Processing of Horticultural Crops (1+2)

Introduction

Introduction
Spoiled food is defined as the food that has been damaged or has undergone changes so as to render it undesirable/unacceptable/unfit for human consumption. Spoilage of processed products is mainly caused by the action of micro-organisms and due to the physical or chemical changes. The reasons for microbial spoilage include under- processing, inadequate cooling, contamination from leakage through can seams and pre-process spoilage. In under-processed cans (receiving low heat treatment) large number of micro-organisms is expected to be involved in spoilage.

Causes of food spoilage
Food spoilage or deterioration may be described as loss of weight, softening, souring, rotting, wilting, moulding or a combination of one or more forms. The major causes of food deterioration are: biotic and abiotic factors.

A. Biotic factors

• Microbial spoilage (yeast, mould, bacteria)
• Enzyme action (activation of enzymes present in foods)
• Insect and rodent damage (attack of insects, rodents and parasites)

B. Abiotic factors

• Factors like temperature, moisture, light, oxygen etc.

A. Biotic factors

1) Microbial spoilage: Micro-organisms like yeast, mould and bacteria spoil food after harvesting, during handling, processing and storage. Some of these micro-organisms can be used to produce desirable changes in foods under controlled conditions such as lactic acid producing bacteria in making cheese and fermented dairy products, yeast as leavening agent and for production of beer and wine, citric acid production by using Aspergillus sp etc.

a) Yeast: Yeasts are those fungi which are not filamentous but unicellular, ovoid or spheroid in shape and reproduce by budding or fusion. Yeasts are both useful as well as harmful in foods. Yeast fermentations are involved in the manufacture of foods such as bread, beer, wines and surface ripened cheese as well as for production of enzymes. Foods are often contaminated by yeasts as they cause spoilage by converting sugar to alcohol and carbon dioxide. Common foods spoiled by yeasts include fruit juices, syrups, molasses, honey, jam, jellies, sauerkraut, wine etc.
Most common yeasts grow in high to low available moisture, but generally require less moisture than majority of bacteria and comparatively more moisture than moulds. The optimum temperature for growth of yeasts is 25-30^oC with maximum limit up to 35-47^oC. Many types of yeast grow best in acidic (pH 4.0-4.5) but not in alkaline medium.

b) Moulds: Moulds are multi-cellular, filamentous fungi whose growth on foods is recognized by its fuzzy appearance. The growth may appear to be white, coloured, dark or smoky. They are strictly aerobes and require oxygen for their growth and multiplication. Moulds grow on almost all kinds of foods ranging from simple to complex. They require less free moisture than yeasts and bacteria. They grow over a wide range of pH (2.0 to 8.5) but majority of them favour acidic pH. Most moulds are able to grow at ordinary temperatures i.e. mesophilic. The optimal temperature for most moulds is 25-30^oC but some can grow at 37^oC or above like Aspergillus sp., while few moulds are psychotropic as they grow well at refrigeration temperature or even below freezing temperature. Most moulds are not harmful. Some moulds in foods are capable of producing toxic materials called mycotoxins. Examples of mycotoxins are aflatoxins produced by moulds growing on groundnut, patulin from Penicillium expansum on apple and other agricultural products such as wheat, millet and rye if they are not dried properly.

c) Bacteria: Bacteria are unicellular micro-organisms and are much smaller in size than yeasts or moulds. They occur in different sizes and shapes and are classified as coccus (spheroidal), bacilli (rod shape or cylindrical) or spirillae and vibrios (spiral shape). They can penetrate the smallest opening, as many can pass through the natural pores of an egg shell. Bacterial growth on the food make it unattractive in appearance, cause discolouration on the food surface, make surfaces slimy or results in undesirable cloudiness or sedimentation. In contrast to yeast and moulds, most bacteria cannot grow in acidic media (pH <4.5). Some bacteria require oxygen for their growth (aerobes) and some cannot tolerate oxygen (anaerobes) while some can grow in an atmosphere devoid of oxygen but can also manage in air (facultative anaerobes). Bacteria can grow and develop rapidly between 20-53^oC. On the basis of temperature requirement, bacteria are categorized as thermophiles (requiring temperature higher than 45^oC), mesophiles (requiring temperature between 20-25^oC) and psychrophiles (requiring temperature less than 20^oC). Bacteria usually cause spoilage in foods which are neutral or low acidic such as vegetables, milk, eggs, meat and fish.
Thus micro-organisms like bacteria, yeast and moulds are the main causes of food spoilage.

