FST 311- PHYSICAL AND COLLOIDAL CHEMISTRY OF FOOD COURSE OUTLINE 1. Water - sructure, chemical and functional properties in Foods, Water activity of food. Applications in food preservation and processing 2. Food Rheology - Definition of common terms: Rheology, stress, stain, shear stress and shear strain - Fluids flowing properties during food processing; plasticity, pseudo elasticity, Elasticity, Viscoelasticity, viscosity and texture - Newtonian and Non- Newtonian liquids, giving specific examples and areas of application in foods - Classification of non-Newtonian fluids (i) Time dependent non-Newtonian fluid: Thixotropic and Rheopetic fluids (2) Time independent Non-Newtonian fluids
Transcript
FST 311- PHYSICAL AND COLLOIDAL CHEMISTRY OF FOOD
COURSE OUTLINE1. Water - sructure, chemical and functional properties in Foods,
Water activity of food. Applications in food preservation and processing
2. Food Rheology
- Definition of common terms: Rheology, stress, stain, shear stress andshear strain
- Fluids flowing properties during food processing; plasticity, pseudoelasticity, Elasticity, Viscoelasticity, viscosity and texture
- Newtonian and Non- Newtonian liquids, giving specific examples andareas of application in foods
- Classification of non-Newtonian fluids
(i) Time dependent non-Newtonian fluid: Thixotropic and Rheopeticfluids
(2) Time independent Non-Newtonian fluids
Shear thining fluids (Pseudoplastic)
Shear thickening fluids (dilatant fluids)
Binghamplastic fluids
- Surface tension of fluids:- Determination andimportance in food processing.
Surfactant: common examples, mode of actionand uses in foods
3. Food colloids: sols, suspensions, emulsions,Gels, foams, formation and their stability in foodsystem.
4. Food additives and toxicants
Reference
1. Peter S. Murano (2003) understanding food scienceand Technology Wadsworth Cengage Learning
2. Pandey, H; Sharma, H.K; Chauhan, R.C; Sharkar, B.Cand Bera M.B (2004). Experiments in food processengineering. CBS New Delhi
3. Fellows P.J. (2000). Food processing Technology-principles and practice Woodhead, CambridgeEngland.
4. Prentice I.H (1984). Measurements in the Rheology offoodstuffs. Elsevier Applied Science. London
5. Fennema, O.R (1976) Principles of Food Science Part 1Food Chemistry. Marcel Dekker, Inc. New York.
WATER
Water molecule structure.
In one Water molecule, H20, the two hydrogenatoms are bounded to one oxygen atom. Eachhydrogen atom of a water molecule shares anelectron pair with the oxygen atom
A charge separation (dipole) occures in whichthe oxygen atom displays a partial negativecharge while the hydrogen atoms are partiallypositive (fig II)
104 150H
HPartial + positive charge (𝛿 +)
Partial + positive charge (𝛿 +)
Partial - ve charge (𝛿 −)
Partial 40% ionic character
The dipolar nature of water affects its physicalcharacteristics like boiling point, freezing pointand vapour pressure because the polarityproduces intermolecular attractive forces
Hydrogen bond in water
Water’s large intermolecular attractive forcescan be explained quite adequately by its abilityto engage in multiple hydrogen bonding on athree dimensional basis. Hydrogen bonding is anelectrostatic attraction between adjacent H20molecules (fig 3)
Fig III: Hydrogen bonding of water molecules in a tetrahedralconfiguration ……… Hydrogen bonds
• Water’s ability to engage in three dimensional hydrogen bondingprovides a logical explanation for many of its unusual properties.For example large values for heat capacity, melting point, boilingpoint, surface tension, heat of fusion, vaporization are all related tothe extra energy needed to break intermolecular hydrogen bonds.
Solvation and Dispersion Action of water .
1. Solubility:- The fact that molecules can form hydrogen bonds inH20 means they can be dissolved or dispersed. This concept istermed solubility. Compounds that hydrogen bond easily to water toform solution or colloidal dispersion are said to be hydrophillic,usually they are charged or polar molecules.
