Dairy and Food Engineering

Lesson 20. Bio-Chemical Deterioration in Food Products

20.1.1.1. Oxidative Rancidity

It occurs in fatty foods with high levels of unsaturation due to breakdown of fats and oils resulting into production of off-flavors and off odour.

Lipid oxidation rate and course of reaction is influenced by light, local oxygen concentration, high temperature, the presence of catalysts (generally transition metals such as iron and copper) and water activity. Control of these factors can significantly reduce the extent of lipid oxidation in foods.

20.1.1.2 Non-enzymatic Browning

This occurs when sugars and amino acids present in the food go through a series of reactions producing a brown colour in the food.

This is referred to as Maillard Reaction. The development of a brown colour and the accompanying flavour in the baking of bread, brewing of beer and roasting of coffee are desirable attributes of this reaction. However it is highly undesirable when it develops in dried milk during storage. Caramelization occurs in products which are high in sugar due to direct or excessive exposure to heat. Some of its examples include syrups, candied fruits, jams and jellies. There is also the oxidation of ascorbic acid on exposure to oxygen that may occur in foods high in this nutrient.

20.1.1.3 Enzymatic Degradation

A typical example is an unappealing brown discoloration, which is seen when peeled ripe bananas or sliced apples, pears or some vegetables are exposed to the air. Enzymatic spoilage also causes the production of off-odours and off-flavours in foods such as meats and meat products. In order to prevent this type of spoilage, the enzyme in the food has to be inactivated before storage.

Effects of Enzyme Action

(A) The ripening process of the banana makes it sweeter, softer, less astringent in taste, and more odorous. This occurs due to hydrolysis (degradation) of the starch (essentially tasteless and insoluble in the water of the banana) to simple water-soluble sugars.

(B) Over-riped tomato gets softened due to hydrolysis of the pectins to their simpler carbohydrate building blocks. Pectin is a water-soluble carbohydrate found in ripe fruits and has strong gelling properties which are used in cooking.

(C) Proteins in foods like cheese, meat, and fish may be hydrolyzed to simpler compounds by the enzymes naturally present. Such chemical changes are often manifested as changes in taste, odor, texture, and so forth.

(D) When freshly harvested products are processed for eating, the normal cellular organization of the tissues may be disrupted, as a result of which the residual enzymes may initiate degradative changes at a very rapid rate. One of the most common examples of such changes is the rapid darkening of freshly peeled potatoes, apples, peaches, and pears. Other oxidizing enzymes induce the common and sometimes intense "hay" flavor of vegetables such as lima beans, corn, and broccoli if they are not cooked soon enough after harvesting.

20.1.2 Effect on Nutritional quality

The four major factors which affect nutrient degradation and can be controlled to varying extents by packaging are light, oxygen concentration, temperature and water activity

20.1.2.1 Vitamins: 

Ascorbic acid is the most sensitive vitamin in foods, and its stability varies markedly as a function of environmental conditions such as pH, concentration of trace metal ions and oxygen. The nature of the packaging material can significantly affect the stability of ascorbic acid in foods. The effectiveness of the material as a barrier to moisture and oxygen as well as the chemical nature of the surface exposed to the food are important factors.

The problems of ascorbic acid instability in aseptically packaged fruit juices have been encountered because of oxygen permeability of the package and the oxygen dependence of the ascorbic acid degradation reaction.

Due to the preferential oxidation of metallic tin, citrus juices packaged in cans with a tin contact surface exhibit greater stability of ascorbic acid than those in enameled cans or glass containers. The aerobic and anaerobic degradation reactions of ascorbic acid in reduced-moisture foods have been shown to be increasing in an exponential fashion over the water activity range of 0.1-0.8.

20.2 Biological Changes

20.2.1 Microbiological

The microorganisms that are principally involved in food deterioration are bacteria, molds, and yeasts. There are thousands of genera and species of microorganisms associated in one way or another with food products. Not all cause food spoilage, and many types are used in preserving foods, such as the lactic-acid-producing organisms of cheese, sauerkraut, and some types of sausage. Other microorganisms are used for alcohol production as in wine or beer making, or for flavor production in other foods.

Microorganism multiplication on or in foods is a major cause of food deterioration. The microorganisms attack virtually all food constituents. Some of them may lead to ferment sugars and hydrolyze starches and cellulose, whereas others hydrolyze fats and produce rancidity, still others digest proteins and produce putrid and ammonia-like odors.

Some produces acid and make food sour, while others produce gas and make food foamy. Some form pigments, and a few produce toxins giving rise to food borne illnesses.