2) Enzyme action: Enzymes are the polypeptides that catalyze a reaction with certain degree of specificity. Many reactions in plant and animal tissues are activated by enzymes. The changes in food during storage can be produced by the enzymes present in the food or by the micro-organisms that contaminate the food. They are responsible for bringing many changes during storage like change in colour, texture and flavour in fresh produce after harvest. Through some of these changes are desirable like ripening of fruits but these changes can also result in food deterioration if they are not halted at appropriate time. Thus, enzymes responsible for deteriorative changes should be inactivated by using a suitable method to prevent food deterioration. The enzymes are proteinaceous in nature and can be denatured by heat. Enzymes can act from 0oC to 60oC, however, 37oC is optimum temperature and the rate of reaction varies directly with temperature. Generally, most enzymes are inactivated by temperature above 80oC. Important enzymes involved in food deterioration include polyphenol oxidase, lipoxygenase, amylase, pectin methyl esterase and poly galacturonase.

3) Insects, parasites and rodents: Insects like worms, bugs, weevils, fruit flies and moths damage the food and reduce its nutrient content and render it unfit for human consumption. Insect eggs may persist in foods even after processing as in flour. Insects in grains, dried fruits and spices are generally controlled by fumigation with fumigants like methyl bromide, ethylene oxide, propylene oxide etc besides eating loss; the insects cause greater damage by making bruises and cuts in the fruits thus exposing the food to microbial attack and resulting in total decay.
Parasitic spoilage occurs in some foods. Parasitic nematode penetrates the hog’s intestine when uncooked food is eaten by the hog’s and finds its way into pork. Rodents like rats cause extensive damage of food grains. Urine and droppings of rodents harbour several kinds of disease producing bacteria and rats spread many human diseases like typhus fever, plague, typhoid fever etc.

B. Abiotic factors
1) Temperature: Low or high temperature apart from its role in food preservation also brings deterioration of foods. The rate of chemical reaction doubles with every 10oC rise in temperature. Excessive heat causes protein denaturation, destroys vitamins, breaks emulsions and dries out food by removing moisture. Similarly, low temperature cause deterioration like freezing and thawing of fruit and vegetables destroy their structure. Many fruits and vegetables are damaged even at refrigeration temperatures (4oC). The deterioration includes off-colour development, surface biting and various forms of decay. Thus banana, tomato, lemons etc are stored above 10oC to avoid chilling injury.

2) Moisture: The presence of water is the most important factor controlling the rate of deterioration of food either by micro-organisms, enzymes or other chemical reactions. Moisture is required for chemical and enzymic reactions and for microbial growth. Foods with high moisture contents deteriorate fast. Perishable foods with high water content like leafy vegetables, juicy fruits, meats and milk deteriorate rapidly. Changes in surface moisture with change in relative humidity cause caking, surface defects, crystallization and stickiness in foods. Condensation of moisture results in multiplication of bacteria and yeasts. Fruit and vegetables give off moisture from respiration and transpiration even when packaged in moisture free packages. Thus control of moisture in foods is necessary to ensure preservation.
The shelf-life of a food can also be measured by the water activity of food. Water activity (aw) is defined as the ratio of the vapour pressure of water in food to the saturated vapour pressure of pure water at the same temperature.

aw = p where, p = vapour pressure of water in food, Pa
p0 p0 = vapour pressure of pure water, Pa

Water activity can also be defined according to Raoult’s law of mole fractions, which refers water activity as the ratio of number of moles of water in a solution to the total number of moles of water and solute in the solution as under:-

This equation can be used to get desired water activity in the food by increasing the number of moles of solute and reducing the mole of solvent either by using drying, concentration, addition of sugar, salt, freezing etc. Thus most of the methods of preservation like drying, concentration, salting, sugar preservation etc are based on management of water activity. Most of micro-organisms fail to grow at reduced water activity and hence aids in preservation. The interaction of aw with temperature, pH, oxygen and carbon dioxide or chemical preservatives influence the microbial growth. Thus, water activity can be used as an important tool in food preservation.

