Food_Feeds (Food Technology & Allied science)

Non-enzymatic deterioration One further category of deterioration worthy of mentioning is that of non-enzymatic browning. An important reaction in foods takes place between the sugar constituents and amine-type compounds, which results in progressive browning and the development of off-flavours. An example of foods in which this type of quality deterioration takes place is dehydrated foods, especially dried potato and vegetables, fruit juices, both dried and concentrated, and wine. These complex chemical reactions are known as Maillard reactions, named after the French chemist who first investigated the interaction of sugars and amines in 1912. Essentially, the aldehyde groups of the reducing sugars react with the free amino groups of amino acids to form furfuraldehyde, pyruvaldehyde, acetol, diacetyl, hydroxydiacetyl and other sugar- degradation compounds that in turn react with amines to produce melanoide-type macromolecules (brown pigments). Despite intensive research into this subject, the only successful way of inhibiting these reactions is by using sulphurous acid and sulphites. The levels of sulphur dioxide allowed in food products are strictly controlled by legislation and also by the amount that can be tolerated before the taste becomes unacceptable. In the case of dried products, these are sulphited immediately after or during blanching. The use of sulphites for this purpose does not involve the antimicrobial protection for which these compounds are used in other applications. It is important to note that sulphite treatment of any fruit and vegetables intended for canning needs to be very tightly monitored in order to avoid the risk of severe accelerated internal de-tinning.

Bacteria Bacteria are single-celled microorganisms that normally multiply by binary fission, which are they divide into two cells following a period of growth. If conditions are favourable for reproduction, one bacterium can divide into two by fission, so that after 11 hrs there can be more than 10 million cells (>1 × 107 ). This is a level where organoleptic spoilage of the food is apparent due to the production of off-flavours, unpleasant odours and slime or it can result in toxin release. There are four stages in bacterial growth: • Lag phase, during which the bacteria are acclimatising to their environment, which can be several hours long; • Log phase, during which reproduction occurs logarithmically for the first few hours. Conditions for growth are ideal during this period; • Stationary phase, during which the bacteria’s reproduction rate is cancelled by the death rate; • Mortality or Decline phase, during which exhausted nutrient levels or the levels of toxic metabolites in the environment prevent reproduction, with the result that the bacteria gradually die off. Bacteria require water, proteins, carbohydrates and lipids for growth. In addition, small quantities of vitamins and trace elements are needed to support and catalyse metabolism. Nutrients must be available in soluble form to aid transport through the cytoplasmic cell membrane. Large and complex organic molecules cannot pass through the membrane without first being broken down into smaller molecules. This is achieved by the bacteria releasing enzymes into the surrounding food that catalyze the breakdown of complex molecules into a form which can be used by the organism. An example of this is the release of amylase by various Bacillus species such as Bacillus subtilis, Bacillus amyloliquefaciens and Bacillus licheniformis, which breaks down complex carbohydrates into simple sugars that can be absorbed through the cell membrane. Water is essential for bacterial growth because it facilitates the transport of small molecules through the outer cytoplasmic membrane of the bacterial cell via osmotic pressure gradients. Bacteria require higher levels of available water than yeasts or moulds. At 20% available water, their growth is good, but it is limited when reduced to 10%, and at 5% there is no bacterial growth. The available water or water activity (aw) is the amount of free water in a food and excludes moisture that is bound and unavailable for the microorganisms to use. Most bacteria cannot grow below an aw of 0.91–0.94

Microorganisms The term microorganism includes all small living organisms that are not visible to the naked eye. They are found everywhere in the atmosphere, water, soil, plants and animals. Microorganisms can play a very important role in breaking down organic material. Temperature regulation is the most commonly used method to kill or control the number of microorganisms present within foods and on packaging material surfaces. Five categories of temperature-sensitive microorganisms are used to define the preferred temperature range for their growth. • Psychrotrophic (cold tolerant), which can reproduce in chilled storage conditions, sometimes as low as 4°C. Having evolved to survive in extremes of cold, these are the easiest to destroy by heat. • Psychrophilic (cold loving), which have an optimum growth temperature of 20°C. • Mesophilic (medium range), which have an optimum growth temperature between 20 and 44°C. These are of greatest concern with packaged foods.

