Foods undergo deterioration to varying degrees in organoleptic properties, nutritional value, safety, and aesthetic appeal. The term food deterioration is often associated with advanced spoilage, which makes food unfit for human consumption. Food is subject to physical, chemical, and biological deterioration. The highly sensitive organic and inorganic compounds of food, and the rapidity with which foods spoil if proper measures are not taken, is indicated in table 1, which lists the useful storage life of typical plant and animal tissues at 21° C. Meat, fish, and poultry can became worthless in 1 to 2 days at room temperature. This is also true for several fruits and leafy vegetables, raw milk, and many other natural products, where field temperatures are higher than 21° C during much of the year in many parts of the world. Much slower rates of deterioration occur with natural or processed foods that are low in moisture, high in sugar, salt or acid, or modified in other ways.
Table 1: Useful Storage Life of Plant and Animal Tissues
|Food Product||Storage Life at 21°C|
|Dried, salted, smoked meat and fish||360 +|
|Dried fruit||360 +|
Source: Desrosier (1977)
The major causes of food deterioration include the following.
These factors are not isolated in nature. Bacteria, insects, and light can also be operating simultaneously to spoil food in the field or in a warehouse. Similarly heat, moisture and air all affect the multiplication of bacteria, as well as the chemical activities of natural food enzymes.
The most important kinds of spoilage are:
Bacteria can grow in fresh foods (meat, fish, milk, vegetables) which are not acidic. Some bacteria can cause infection and food poisoning as well as spoilage. A number of bacteria can form spores, which are less easily destroyed by preservation techniques; they can start to grow again after insufficient heat treatment.
Not all micro-organisms cause spoilage. Some cause desirable changes in fish and meat. An example of this is the fermentation of fish, for example resulting in fish pastes or sauces. These changes are caused by useful microorganisms, of which there are many kinds. Micro-organisms are usually not visible to the naked eye, which means that serious infections and food poisoning can be caused without the food being visibly changed.
The following factors influence the growth of bacteria and the speed with which rotting takes place.
Damage: The skin of food, for example, is a protection against bacterial growth in the flesh. By damaging the skin, which functions as a barrier, nutrients are released. Bacteria can enter the flesh and start to grow.
Water content (internal water content and humidity): Fruits and vegetables may contain 90 or even 95% water. Fish consists of an average 70% water; in fatty fish this percentage is about 65% and in lean fish about 80%. Beef consists of 65% and pork of 60% water on average.
Oxygen content: Strictly aerobic microorganisms need oxygen for their growth, while strictly anaerobic micro-organisms can only grow in the absence of oxygen.
Acidity: The acidity of a product is indicated by its pH. Fish and meat have a neutral pH, i.e. 7. Bacteria only grow between a minimum pH of 4.5 and a maximum of 8–9 with an optimum of 6.5–7.5. As a result, fish and meat are very susceptible to spoilage. Foods such as tomatoes, apple and citrus fruits contain natural acidity, (low pH<4.5) with varying degrees of preservative power. When fermenting fish and meat, the pH is deliberately kept low, so that only useful microorganisms affect the product and not those bacteria, which cause spoilage.
Specific chemical composition: Bacteria need sources of energy and nitrogen. Mineral and vitamins are also important for growth.
Temperature: The range of temperatures for the growth of most micro-organisms and bacteria is between 7°C and 70°C, but the range within which some will survive is much greater. With freezing, micro-organisms are inactivated and with long-term heating most micro-organisms will eventually die, usually above 80°C. Spores however are often resistant to temperatures above 100°C. The time between contamination and processing or consumption is also of importance. Some micro-organisms grow faster than others, which means the number of micro-organisms and the amount of toxins can vary. At 37°C certain bacteria can multiply from 1,000 to 10,000,000 individual organisms in seven hours. The actual rate at which bacteria grow depends on a combination of the factors mentioned above. A watery product at 25°C will spoil much quicker than a dry, acidic product at 5°C.
How does contamination take place?
Contamination can come from people's (germs on skin, intestines, cuts, throat, hands), also from soil, dust, sewage, surface water, manure and other spoiled foods. Badly cleaned apparatus, domestic animals, pets, vermin or unhygienically slaughtered animals can also cause contamination.
Contamination after a preservation treatment has been carried out is especially dangerous. An example of this is the recontamination of cooked meat by placing it on the same plate on which fresh raw meat was kept.
How do one prevent contamination? Hygiene!
Food preservation may be defined as the set of treatment processes that are performed to prolong the life of foods and at the same time retain the features that determine their quality, like color, texture, flavor and especially nutritional value.
Food preservation processes have different time scales, ranging from short periods needed for home cooking and cold storage methods, to much longer periods of time required by strictly controlled industrial procedures such as canning, freezing and dehydration.