2. Hydration:- Is the process by which water molecules surround andinteract with solutes by acting as a solvent. When a salt such assodium chloride becomes hydrated the electrostatic attraction of Naand Cl ions for one another weakened, the ions interact with H20molecules and go into solutions as solutes with negative end of thewater molecules attracts to the Na+ and positive end of the watermolecules attracts to the Cl- ions e.g Proteins, certain Vitamins,phospholipids, sterols (with hydrophyllic and hydrophobic). Foodcompounds that have charged functional group such as CO , NH3
+ POare water soluble for the same reason.
3. Formation of micelles structure:- Water alsodisperses amphiphillic molecules. In water they formmicelles. Micelles are dusters of molecules in whichthe hydrophobic groups are directed away from thewater, while the polar (charged) groups are exposed onthe external surface.
The non polar hydrophobic groups form a stable innercore due to forces called Hydrophobic interactions(seethe next slide).
Formation of Micelles
FUNCTIONAL PROPERTIES OF WATER1. Water is a component of colloidal dispersions
2. It is a fat replacer ingredient and a zero calorie ingredient infood
3. It is a medium of heat transfer because of its high thermalconductivity
4. It serves as a medium for chemical reactions since reactiontakes place in solution
5. It is a medium for microbial growth
6. Water is a plasticizer-a substance that when added to a foodsystem, makes it softer
7. It is a reactant or a product in chemical reactions such ascondensations and hydrolysis.
Water Activity of foodsSignificance of water in foodsWater is present in most natural foods to the extent of 70% oftheir weight or greater. Fruits and vegetables may contain 90%or even 95% water. Cooked meat from which some of the waterhas been driven off still contains about 60% water. Water affectsthe texture of foods e.g raisin (dehydrated grape) and prune (adried plum). The form in which water occurs in foods dictatesthe physical properties of the food for example, fluid milk andapples contain approximately the same amount of water buthave different physical structures.Water affects the keeping qualities of food, which is one reasonfor removing it from foods (by evaporation, drying,concentration, frozen, emulsification) water is most active infoods in liquid form.As a liquid in foods, it is the solvent for numerous foodchemicals and thus promotes chemical reactions between thedissolved constituents.Water is also necessary for microbial growth.
It may also be necessary to remove water form food (in addition topreservation) to reduce the weight and bulk of the food and thussave on packaging and shipping costs.
Water exists in foods in various ways- as a free water (e.g in tomatojuice), a droplets of emulsified water (e.g butter), water tied up incolloidal gels (e.g gelatin desserts), a thin layer of adsorbed wateron the surface of solids (contributing to caking as in dried milk),and as chemically bound water of hydration (e.g sugar crystals)
Some of these bound water forms are extremely difficult to removefrom foods (even by drying)in order to preserve such foods. Manyskills in food processing involve the removal of these bound waterto a save levels in foods. In some cases the storage stability of adehydrated item is enhanced by leaving a trace of moisture,equivalent to a mono molecular layer of water, to coat all externaland internal surface. This monomolecular layer of water then mayserve as a barrier between atmospheric oxygen and sensitiveconstituents in the food which otherwise would be more easilyoxidized
• Water activity (Aw)
Moisture content of foods refers to the absolute amount of waterpresent in a food. Water activity (aw or Aw) is a measure of theavailability of water molecules to enter into microbial, enzymatic orchemical reactions. This availability affect the shelf life of a food.Water activity is calculated as the ratio of the water vapourpressure of the substance divides by the vapour pressure of purewater at the same temperature.
aw=𝑝
𝑝𝑜
p= vapour pressure of the food
po= vapour pressure of pure water at the same temperature. Thewater activity of pure water is 1.0 according to this equations. Allfoods, since they contain some non volatile substances, will have aw
values less than 1. In simpler terms aw is a measure of relativehumidity. By multiplying aw by 100, the relative humidity (RH) ofthe atmospher in equilibrium with the food (RH%) or is obtained.
• RH(%)= 100 X aw
Food Moisture (%) Water activity
Ice at o0c 100 1.00
Fresh meat 70 0.985
Bread 40 0.96
Flour 14.5 0.72
Ice at -500C 100 0.62
Raisins 27 0.60
Macaroni 10 0.45
Potato chips 1.5 0.08
The Table below gives the moisture content and water activity of several foods
Water activity has to do with the form in which the water exists in foodi.e
Free water
Adsorbed water
Bound water
• Free water: Is highly entrapped and therefore easily pressed fromfood matter, when this is done, the water can be seen and felt. Freewater acts as a dispersing agent and solvent and can be removed bydrying.
