, Pasteur beganresearching the cause of bitterflavors in wine. He found thatmicrobes were the cause. Hedeveloped the pasteurizationprocess to protect wine fromthese microbes. He applied thismethod to the preservation ofmilk and beer as well as wine.
398 Unit V Food Microbiology: Living Organisms in Food 1'~
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months when properly stored. Cucumbers because portions of the casein molecules that
1will spoil in a week or so in the refrigerator. form the curds are attracted to fat. When thePickles will keep in unopened jars on the shelf water-soluble whey is drained off, the fat con- ~for a year or more. centration is increased. Calcium is also con- 1
J
A second benefit of fermentation is variety. centrated in cheese. One ounce of Cheddar or 'J~
Fermented products have added a wider Monterey Jack cheese contains as much calcium irange of food options to diets around the as six ounces of milk. The iron in one cup of ,!
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world. raw cabbage is 0.4 mg. One cup of sauerkraut ~
Icontains 3.47 mg of iron. J
Nutritional Changes in Fermented1
Other ingredients added during process- 1~
Products ing also affect the nutritional quality of fer- (
:1
The nutritional value of fermented foods mented foods. For instance, all pickles have 1~
often differs from the value of their unfer- more sodium than cucumbers. This is because i
mented counterparts. The types of microbes of the salty brine used during fermentation.added and the energy sources they consume Soybeans contain 1 mg of sodium per cup.affect the nutrient content. For instance, "Light" soy sauce contains as much as 530 mg
~l"l cheeses are higher in fat than milk. This is of sodium per tablespoon. See 17-16.11 111
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141111What Microbes Do to Food Value
Iak~ Serving Protein Carb Fat Sodium Calcium Iron VitA VitCFood Size Cal (g) (g) (g) (mg) (mg) (mg) (RE) (mg)