The preservation of fruits and vegetables entails the partial utilization of the raw material. In some cases, during the process it becomes necessary to add a packing medium, e.g., syrup or brine, while in others the raw material is used alone, as in frozen products. The raw material may be processed differently, depending upon the product to be obtained, e.g. vegetables in sauce, jellies, pickles and juices, for instance. The same raw material may be processed in different ways, as a result of which different products will be manufactured. In general terms, some processing methods are mentioned as below.
Preservation Methods by Chemical Action
Acids, salts and sugars are the principal food preservatives of a chemical nature. Sodium chloride is perhaps the oldest compound serving as a preservative. Acids, chiefly lactic, but occasionally including propionic, are produced. Acetic acid in the form of vinegar is used in the manufacture of several pickled products. Benzoic acid, sodium salts - sodium propionate, diacetate and sulfur dioxide, and sodium chloride are added to foods to prevent spoilage. Sugars are employed in the manufacture of jelly jams, preserves, sweetened condensed milk, sweet pickles, and other products aiding the preservation of the products into which they are incorporated. Before being considered for use in food, a chemical preservative must fulfil certain conditions, which are:
Preservation by the addition of sugar: Sugar is generally added in the processing of jams, jellies and sweet. The fruit must be boiled, after which the sugar is added in variable amounts, depending upon the kind of fruit and the product being prepared. The mixture must then continue to boil until it reaches a high level of soluble solids, which allows for its preservation. The addition of sugar combines with certain fruit substances to produce a gel - like consistency, which characterizes the texture of jams and jellies. To achieve this, appropriate acidity levels and sugar content, together with pectin, form a proper gel.
Benzoic acid and benzoate: Benzoic acid is a stable, white, granular or crystalline powder possessing a sweet, stringent taste. The sodium salt is more soluble in water (62.5 g in 100 ml) at 25° C than benzoic acid. For this reason it is the preferred form for industrial use.
Sodium benzoate has an optimum pH range between 2.5 and 4.0. If the pH of the food product is above pH 4.5, acidification may be desirable; the benzoate can be 100 times more active at this optimal pH as compared to pH above 6.0. The microbial level decides the amount of benzoate required. Temperature also plays a part; cold-stored juices need less benzoate. Some fruit juices at 30° C require as much as 0.05% to prevent fermentation. Consequently, juice should be kept cool or have lower numbers of yeast organisms. For preservation of a wide variety of foods, sodium benzoate or benzoic acid is used in amounts of less than 0.1 per cent.
Sulfur dioxide: Sulfur dioxide is used to treat fruits and vegetables before and after dehydration to extend the storage life of fresh grapes, and to prevent the growth of undesirable microorganisms during winemaking and the manufacture of juices.
Sulfur dioxide is more effective against mold spores and bacteria than against yeast; therefore it is combined with sodium benzoate, which is more effective against yeast, for fruit squashes. Sulfur dioxide is also used as a preservative in manufactured meats, sausage, and soft cheeses. In meat the flesh color is stabilized.
Sulfur dioxide is added to dried fruit in amounts up to 3000 PPM; less in dehydrated vegetables. During their storage, sulfur dioxide slows deteriorate changes, such as severe darkening in color and off-flavors. Its reducing action is valuable in preventing the loss of ascorbic acid in dried fruits and vegetables and the disappearance of beta-carotene (pro-vitamin A) in vegetables. Frequently as much as 90 percent sulfur dioxide is removed by steam during the cooking of dried fruits and vegetables.
Sulfur dioxide is usually applied to vegetables after blanching and before dehydration, in the form of Sodium metabisulphite solution. The uses of sulphites, or sulfur dioxide, to treat vegetables prior to dehydration, aids in the prevention of deteriorative changes during dehydration and storage. Sulfur dioxide is a useful agent for the prevention of browning reactions in dried fruits. It should be declared on the label, for those consumers who may be sensitive to it, e.g. people with an asthmatic condition.
Treatment with acid: Most foods may be preserved by heat treatment when they have a pH lower than 4.0. For this reason several methods have been developed which control the pH through the production of acid, or the addition of some organic acid, like acetic, citric and even lactic acid. The acidification of low-acidity vegetables to less than pH 4.5 for commercial sterilization-based processing, with brief sterilization periods at temperatures of 100°C, is a very practical method to employ on a small-scale and even home processing.
Packaging and Packaging Materials
If the climate is very dry, it may not be necessary to package dried foods as they may not pick up much moisture from the air. However a humid climate results in dried foods gaining moisture and going moldy. The stability of dried foods depends not only on the humidity of the air, at which a food neither gains nor loses weight, but also on the type of food. Different foods can be grouped according to their ability to absorb moisture from the air. The two groups are hygroscopic, which absorb moisture easily and non-hygroscopic, which do not absorb moisture. The classic example is salt and pepper, where salt is very hygroscopic and pepper is non-hygroscopic, but similar examples exist for fruit and vegetable products. This difference determines the packaging requirement for different fruit and vegetable products. The moisture content at which a dried food is stable is known as the Equilibrium Moisture Content and examples of this for different fruits and vegetables are mentioned below, together with the packaging requirement for different groups of foods.