• Adsorbed water: or structural water associated in layers viaintermolecular hydrogen bonds around hydrophilic food molecules
• Bound water: Or water of hydration exists in a tight chemically boundsituation, such as within a arystalline structure via H2O ion and waterdipole interactions, bound water does not exhibit the typicalproperties of water, failing to freeze at OoC and failing to act as asolvent.
Water in foods exerts a vapour pressure the size of the vapour pressure dependson
a. The amount of water present
b. The temperature
c. The concentration of dissolved solute (particularly salts and sugars) in thewater. The more the dissolved solute, the less volatile the water and the lessthe vapour pressure.
• Equilibrium moisture content of foods
• At a constant temperature, the moisture content of food changes until it comesinto equilibrium with water vapour in the surrounding air. The food thenneither gain nor loses weight on storage under those conditions. This is calledthe Equilibrium moisture content of the storage atmosphere is known as theequilibrium relative humidity.
• BET mono layer value
A proportion of the total water in a food is strongly bound to specific sites,however when all sites are occupied by adsorbed water, the moisture content istermed BET (brunauer-Emmett-Teller) monolayer value. The BET monolayer valuerepresents the moistiure content at which the food is most stable. At moisturecontents below this level, there is a higher rate of lipid oxidation and at highermoisture contents maillard browning, enzymatic and microbiological activities arepromoted.
• Measurement of Aw in foods1. Measurement of Aw based on the direct
relationship between aw and ERH2. Measurement of the ambient relative humidity in
food storage areas3. Measurement of total moisture content of a food
material and relating this moisture to aw using apre-determined sorption isotherm curve for thefoods
• Moisture sorption isotherm (MSI)MSI are graphs of data that interrelate the water(moisture) content of a food with its water activity ata constant temperature. It indicates the water activityat which a food is stable and allows prediction of theeffect of changes in moisture content on aw andtherefore on storage stability (see below)
Moisture sorption isotherm (MSI)
MSI are graphs of data that interrelate the water (moisture) content ofa food with its water activity at a constant temperature. It indicatesthe water activity at which a food is stable and allows prediction of theeffect of changes in moisture content on aw and therefore on storagestability (see below)
Water activity
AA
BB
CC
DD
100
x
0 0.25 0.5 0.75 1.0
Moisture sorption isotherm (MSI)plot to show how wateractivity and moisture differ. For example, a food productmight have a relatively low constant moisture level (x) andyet increase significantly in water activity (Point A to B).during storage of a day food in a humid enviroment asmore moisture uptake occurs (points B to C to D) theproportion of bound water decreases and the amount offree water increase. This changes a hard crisp product toone that is soft and moist. Each food has unique set ofsorption Isotherms at different temperatures.Practically, reduction in aw can be obtained by drying,adding water soluble salts and sugars, and by freezing offoods.
• Effects of aw on foods
Almost all microbial activities are inhibited belowaw of 0.6, most fungi are inhibited below aw of 0.7,most yeasts are inhibited below aw of 0.8 andmost bacteria below aw of 0.9. The interaction ofaw, with temperature, pH, Oxygen and carbondioxide or chemical preservatives has animportant effect on the inhibition of microbialgrowth (huddle effect).
Most enzymes are inactivates when the aw fallsbelow 0.85. Non enzymatic or maillard reactionsare strongly dependent on aw and reach amaximum rate at aw of 0.6-0.7.
Food RheologyFood Rheology is the branch of food science that is concernedwith the flow and deformation characteristics of food materialsunder the applied force. Rheological measurements includemeasure of viscosity, fluidity, elasticity and plasticity of non solids.The study of rheology is relevant because:1. It can provide information about a food’s structure2. It can aid in the design of food processing equipment3. It can provide vital information related to a food’s shelf life4. Rheological data can be correlated to sensory data5. It can help in product development to fine tune foods to
consumer’s taste.In rheological measurements we talk about stresses or strains.A stress (pressure, mechanical action) is defined as force per unitarea of food materials, which cause them to move or to deform(become compressed expand, bend or break).