3.) Oxygen and Light: Air and oxygen bring about a number of destructive changes in food components such as destruction of food colour, flavour, vitamin A, C and E and other food constituents. As oxygen is required for growth of moulds, its removal from the food by deaeration, vacuum packing or flushing containers with nitrogen or carbon dioxide or by using oxygen absorbing chemicals like antioxidants helps in food preservation.
Vitamins like B2, A, C and food colours in the food are deteriorated in the presence of light. Foods can be protected from light by using impervious packing or keeping them in containers that screen out specific wavelengths. Further, all the food deteriorative factors are time dependent. The longer the time, the greater the destructive influences. Therefore, optimum time of storage for food commodities need to be worked out.

Spoilage of canned food products: The spoilage of canned products during storage may be caused due to two main reasons:

• Chemical reaction on the can producing hydrogen swells or perforations, and
• Chemical action on the fruit or vegetable resulting in discolouration or loss of flavour etc.

The appearance and smell (odour) of the spoiled can is different from that of normal unspoiled can and such cans should never be tasted. It is therefore, essential to identify the can from the external appearance and diagnose the cause of the spoilage.

I. Identification of spoilage on the basis of external appearance of cans
The extent of spoilage of heat processed product can be identified by observing the external appearance of the unopened can. The ends of a normal can of food after processing are flat or slightly concave having partial vacuum inside. With the development of pressure inside the can, the can goes through a series of distortions due to increasing pressures. The conditions of such cans are designated by following terms:

1. Flipper: A can with a mild positive pressure having both ends flat. One end of this can will become convex when the side of the can is struck sharply or when the temperature of the contents is increased.

2. Springer: It is the can with both ends bulged, but one or both ends will stay concave. If swollen end of can is pushed in, an opposite flat end will pop out. Both Flipper and Springer indicates the initial stage of hydrogen swell but may also be caused by insufficient exhausting or over filling of the can. Later on it results in denting of can with change in temperature.

3. Soft swell: It refers to a can with both ends bulged, but the gas pressure is low enough to permit the ends to be dented by manual pressure.

4. Hard swell: The can with hard swell has both ends bulged. It contains such a high gas pressure that neither end could be dented by hand. Oftenly, the high gas pressure distorts the ends or side seam of the cans and finally the can bursts from side seam or through the seal at ends. The decomposed food in the can has an offensive and sour odour and the product is generally discoloured. It is not fit for consumption and may contain toxins produced by Clostridium botulinum.

5. Breather: A breather is a can with a minute leakage that permits air to move in or out but does not necessarily allow micro-organisms to enter. In these cans, there is no vacuum and the pressure inside the can is equal to that of the atmosphere. The food remains fit for consumption.

6. Leaker: A very small leakage in the can may be due to faulty seam, or pin hole as a result of corrosion inside the can or rusting of can from outside.

7. Flat sour: It may be caused by under-sterilization. The product has a sour odour and its acidity is much greater than that of the normal product.

8. Bursting of can: Bursting of cans is due to excess of pressure caused by the gases inside, produced by decomposition of food by micro-organisms, or by hydrogen gas formed by chemical action of acids of the food on the tinplate. The canned product becomes a total loss.

II. Identification of spoilage on the basis of appearance of glass containers
The glass containers like bottle, jar, carboys etc of food under gas pressure may have its closure bulged or popped off or may show leakage of food through the broken seal. The microbial growth can also be seen through the glass container in the form of gas bubbles, cloudiness and films of growth.