FOOD SPOILAGE AND SPOILAGE MICROORGANISMS

• Food spoilage is defined as damage or injury to food causing disagreeable change in food rendering in unsuitable for human consumption.

• Spoilage result in the change in smell, taste, colour , touch or sight.

• Food must be considered spoiled if it is contaminated with pathogenic microorganisms or various poisonous agents, such as pesticides, heavy metals etc.

Food Types ---

On the basis of susceptibility of spoilage, foods can be grouped as:

1. Perishable foods: Those foods which spoil quickly within few days.

They must be kept refrigerated or frozen, perishable foods include dairy products, meat, poultry, fish.

2. Semiperishable foods: Those foods have relatively long shelf life; few weeks or months. e.g bread, butter, cake , many canned fruits, potatoes and some apple varieties.

3 Stable or Nonperishable food : Those foods having very long shelf life for months or years . e.g canned fruits and vegetables, dried fruits and vegetables, pea nut butter . Dry beans , salt, sugar, flour.

EFFECT OF FOOD SPILAGE:

Microbial food spoilage occurs as a consequence of either microbial growth in a food or release of extracellular and intracellular (following cell lysis) enzymes in the food environment.

Signs of spoilage of different types of food: Changes in color, Changes in odour , Changes in texture e.g slime formation. Accumulation of gas or foam, liquid, biogenic amines, toxins etc. Decomposition of proteins, carbs, and vitamins.

Spoilage by microbial growth occurs much faster than spoilage by microbial extracellular or intracellular enzymes in the absence of viable microbial cells.

Sources of microorganisms in food:

The primary sources of microorganisms in food include: 1. Soil and water 2. Plant and plant products 3. Food utensils 4. Intestinal tract of man and animals 5. Food handlers 6. Animal hides and skins 7. Air and dust

MAJOR CAUSES OF FOOD SPOILAGE:

1. Growth and activity of microorganisms. 2. Action of native enzimes 3. Insects , rodent and parasites 4. Chemical reactions of the constituents of food 5. Enviromental factors 6. Time

1. Growth and activity of microorganisms.

Bacteria, yeasts and molds are the major causes of food spoilage. They produce various enzymes that decompose the various constituents of food.

Bacteria : Bacteria spoil foods with relatively high water activity such as milk and products and low acid foods like vegetables and meat. Both sporulating and non sporulating bacteria can spoil food under wide variety of conditions.

E.g.Bacillus, Salmonella, Staphylococcus, Shigella, Acetobacter, Alcaligens, Clostridium, Serratia, Erwinia etc.

Yeast: Yeasts are responsible for the decomposition of food with a high sugar content and cause fermentation. Two types of yeast are involved– True yeast and false yeast. True yeast metabolizes sugar producing alcohol and CO2 . (Fermentation) False yeast( Fungi imperfecti) grows as a dry film on highly acidic food surface and foods having high sugar.

E.g. Candida, Torula, Rhodotorula, Saccharomyces, Schizosaccharomyces, Hansenula, Trichosporon etc.

Moulds: Molds grow in filaments forming a tough mass which is visible as mould growth. They form spores. Mould spores when dry float thr’ air and germinate upon getting suitable condition.

E.g. Aspergillus, Fusarium, Alternaria, Mucor, Botrytis, Rhizopus, etc.

Both yeast and moulds can survive and grow in high acidic foods like pickles, jams jellies, fruits tomatoes etc. Both yeast and moulds are destroyed on heating.

2): Action of native enzymes: The activity of native and endogenous enzymes present in plant and animal food gets intensified after their harvest or slaughter. These enzymes cause undesirable changes in colour, texture and flavour with time. These enzymes are destroyed by heat, radiations or chemical treatment.

• Ripening: Enzymes present in the food cause food to ripen. If not consumed the food become over-ripe and eventually decay. For e.g. during the ripening of banana its starch changes to sugar, colour changes and texture softens.

• Browning : When certain foods are cut and the surface is exposed to air, enzymes cause them to turn brown. e.g. apples,

• Enzymatic rotting : Enzymes in fish cause deterioration even at low temperatures, which is called rotting. 3)--Insects, parasites and rodents Infestations (invasions) by insects and rodents also account for huge losses in food stocks. Insects destroy cereals grains, fruits and vegetables not only by consuming the food but contaminating the food by various microorganisms and facilitate its spoilage . Rodents apart for consuming considerable quantity of food also contaminate the food through their droppings , urine and filth . Rodents are also carrier of pathogenic bacteria . 4). Chemical Reaction The quality of foods deteriorates due to chemical reactions of the constituents of food . For e.g. the unsaturated fatty acid components undergo oxidation due to exposure to atmospheric air giving rise to oxidative rancidity in fat rich foods . Free fatty acids may also be released due to hydrolytic reactions causing odour as well as undesirable changes in the texture of food .