Strain: Strain is a result of stress and may take theform of deformation in the case of solid material.Fluid foods show continuous deformation at a flowrate that is proportional to the amount of stressapplied to them. Fluids do not recover after a stressis removed, instead fluids respond to stress bychanging shape which means movement of flowaway from the stress. solids respond to stress byexhibiting strain, deformation or rupture.Shear: It refers to the relative motion of one surfacewith respect to another surface in parallel to it, whichcreates a zone of shearing action on any substance(fluid or solid) located between the moving surfaces.The stress as a result of applying shearing force iscalled shear stress and the corresponding strain iscalled shear strain
Material flows properties during food processing
1. Plasticity:- A plastic solid shows continuous deformationat a rate that is proportional to the amount of stress applied.Although there is some recovery after the stress is removed.A material that exhibits plastic flow resists change inposition until a minimum force is applied, at which point(yield point) can be made to flow. e.g soft cheeses, cakefrosting etc.
2. Pseudoplastic fluid:- Viscosity decreases as the shearrate increases. Examples are banana puree, orange juiceconcentrate, Gum Tomato Purce
3. Elasticity:- Elastic food materials will return to theiroriginal shape when the deforming stress is removed
Material before stress is
applied pseudoplasticityApply Shear Stress Material deformed by stress
Material before stress is
applied
When shear stress is applied
Materials recovered when stress is
removed
Example: bread dough
4. Visco Elasticity:- Visco elastic food has viscous andelastic properties exhibited at the same time. Itexhibits both flow and deformation. Examples arebread dough, cheese, gelled foods (like jam)
5. Viscosity:- It is defined as a liquid’s internalresistance to flow. For all liquids viscosity decreaseswith an increase in temperature, and for most gases itincreases with increase in temperature. The absoluteviscosity can be calculated in terms of shear stress (theforce acting in the plane of a fluid) and shear rate (thevelocity experienced by the fluid between movingplates)
n =𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 (𝑟)
𝑠ℎ𝑒𝑎𝑟 𝑟𝑎𝑡𝑒 (𝐷)(mpa.sec)
The units: mpa.sec (milli pascal seconds) or poise.
Viscosity is an important characteristics of liquid foods in manyareas of food processing. For example the characteristics mouthfeel of food product such as tomato ketchup, cream, syrupyoghurt e.t.c is dependent on their consistency or viscosity.
The viscosity of many liquids changed during heating, cooking,concentration etc and this has important effect on for examplethe power needed to pump this product.
Newtonian and non- Newtonian fluid
• Newtonain foods: are homogeneous mixtures that exhibit nochange in viscosity as the rate of shear (applied mechanicalforce) is increased. Examples are: water, fruit juice, carbonatedbeverages, sugar solution, milk, honey, vegetable oils e.t.c. Theviscosity of Newtonian fluid is dependent only on temperaturebut not on shear rate and time.
In Newtonian liquid, the graph of shear stress against shear rate islinear, passing through the origin.
Shear stress
(mpa)y
x
Shear rate (1/5)
Slope =
x
y
x
y
Absolute Viscosity (mpa.sec)
Non- Newtonian fluids/ foodsThese are heterogeneous mixtures that exhibit achange in viscosity as the rate of shear (appliedmechanical force) is increased. Non-Newtonians fluidscan be classified into 2 categories.A. Time independent Non-Newtonian fluidsB. Time dependent Non-Newtonian fluidA. Time independent Non- Newtonian fluids: theirviscosities are not dependent on time but on shear rateand temperature. They are:
Vis
cosi
y
Newtonian Liquid
Shear rate (1/5)
Pseudoplastic:- Their viscosity decreases with increased in shear rate.They are also described as “shear-thinning” fluids
Vis
cosi
ty
Psendo Plastic
Shear rate (1/S)
Sh
ear
Str
ess
Psendo Plastic
Shear rate
Examples are: Fruit Juice concentrates, vegetable soups, puree,apple sauce, guar and xanthan gum, thickening products etc.II. Dilatant fluid: The viscosity of these foods fluids increaseswith increased shear rate. They are also called “shear –Thickening” fluids
These fluids exhibit a so-called yield value i.e. a certain shear stress
must be applied before flow occurs; once flow starts shear rate has
no effect.
