PRINCIPLES OF FOOD SPOILAGE

SPOILAGE

Microbial growth- microbial food spoilage Changes in color, odor, and texture

Formation of slime

Accumulation of gas

Release of liquid

SPOILAGE MICROFLORA

Food types

Food environment Temp, aw, pH etc.

Microbial types

CLASIFICATION OF FOODS DEPENDING ON STABILITY

1. Highly perishable foods 1. No preservation process

1. Heating, food additives, freezing

2. Shelf-life-1-10 days

3. Meat, poultry, fish, raw milk, eggs, fruit and vegetables

2. SEMIPERISHABLE FOODS

Lightly processing pasterization of milk

Refrigeration

Shelf-life- 1-3 weeks

Frozen foods, hard cheese and fuits

3.SEMISHELF-STABLE FOODS

Two or more preservation process can be used

Vacuum packaging the cooked products

1-4 months

4. SHELF-STABLE FOODS

Highly processed Heat prpcessed-strelized and hermeticaly sealed

Suga, flours, dry foods, vegetable cans

MICROBIAL SPOILAGE

a)Kinds and numbers of M.O.

Main sources of M.O.: People, raw materials, ingredients, equipment, air, dust and pests.

Initial numbers of M:O. Can be kept low by

Good personal hygiene

Santizing equipment

Controlling temperature

Using clorinated water

Sanitary conditions during handlng the food

NUMBER OF M.O

Washing the fresh foods can reduce numbers of M.O.

Food with 100 M.O. Shelf life-12 days

Food with 5000 M.O shelf life-7 days

Contamination

Growth Type of organisms

Utilize nutrient

Competion

Bacteria hgrow faster than yeast and molds

İntial number

Environmental condition

pH, aw, oxygen , inhibitory substance

Pretreatments Remove or destroy M.O.

Heat stable enzymes-no cell left by heating

Thermoduric M.O.

B)Food residiues

On the equipment or wall, floor Must be clean troughly

C)Moisture content

Food with high water activity- must be refrigerated

D) Time-generation time

Bacteria with shorter generation time dominate in growth environment

Pseudomonas-shortest generation time

E) Temperature

The most important factor- opt. growth temp.

The shelf life of a food is doubled for every 10C decrease in storage temp.

F) Significance of microbial type

Bacterial spoilage yeast spoilage - mold spoilage

PRODUCTS OF SPOILAGES Odor- production of volatile compound

Ammonuim, H2S, indole

Color- pigment production,

Serratia mercescens produce red pigment

Oxidation of natural color compounds Myoglobin

Texture-break down of pectin or protein Vegetable softening meat softening

Gas- CO2, H2, H2S Slime-

production of dextran (Leuconostoc mesenteroides) too many bacterial cells

Loss of liquied (meat)-low water holding capacity Taste-

Souring-organic acid production Bitterness-peptid, amino acids, fatty acids

METABOLISM OF CARBOHYDRATES

Glucose anaerobicaly decomposed in six main taypes of fermentation:

1. An alcoholic fermentation 1. Sacharomycess cerevisiae

1. Ethanol +CO2

METABOLISM OF CARBOHYDRATES

Simple lactic acid fermentation Homofermentative-lactic acid main

products

Mixed lactic acid fermentation Heterofermentative Lactic acid, acetic acid, ethanol, CO2

Colfiform type fermentation Lactic, acetic, ethanol, hydrogen, acetone,

butandiol, CO2

Propionic fermentation Propionibacterium-propionic, acetic,

succinic acid, CO2

Butyric-isopropyl fermentation Anerobic m.o.- clostridium, butyric, acetic

acid, hydrogen, acetone, butylene, butanol, proponal

METABOLISM OF FOOD NUTRıENTS

Metabolism of Nitrogenous compounds Proteins peptides amino acids

Peptidases catalyze the hydrolysis

Peptides gives bitter taste

Some of the amino acids can give undesirable taste

Decomposition of proteins or amino acids may results in the production of odors Called putrefaction

Sulfur containing products, H2S

Hydrogen

Amines

histamine, tyramine, piperdine, putrecine and cadavarine

ammonia, metyl, ethyl sulfides, indole

ORGANIC ACIDS

Can present in foods as salts Lactate, citrate

M.O. Oxidise organic acids to carbohydrates Causing more alkaline pH

Film yeasts anarerobicaly oxidise organic acids to CO2 and water

LIPIDS Fats are hydrolysed

by microbial lipase to glycerol and fatty acids

Some M.O. degrate phospholipids to their phosphate, glycerol, fatty acids and nitrogenous base

LIPIDS

M.O. hydrolyse lipoproteins to proteins, cholesterol esters and phospholipids

PECTIC SUBSTANCES

Protopectin is converted to pectin, a water soluble polymer of galactrnic acid

It gels with sugar and acid

PECTIC SUBSTANCES

Pectine esterases hydrolyse the methyl ester linkage of pectin to yield pectic acid and methanol

Polygalacturanase: break the linkage between galactronic acid units of pectin or pectic acid to yield short chains and D-galactronic acid

OTHER COMPOUNDS

Alcohols usually are oxidized to the organic acids Athanol acetic acid

Acetaldehyde acetic acid (oxidised) or ethanol (reduced)

Glycerol dissimilate to glucose

PREFERENCES FOR METABOLISM

M.o. Use carbohydrates first nonprotein nitrogenous (NPN) Proteinaceous compounds Lipids Small molecules are prefered tp large ones For example: monosaccharides are prefered to

polysaccahrides Example: Pseudomonas grow aerobically in fresh

meat Metabolise first glucose Then free amino acids and other NPN Then they will produce extracellular proteinases to

break down proteins They will produce lipase also

PREFERENCES FOR METABOLIS

In milk: Lactose is the main carbohydrate

Only lactose utilizing m.o. can produce acid and gas

Non lactose-utilising m.o. use NPN and proteinaceous compounds to produce amines, NH3, inole