When food is contaminated under natural conditions, several types of organisms will be present together. Such mixed organisms contribute to a complex of simultaneous or sequential changes which may include acid, gas, putrefaction, and discoloration.

Foods are frequently classified on the basis of their stability as non-perishable, semi-perishable and perishable. Hermetically sealed and heat processed (e.g. canned) foods are generally regarded as non-perishable. Spoilage may also take place when the canned food is stored at unusually high temperatures: thermophilic spore-forming bacteria may multiply, causing undesirable changes such as flat sour spoilage.

Majority of foods (e.g. meat and fish, milk, eggs and most fresh fruits and vegetables) are classified as perishable unless they have been processed in some way. Often, the only form of processing which such foods receive is to be packaged and kept under controlled temperature conditions.

Low moisture content foods such as dried fruit and vegetables and frozen foods are classified as semi-perishable.

Table 20.1 Major modes of deterioration, perishables.

Perishables	Mode of deterioration (assuming an intact package)	Critical environmental factors
Fluid milk and dairy products	Bacterial growth, oxidized flavour, hydrolytic rancidity	Oxygen, temperature
Cheese	Rancidity, browning, lactose crystallization, undesirable mold growth	Temperature, relative humidity
Ice cream	Graininess cause by ice or lactose crystallization, texture	Fluctuating temperature (below freezing)
Fresh red meat	Bacterial growth, loss of red colour	Oxygen, temperature, light
Fresh poultry	Bacterial growth, off- odour	Oxygen, temperature, light
Fresh fish	Bacterial growth, off- odour	Temperature
Fresh fruits and vegetables	Respiration compositional changes, nutrient loss, wilting, brushing, microbial growth	temperature, relative humidity, light, oxygen, physical handling
Frozen fish, meats, poultry	Rancidity, protein denaturation, colour change(freezer bum), toughening	Oxygen, temperature, temperature Fluctuations
Frozen fruits and vegetables	Loss of nutrients, yeast growth, loss of texture, flavour, odor, colour and formation of package ice	Oxygen, temperature, temperature Fluctuations
Frozen concentrated juices	Loss of cloudiness, yeast growth, loss of vitamins and loss of flavour or colour	Oxygen, temperature, temperature Fluctuations
Frozen convenience foods	Rancidity in meat portion, weeping and curding of sauces, loss of colour, loss of flavour, package ice	Oxygen, temperature, temperature Fluctuations

 

Table 20.2 Major modes of deterioration, semi perishables

Semi Perishables	Mode Of Deterioration (Assuming An Intact Package)	Critical Environmental Factors
Fresh bakery products	Staling, microbial growth, moisture loss causing hardening, oxidative rancidity	Oxygen, temperature, humidity
Breakfast cereals	Rancidity, loss of crispness, nutrient loss, breakage	Relative humidity, temperature, rough handling
pasta	Texture change, staling, vitamin and protein quality loss, breakage	Relative humidity, temperature, light, oxygen, rough handling
Fried snack foods	Rancidity, loss of crispness, breakage	Oxygen, light, temperature, Relative humidity, physical handling
Dehydrated foods	Browning, rancidity, loss of colour, loss of texture, loss of nutrients	Relative humidity, temperature, light, oxygen
Nonfat dry milk	Flavour deterioration, loss of solubilization, caking, nutrient loss	Relative humidity, temperature
Coffee	Rancidity, loss of flavour and odor	Oxygen, temperature, lights, relative humidity
Tea	loss of flavour, absorption of foreign odors	Oxygen, temperature, lights, humidity
Canned fruits and vegetables	Loss of flavour, texture, colour and nutrients	Temperatures

 

The species of micro-organisms which cause the spoilage of particular foods are influenced by two factors: a) the nature of the foods and b) their surroundings. These factors are referred to as intrinsic and extrinsic parameters.

       The intrinsic parameters are an inherent part of the food:

• pH,

• water activity,

• nutrient content,

• antimicrobial constituents and

• biological structures.

     The extrinsic parameters of foods are those properties of the storage environment that affect both the foods and their microorganisms:

• temperature,

• relative humidity and

• gas compositions of the surrounding atmosphere.

The protection of packaged food from contamination or attack by micro-organisms depends on the mechanical integrity of the package (e.g. the absence of breaks and seal imperfections), and on the resistance of the package to penetration by micro-organisms.

Extensive studies on a variety of plastic films and metal foils have shown that microorganisms (including mounds, yeasts and bacteria) cannot penetrate these materials in the absence of pinholes.