Vis
cosi
ty Bingham Plastic
Shear rate
Sh
ear
Str
ess
Shear rate
Examples: tomato Paste, French Dressing, TomatoKetchup
B. Time dependent Non-Newtonian fluids
In these fluids, the viscosity is dependent ontemperature, shear rate and time
I. Thixotropic fluids: These fluids are also knownas time thinning flows i.e their viscosity decreaseswith time
Vis
cosi
ty Thixotropic fluids
Shear rate
Shea
r S
tres
s
Shear rate Example: Yogurt
II. Rheopectic fluids: These fluids are also known as timethickening i.e their viscosity increase with time
Vis
cosi
t
y
Rheopetic fluids
Shear rate
Shea
r st
ress
Shear rate
Food Texture
Texture can be regarded as a manifestation of the rheological properties ofa food. It affects processing and handling, influence food habits and affectshelf life and consumer acceptance of foods. Building up an image of thetextural properties of the food takes place in several stages.
1. An initial assessment of hardness, ability to fracture and consistencyduring the first bite.
2. A perception of chewiness, adhesiveness and gummness duringchewing, the moistness and greasness of the food together with anassessment of the size and geometry of individual pieces of food.
3. A perception of the rate at which the food breaks down while chewing,the types of pieces formed, the release or absorption of moisture andany coating of the mouth or tongue with food.
Force, distance, time energy and ratio measuring instruments are the basisof the most common method used in measuring food texture and otherrheological properties e.g tenderometer, (force) penetrometer/ distancemeasuring), Ostwald viscometer (time),Farinograph /Amylograph (Energymeasurement) Brookefield viscometer (force).
• SURFACE AND INTERFACE.Various types of interfaces can exist between twophases, the main ones being gas-solid, gas-liquid,liquid- solid, and liquid –liquid. If one of the phases isa gas (mostly air), one usually speaks of a surface. Wecould have solid interface (one of the phases is a solidand a fluid interface between 2 fluids gas- liquid orliquid- liquid). A solid interface is rigid, while a fluidinterface can be deformed.• Surface and interfacial tensionAn interface between 2 phases contains an excess offree energy. Molecules in the interior (bulk) of a liquidphase is attracted in all direction to other molecule inits immediate environment, the attractive forces arebalanced (see below).
Air
Liquid I
Liquid II
Molecules at a surface or interface are not surrounded completely byother molecules of the same type and the same physical state andthus the net attractive force for each molecule is directive toward theinterior of the phase in which the molecule resides, the inwardattraction tends to reduce the number of molecules at the surface orinterphase and as a result the surface or intersurface area is reducedto a minimum. The forces causing a reduction in surface or interfacialarea are called surface tension or interfacial tension (y). The unit ofmeasurement is dynes/cm at constant temperature, pressure andconcentration.
. Y=𝑓
𝑙(𝑓𝑜𝑟𝑐𝑒
𝑙𝑒𝑛𝑔ℎ𝑡).
Surface or interfacial tension can also be defined as the work requiredto overcomes the force of inward attractions so that the interiormolecules can move to the surface region (to increase the area ofsurface).The intermolecular attractions which are responsible for surface andinterfacial tensions of liquids involved H-bonding and dispersion forces,both of the forces are important forces for non polar liquids (e.g triglycerides)
Factors affecting the surface tension of fluids:1. Temperature:The surface tension of almost all liquid drops linearly
with increasing temperature.2. Presence of solute: with solutions of inorganic salts and compounds
with a large no of hydroxyl groups (e.g sucrose)surface tensionincreases slightly as the concentration of the solute increases.
3. Presence of surfactants in solution reduces surface tension.Examples in foods are: Protein, phospholipids, salts of fatty acids,monoglycerides, diglycerides etc
Industrial application of surfactants1. They are being used as stabilizers and emulsifiers in food industries
e.g lecithin2. Wetting/ cleaning agents (see previous note)References:1. Potter and Hotchkiss (ed) (2--7). Food science C.B.S publishers, Inc
Maryland2. Ihekoronge and Ngoddy (1985). Integoated food science and
Technology for the tropic Macmillan education Ltd. London andOxford.
Mode of actions of cleaning agents
Object to be cleaned
Hydrophobic (R)Grease
Hydrophyllic (M)
The grease is pulled away from the surface to be cleaned bythe key mechanism shown. The process of any cleaningagent is to supply compounds with hydrophobic andhydrophyllic groups which will also appreciably decreasesurface tension and increase wettability.
Classification of cleansing agent
1. Soaps:- Compound of the type R.COO.M where R.COO isfatty acid radical representing oleic, stearic, palmitic,lauric and myristic. These are usually present in soaps asmixtures based on glyceride raw materials. M is an alkalielement such as Na or K.