5). Enviromental factors Enviromental factors like air, light , temperature and moisture play important role in the spoilage of food . A brief account of various environmental factors is given below – • Air and oxygen can have determined effects on vitamin A, C, food colour , flavours and other constituents. • Atmospheric oxygen can react with some food components which may cause rancidity or colour changes. • Light destroy riboflabin , vitamin A, vitamin C, and also promotes light induced oxidation reactions affecting flavour and colour of food . • Temprature and moisture affect the quality of food in various ways including the role in the growth of microorganism. 6) Time The quality of food remains at its peak for some time soon after its harvest / slaughter and thereafter as the time progress, the deterioration in the quality of the food also progress. Factors affecting microbial Spoilage

There are several factors which influence the number and types of microorganisms in food spoilage. They are divided into two categories— Intrinsic and Extrinsic Factors

(a) Intrinsic factors:

These are inherent part of the food. They include:

➢ Hydrogen ion concentration (pH),

➢ Oxidation-reduction potential of food

➢ moisture content,

➢ nutrient content of the food,

➢ antimicrobial substances ad

➢ biological structures.

1. Hydrogen ion concentration (pH) pH of the food plays a major role in food spoilage. Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 and 7.5. Some bacteria can grow within a narrow pH range of 4.5 and 9.0, e.g. Salmonella. Other microorganisms especially yeasts and molds and some bacteria grow within a wide pH range, e.g. molds grow between 1.5 to 9.0, while yeasts grow between 4.5 and 6.0

Food type Range of pH values

Beef 5.1 - 6.2

Chicken 6.2 – 6.4

Milk 6.3 – 6.8

Cheese 4.9 - 5.9

Fish 6.6 - 6.8

Oyester 4.8 - 6.3

Fruits < 4.5 (most < 3.5)

Vegetables 4.0 – 6.1

The above table shows that most of the fruits are spoiled by yeast and molds and not by bacteria. Vegetables have higher pH than fruits and hence can be spoiled by bacteria also. Sae foods and meat can be spoiled by bacteria, yeast as well as moulds.

2 . Oxidation-reduction potential of food

The O/R potential of a system is expressed by the symbol Eh.Microorganisms exhibit varying degrees of senstivity to the Oxidation-Reduction (O/R) potential of the medium. The O/R potential of an element or compound may be defined ad theease with which it gains or loses electrons. When a substance gains electrons the substrate is said to be reduced or when it loses the electrons it gete oxidized. When electrons get transferred from one substrate to another , a potential difference is created between the two substrates which can be measured and expressed as mv (millivolts). The more highly reduced a substrate is, the more negative will be its electrical poteneial.

Anaerobic organisms requires low (negative) O/R potential such as Closridium requires Eh of about -200 mv, while areobs and facultative anaerobes requre high (positive) O/R potential such as Bacillus requires Eh of about +200 mv.

Plant foods especially plant juices have Eh values in range of +300 to +400mv. Thus one can execpt predominance of aerobic bacteria and moulds in plant juice which are responsible for their spoilage . Solid meats have Eh values around -200 mv (anaerobes predominate), while minced meats have Eh around +200 mv (aerobes predominate). On the surface of meat areobic bacteria may be found while in the inner tissues anaerobes eill predominate and bring about putrefication.

3 .Moisture content

The effect of moisture is measured in terms of water activity: -the amount of free water in a food medium. The amount of free water is important for growth of microorganisms. If there is lack of this free water microorganisms will not grow.

Water activity is defined as the vapour pressure of a food substance to that of water at the same temperature. (Aw = VPFood/VPWater)

• The water activity is therefore equal to 1.0. Food products have a water activity of less than 1.0.

• Growth of microorganisms is greatly affected by the level of water activity(Aw) in the food. Inhibition of growth occurs if the water activity for food is lowered beyond an organism’s minimum level of water activity that is necessary for growth. Microorganisms have varied minimum water activity requirements that supports their growth in food.

Water activity required for growth of some microorganisms.

Organisms Water activity Most food spoilage bacteria 0.91 Most food spoilage yeasts 0.88 Most food spoilage moulds 0.80 Halophilic bacteria 0.75 Xerophilic moulds 0.65

Water activity of some food products.

Food Water activity Fresh vegetables, meat, milk, fish 0.98 Cooked meat, caramels 0.95-0.98 Flour, rice, beans, cereals 0.80-0.87 Dried fruits, caramels 0.60-0.75 Spices, Milkpowder 0.20-0.60

Spoilage bacteria cannot grow in food having water activity less than 0.91while moulds cannot grow in food having water activity less than 0.80.Thus microorganisms cannot grow without water. Hence water activity of foods is lowered in order to preserve them from spoilage.

• A saturated salt solution has a water activity of 0.75. Salting and drying reduces the water activity of a food product.