20.2.1.1 Bacteria

Bacteria are unicellular microorganisms of many forms, spherical, cocci, rod shaped bacilli and spiral shaped spirilla. Some bacteria produce spores which are remarkably resistant to heat, chemicals, and other adverse conditions. Bacterial spores are far more resistant than yeast or mold spores, and more resistant to most processing conditions than natural food enzymes. Most of the bacteria are in the order of one to a few microns in cell length and somewhat smaller than this in diameter. (A micron is one-thousandth of a millimeter (0.001 mm) or about 0.00004 inch.) All bacteria can penetrate the smallest of openings, and many can pass through the natural pores of an egg shell once the natural bloom of the shell is worn or washed away.

 20.2.1.2 Molds

Molds are larger than bacteria and yeast and more complex in structure. They grow by a network of hair-like fibers called mycelia and send up fruiting bodies that produce mold spores referred to as conidia. The blackness of bread mold and the blue-colored veins of blue cheese are due to the conidia. The mycelia are a micron or so in thickness and, like bacteria, can penetrate the smallest opening; or in the case of weakened skin or shell can digest the skin and make their own route of penetration.

20.2.1.3 Yeasts

Yeasts are somewhat larger than bacteria, of the order of 20 microns in individual cell length and about half this size in diameter. However, yeasts are smaller than molds. They are associated with nearly all types of food products. Foods such as fresh vegetables, meat, poultry, and cheese often contain yeasts, but in these foods, bacteria outgrow the yeasts. When bacterial inhibitors are added, yeasts can dominate. Some yeasts are found in foods such as honey, molasses, sugar, and fruit. Salt-tolerant yeasts grow as films on brine food and on salted food and ham.

20.2.2 Macrobiological

20.2.2.1 Insect Pests

Even though warm humid environments promote insect growth, most insects will not breed if the temperature exceeds about 35 C° or falls below 10 C°. They can’t reproduce satisfactorily unless the moisture content of their food is greater than about 11%.

The main categories of foods subject to pest attack are cereal grains and products derived from cereal grains, legumes, dairy products such as cheese and milk powders, dried fruits, dried and smoked meats and nuts.

The presence of insects and insect excrete in packaged foods may render products unsaleable, causing considerable economic loss, as well as reduction in nutritional quality, production of off-flavours and acceleration of decay processes due to creation of higher temperatures and moisture levels.

Early stages of infestation are often difficult to detect; however, infestation can generally be spotted not only by the presence of the insects themselves but also by the products of their activities such as webbing, clumped food particles and holes in packaging materials.

Unless plastic films are laminated with foil or paper, most of the insects are able to penetrate them quite easily. As the rate of penetration is directly related to film thickness hence, thicker films are more resistant than thinner films, and oriented films tend to be more effective than cast films.

Generally, the penetration varies depending on the basic resin from which the film is made, on the combination of materials, on the package structure, and of the species and stage of insects involved. The relative resistance to insect penetration of some flexible packaging materials is as follows:

• excellent resistance: polycarbonate; poly-ethylene-terephthalate;

• good resistance: cellulose acetate; polyamide; polyethylene (0.254 mm); polypropylene (biaxially oriented); poly-vinyl-chloride (unplasticised);

• fair resistance: acrylonitrile; poly-tetra-fluoro-ethylene; polyethylene (0.123 mm);

• Poor resistance: regenerated cellulose; corrugated paper board; Kraft paper; polyethylene (0.0254 - 0.100 mm); paper/foil/polyethylene laminate pouch; poly-vinylchloride (plasticized).

          Some simple methods for obtaining insect resistance of packaging materials are as following:

• select a film and a film thickness that are inherently resistant to insect penetration;

• use shrink film over-wraps to provide an additional barrier;

• Seal carton flaps completely.

20.2.2.2 Rodents

Rats and mice carry disease-producing organisms on their feet and/or in their intestinal tracts and are known to harbour salmonella of serotypes frequently associated with food-borne infections in humans. In addition to the public health consequences of rodent populations in close proximity to humans, these animals also compete intensively with humans for food.

Rats and mice gnaw to reach sources of food and drink. Their incisor teeth are so strong that rats have been known to gnaw through lead pipes and unhardened concrete, as well as sacks, wood and flexible packaging materials.

Proper sanitation in food processing and storage areas is the most effective weapon to fight against rodents, since all packaging materials apart from metal and glass containers can be attacked by rats and mice.