2. Detergents:- Synthetic organic chemicals whichpromote better surface tension lowering than soaps.Detergents compound, which can be oxidized to simpleand products (biologically soft syndets) are preferable.
Types of detergent
1. Anionic (give R- in water) e.g
• Sulfated fatty alcohols
• Sulfonates e.g aryl-benzen-sulfonats (ABS
• Sulfated esters and acids: Has good stability in hard water but not under acid and alkaline conditions
2. Cationic (give R+ in water)
• Have no strong detergent characteristics but do have germicidal properties e.g Quaternary ammonium compounds
3. Non ionic: Produces little foam but possessesexcellent soil removal and grease emulsificationxtenstics e.g Akyl-aryl-ethylene oxide derivatives,ailphatic polyhydric alcohol esters and fatty acidamides.4. Detergent builders:• Phosphates such as sodium tripoly phosphate and
tetrasodium pyrophosphate. Used 30-50% indetergent formulation to extend foam.
• Na carboxymethyl cellulose (1-3%) to improve soiland dirt suspensions.
• Fluorescent dyes as brightners.• Na silicate added to avoid Aluminium corrosion in
washing machines• Zeolite as alternative phosphorus free builders
FOOD ADDITIVES
• Food AdditivesA food additive is any substance not naturallypresent in a food but added during its preparationand remaining in the finished product, also in thiscategory is any substance naturally present but witha concentration increased by fortification. Onemajor function, but not the sole function, of foodadditives is to aid in preserving foods. Otherchemical added to foods are not preservative butare added for functional properties associate withfood colour, flavor or texture.
• The use of chemical additives in food processing
1. Maintenance of nutritional quality, such as by the use of antioxidants
2. Enhancement of keeping quality or stability, with resulting reduction in food wastage,through the use of antioxidants, antimicrobial agents, inert gases, meat cures etc
3. Enhancement of attractiveness of foods by means of colouring and flavoring agents,emulsifiers, stabilizer, thickeners, chamfers, and bleaching agents
4. Providing essential aids in food processing agents which function in this capacity includeacids, alcohols, buffers, sequestrates and various other types of chemicals
Broad classes of international additives
1. Preservatives:- Inhibits the growth of bacteria, yeasts and or molds. Examples are sodiumbenzoate (in soft drinks and acid foods), sodium/calcium propionate (in bread and cakesas mold inhibitor) and sorbic acid used in cheese and in moist dog foods to control mold.Ant browning agents e.g S02
2. Antioxidants: Are used to prevent oxidation of fats by molecular oxygen. Without themfat containing foods could not be stored for long without developing rancidity e.g BHA,BHT, Ascobic acid, stannous chloride, Tocopherols So2
3. Sequestrates: These are chelating agents or sequestering compounds which serve toscavenge metal ions. They do this by combining with trace metals such as iron and copperand remove them from solution e.g Ethylene diamine tetra acetic acid (EDTA), polyphosphate citric acid. The trace metals need to be removed because they are activecatalyst of oxidation, leading to off-colour or flavor reactionss in foods
4. Surface activate agents: These include the emulsifiers use to stabilize oil-in-water and water-in-oil mixtures, gas-in-liquids mixtures. In addition to emulsifierof natural origin such as lecithin and synthetic ones (monoglycerides anddiglycerides), other emulsifying agents include certain fatty acids and theirderivatives, bile acids (in digestion,) defoaming compounds and detergentchemicals
5. Stabilizers and thickners: e.g gums, starches, dextrin, protein derivative andother additives that stabilize and thicken foods by combining with water toincrease viscosity and to form gels.. Example are Gum-arabic, CMC, carrageenan,pectin, amylose, gelatin and others.