• By increasing the concentration of sugar or salt in the food material. The solute and ions bind the water in the solution and make the water unavailable to the microorganisms growing there.

• The presence of hydrophilic colloids make the water unavailable. For e.g. 3 to 4% agar in the medium is sufficient enough to restrict the growth of bacteria.

• Water of crystallisation is not available to microorganisms. For e.g. when water is present in the form of ice then cannot be used by the microbial cells.

4. Nutrients content of the food

• Microorganisms require proteins, carbohydrates, lipids, water, energy, nitrogen, sulphur, phosphorus, vitamins, and minerals for growth. Various foods have specific nutrients that help in microbial growth. Foods such as milk, meat and eggs contain a number of nutrients that are required by microorganisms.These foods are hence susceptible to microbial spoilage. 5. Biological structures

• Some foods have biological structures like thick and protective covering that prevent microbial entry. For example, meat has fascia, skin and other membranes that prevent microbial entry.Eggs have shell and inner membranes that prevent yolk and egg white from infection. Scales of fishes, hides of animals, shells of nuts like walnut, almons etc. Protect them from microbial attack.

• Sometimes to protect the food artificial coating may be given.

6. Antimicrobial constituents

• Antimicrobial substances present in foods inhibit microbial growth. 1. Various foods have inherent or natural antimicrobial substances that prevent (inhibit) microbial attack. Such inhibitors are like lactinin and anti-coliform factors in milk and lysozyme in eggs, eugenol in cloves etc. 2. Inhibitory substances are sometimes produced by microorganisms growing in food like nisin, propionic acid alcohol inhibits the growth of other organisms.. E.g. Alcohol produced by yeast in wine is inhibitory to other microorganisms. 3. Heating the food: Heating of food also leads to the formation of certain inhibitory substances, like heating sugar syrup leads to formation of furfural which inhibits sugar fermentation.

(B). Extrinsic factors

• These are the factors external to the food that affect microbial growth. They include:

1. Temperature of storage,

2. Presence and concentration of gases in the environment

3. Relative humidity of food storage environment.

1. Temprature

• The growth of microorganisms is affected by the envirnmental temperatures. Various microorganisms are able to grow at certain temperatures and not others. Bacteria can therefore be divided into the following groups depending upon their optimum tmperature of growth. (i). Pyshrophilic microorganisms

• These grow best at about 20oC but also down to -10oC in unfrozen media. • Psychrophilic bacteria can cause food spoilage at low temperatures. • Several of the microorganisms found in the soil and water belong to this group.

(ii). Mesophilic bacteria • These organisms grow between 25oC and 40oC, with an optimum growth temperature close to 37oC • Some such as Pseudomonas aeroginosa may grow at even lower temperatures between 5-43oC • None of the mesophilic bacteria are able to grow below 5oC or above 45oC. • Most pathogenic bacteria belong to this group

(ii). Thermophilic bacteria. • These grow at temperatures above 45oC. Often their optimum growth temperatures is between 50oC and 70oC. • Growth of some bacteria occur at 80oC. • Bacteria in this group are mainly spore formers and are of importance in the food industry especially in processed foods.

Note that: The effect of temperature on microbial growth also depends upon other environmental conditions such as: o Growth factors in the nutrient medium, o pH of the food, and o Water activity.

2. Concentration of gases in the environment

This relates to the presence and concentration of gases in the food environment. Various microorganisms require for growth, either high oxygen tension (aerobic), low oxygen tension(microaerobic) or absence of oxygen (anaerobic). Some microorganisms may grow either in high oxygen tension, or in the absence of oxygen (facultative anaerobes).

• Anaerobic or facultatively anaerobic sporeformers are most likely to grow in canned foods . • Microaerophilic bacteria are most likely to grow in vacuum packed foods since they have low oxygen tension, while • Aerobic bacteria are likely to grow on the surface of raw meat. • Aerobic molds will grow in insufficiently dried or salted products. Altering the gaseous environment helps in preserving the food. CO2 has been used to retard the surface spoilage of beef. Growth of fungi on various such as apples and pears. O3 which is a strong oxidizing agent has been found effective against many food spoilage microorganisms.

3. Relative humidity Relative humidiy is the amount of moisture in the atmosphere or food environment. Foods with low water activity placed at high humidity environment take up water, increase their water activity and get spoiled easily. For example, dry grains stored in a environment with high humidity will take up water and undergo mold spoilage.

LIST OF SPOILAGE MICROORGANIMS:

Spoilage of Canned Foods • Thermophilic sporeformers – Can cause some types of spoilage of low acid (high pH) foods (such as corn, beans, peas) when the cans are temperature abused at 430C and above, even for short duration • Spoilage due to insufficients heating – Clostridium and some Bacillus spp. • Spoilage due to container leakage – Damage and leakly containers will allow different types of microorganisms to get inside from the environment after heating