Dried fruits and vegetables are usually packaged in one of the many different types of plastic film. The selection of the correct type of packaging material depends on a complex mix of considerations.
There is a very wide range of packaging materials that can be used for foods. The following is a very brief description of some of the more important points concerning the most widely available types of packaging materials in most developing countries.
All types of plastic film, with the exception of un-coated cellulose or foil, can be sealed using a heat sealer. Care should be taken to ensure that there is no product dust on the inside of the package where the seal is to be, as this will prevent proper sealing.
Solid products, such as pickles and chutneys are usually filled by hand using scoops or ladles into jars, plastic pots or bags. This is a time-consuming operation, which may require a large staff input. However, in most small-scale operations, this is the only realistic option because mechanical fillers for these types of product are very expensive and usually operate at top high a throughput.
Although liquid products can also be filled by hand using jugs or ladles, in contrast to solid products, there are a number of small liquid fillers available, which are affordable by many small-scale producers.
Jars and bottles are available in countries that have a glassworks, or have access to an overland supply from a neighboring country. Because of their heavy weight, high bulk and fragility, glass containers are expensive to transport long distances and are frequently not available to producers in developing countries. Where they are available, they are usually reused and great care is needed to ensure that they are properly cleaned. New and reused containers should be sealed with new caps, lids or corks in order to obtain an adequate seal.
Plastic pots and bottles are suitable for some types of foods and they are becoming increasingly common as a result of their lower production and distribution cost. Pots can be either heat sealed with a foil lid or with a snap-on plastic lid. The most common types of plastic film in developing countries are polythene and polypropylene, although increasingly there are agents who can supply more sophisticated and expensive imported laminates.
Small laminated plastic/foil/cardboard cartons for UHT juices are appearing in many countries but these are usually imported under licence to large scale juice manufacturers and not available to small scale processors. Additionally, the UHT technology is not suitable for small-scale production. Other cardboard and paper packaging is more widely available and can usually be printed by local print companies.
Other, more traditional types of packaging such as leaves, jute, wood and pottery are not usually able to convey an image of modern or hygienic products.
Nutritive Aspects of Food Constituents
In addition to building and maintaining the body, energy for the body's functions comes from the food consumed. There is increasing evidence that nutritional status and specific nutrients influence mental processes and behavioral attitudes. The nutrients in food, required in balanced amounts to produce and maintain optimum health, belong to the broad groups of carbohydrates, proteins, fats, vitamins, and minerals.
The major sources of energy for humans are carbohydrates, fats, and proteins. These nutrients have additional specific functions, but their conversion to energy in the form of calories, is of fundamental importance.
Calories are needed to satisfy the bodies' energy requirements for the production of body heat, synthesis of body tissue, and performance of work. Carbohydrates such as sugars and starches, which are fully digested and oxidized by humans, provide about 4kcal/g of energy. Most fats are digested to the extent of 95% yielding 9kcal/g. Proteins, due to incomplete digestion and oxidation, also yield an energy equivalent of about 4 kcal/g. On an equal weight basis; fat generally yields 2.25 time as many calories as protein or carbohydrates.
Vitamins are organic materials, other than essential amino acids and fatty acids that must be supplied in small amounts to maintain health. An exception to this is vitamin D, the only major vitamin the human body is capable of manufacturing.
|Vitamin K||helps clots blood and keeps bones strong, but it doesn't absorb into the body so easily.|
|Source:||Cabbage, cauliflower, spinach, other green leafy vegetables, cereals, soybean and the bacteria lining the intestines also produce Vitamin K.|
|Vitamin A||helps form and maintain healthy teeth, skeletal and soft tissue, mucous membranes and skin. It also improves vision and is required for reproduction and lactation. Beta-carotene is a precursor to vitamin A.|
|Source:||Eggs, meat, cod, fish oil, milk, cheese, cream, liver and kidney. Sources for beta-carotene: carrot, pumpkin, sweet potato, winter squash, apricot, pink grapefruit, cantaloupe, broccoli, spinach, most dark green leafy vegetables etc.|
|Vitamin D||helps the body absorb and maintain adequate levels of calcium and phosphorus.|
|Source:||Cheese, butter, margarine, cream, fortified milk, fish, oysters, fortified cereals and sunlight.|
|Vitamin C||is important for healthy teeth and gums, and helps absorb iron, maintains connective tissues and the immune system, and heals wounds. The non-absorbed vitamin is passed out in urine, though recent research shows that too much might cause gout.|
|Source:||Green pepper, herbs (e.g. mint) citrus fruit, strawberry, tomato, broccoli, turnip, other greens, potato, cantaloupe, fish, milk etc.|
|Vitamin E||acts as an antioxidant to protect body tissue from toxins, improves complexion, forms red blood cells and aids vitamin K.|
|Source:||Wheat germ, corn, nuts, seeds, olives, spinach, asparagus, other green leafy vegetables and vegetable oils.|
The Food Pyramid