6. Bleaching and maturing agents, starch modifiers. Freshly milled flour has ayellowish tint and suboptimum functional baking qualities. Both the colour andbaking properties improve slowly in normal storage. These improvements can beobtained more rapidly and with better control through the use of certainoxidizing agents such as Benzoyl peroxide (bleaches the yellow colour). Oxides ofnitrogen, chlorine dioxide, and other chlorine compounds bleach the colour andmature the flour. Starch modifiers are Hypochlorite, which oxidises starch tohigher degree of H20 solubility
7. Buffers acids, Alkalis: They are pH-adjusting and pH- controlling chemicals,buffer, acids and alkalies affect an endless number of food properties. Acids couldbe derived from natural sources e.g fruits and fermentation
8. Food colours:- Added to foods to produce appetizingand attractive qualities in foods. It could be natural orsynthetic colours. Example of natural colours: Extract ofannatto, caramel, carotene and saffron. Synthetic coloursgenerally excel in colouring power, uniformity, stability andlower cost.9. Artificial sweetners: Are substances containing few orno calories at the levels required for sweetness e.gAspartame (reduce calories sweetners) , saccharine (nocalories10. Nutritive additives: Vitamins, minerals, and othernutrients are added as supplements and enrichment to ano of products.11. Flavoring agents: e.g spices, herbs, essential oils, andplants extract, Synthetic flavours include Benzaldehyde(wild cherry or almond) ethyl butyrate (pineapple), methylanthranilate (grape) and methyl salicylate (wintergreen),iso-amylacelate (banana) and other flavor potentiators
Others
Ammonium sulphate- Promote growth of bakersyeast, CaCl2- firming agent for fruits and vegetables
Calcium phosphate – Anticaking agents for salt andgranular foods
Bentonite - clarifying agent for wine
Sodium nitrate- meat curing agents
Oxystearin –Crystallization inhibitors
Gibberelic acid- plant growth stimulant used inmalting balley
FOOD COLLOIDS, EMULSIONS AND FOAMS
Food colloids, emulsions and foamsFood colloids are surface active ingredients such as fatty acids,glycerides, phosphatides, polysaccharides and proteins. In terms ofsize, a colloids is a particle that is too large to dissolve and become thedispersed phase of a true solution. Colloidal particles may exist ascharged particular (ions) as molecules or as clusters of these calledaggregates. The particles in a colloidal dispersion do not settle out asthey would in a suspension.• Types of colloidal system1. Emulsion2. Foams3. Gels4. Soils
1. An emulsion: An emulsion is a colloidal dispersion of 2 liquids, usuallyoil and water, that are immiscible (not mixable). If the mixture is shakenvigorously the 2 liquids become dispersed in each other and an emulsion issaid to be formed.• Types of emulsiona. Oil-in-water (o/w) emulsion: Small oil droplets formed the dispersed
phase through the water e.g milk cream, ice creamb. Water-in-oil (w/o): Is one in which small water droplets are dispersed
through the oil e.g butter or margarine. Emulsions are opaque to lookat, because the particle size in the dispersed phase is large enough todeflect rays of visible light.
• Emulsion stabilityEmulsions are thermodynamically instable food systems. However, withaddition of surface active molecules (surfactants) or thickening agents, thephases can be stabilized. Emulsifiers contain hydrophilic and hydrophobicregions in their structure and they work by attaching to the surface ofdroplets in an emulsion and thereby reduces the surface tension (seebelow)Surface tension: forces that cause 2 dissimilar liquids to separate fromeach other.
`…`
.```oil
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Water
Oil-in-Water emulsion Water-in-oil emulsion
Hydrophobic end
Hydrophillic end
At oil/w interfaces emulsifiers, hydrophilic part is directed to the waterwhile hydrophobic is directed to the oil to minimize the contact areabetween phobic and phallic phases to reduce surface tension.2. Foams: In foam, Gas is the dispersed phase and liquid a continuousphase. Foams are not permanently stable and are termed metastable e.gice-cream, beaten egg white, over time, a foam achieves equilibriumunder the force of gravity through the process of drainage of liquid fromthe foam. Surfacfants and proteins inhibit drainage by differentmechanisms, proteins acts as an emulsifier to trap the tiny air cells.Gellatin maintain foam stability by increasing viscosity and preventscrystallization of sucrose.3. Gels: In gel, a liquid is dispersed in a solid e.g cook egg, palp, jellies.Colloidal gels form when colloidal molecules or particles associate in aliquid such that the solvent becomes immobile. Examples of plant-derived gelling agents are Gums, pectin, starches, soyabeans (coagulatedby heat to form gel). Animal proteins e.g gelatin, casein (in cheese), eggproteins form gel.4. A sol: In sol, solid is dispersed in a liquid (a continuous phase) e.gstarch, ogi, in a sense, the opposite of a gel is sol. A starch suspension inwater becomes a soil when heated due to the process of gelatinization.On cooling a gelatinized starch/Ogi sol converts into a gel.