Packing the fruit in an atmosphere that excludes air will reduce the extent of the browning reactions but can lead to quality problems and does not provide a viable solution. Enzymes are also produced during microbial spoilage of foods and are involved in the breakdown of texture. Many of the microorganisms that secrete enzymes are moulds; however, there are bacterial species (e.g. Bacillus subtilis, Bacillus amyloliquefaciens and Bacillus licheniformis) that produce heat-stable amylases. Amylase enzymes degrade starches, particularly naturally occurring starches, with the effect that the viscosity of the food is reduced as the macromolecular starch granules are broken down into their constituent sugars. Moulds are of particular concern for packaged foods because it is common for many species to produce spores as part of their reproductive cycle. Spores are easily carried in the air and can contaminate inside surfaces of exposed packaging. Other complex reactions can occur, for example Rhizopus species causes softening of canned fruits by producing heat-stable pectolytic enzymes which attack the pectins in the fruit. Mucor piriformis and Rhizopus species also cause the breakdown of texture in sulphite-treated strawberries as a result of similar production of enzymes. Byssochlamys species have been considered responsible for the breakdown in texture of canned foods, particularly strawberries, in which it is commonly found. This is a heat-resistant mould that requires temperatures in excess of 90°C for several minutes to adequately destroy it. Failure to inactivate enzymes completely often shortens the storage life of packaged foods. This problem is rare in case of canned foods but is common with frozen fruits and vegetables that receive only a blanching process prior to freezing. Blanching is intended to inactivate the majority of the enzymes, without imposing excessive thermal damage to the food, and hence it uses relatively mild temperatures (90–100°C) and short heating times (1–10min). The renewed activity that often seems to be present in the thawed food after frozen storage is attributed to enzyme regeneration.

Agents of food deterioration 2.1 Enzymes Enzymes are complex globular proteins found in living organisms and act as catalysts for speeding up the rate of biochemical reactions. Enzymes are naturally present in foods and can potentially catalyse reactions which could lead to food deterioration. It is usually necessary to inactivate enzymes (by denaturing the protein) present in food and on packaging surfaces using heat or chemical means to preserve and extend the shelf life of foods. Fruit and vegetables are major sources of enzymes which are associated with food spoilage. Enzymes associated with the deterioration of fruits and vegetables include peroxidase, lipoxygenase, chlorophyllase and catalase. Peroxidase is relatively heat-resistant and comprises a mixture of several enzymes of varying heat resistance. Some of these can be inactivated by mild heat treatments, whereas some require severe heat treatment. During the ripening of fruit, the activity of some enzymes (e.g. pectinesterase and polygalacturanase) increases and consequently causes a softening of the tissue. In potatoes, enzyme inhibitors play an important role in balancing the rate of biochemical reactions in relation to sugar accumulation. This has commercial importance for the storage and conditioning of potatoes prior to processing, where the presence of reducing sugars is undesirable because they can lead to enhanced browning reactions that cause discoloration. The active enzyme is invertase, which produces reducing sugars from sucrose, especially at low temperatures. At higher temperatures the inhibitor is active and reducing sugars are prevented from forming. Another problem with fruits and vegetables is enzymatic browning which results from damage or cutting of the surface and exposure to the air. This is due to the action of polyphenoloxidase, which in the presence of air oxidizes phenolic constituents to indole quinone polymers. These reactions are undesirable and several methods are used to prevent this type of browning including the use of citric acid, malic acid or phosphoric acid to inactivate the enzyme, or alternatively preventing oxygen from coming into contact with the food by immersion in brine.

Deteriorative Reactions In Food System Food deterioration can be defined as the breakdown of food by agents of either microbiological or chemical origin, either directly or indirectly from products of their metabolism. Contamination of packaged foods can take place from microbiological, chemical and physical sources. Microbiological sources can be present in foods prior to packaging or on surfaces of packaging materials; hence, the shelf life will depend on their types and numbers, in addition to the hurdles to growth offered by the preservation techniques. Chemical sources often enter from enzymes released by microorganisms, to catalyse the rate at which food substrates decompose into smaller compounds which can move through cellular walls of a microorganism. Physical contamination may carry and introduce microorganisms which could cause food deterioration, but does not play a role in deterioration.