ENZYMES enzymes Enzymes are biological catalyst They help
ENZYMES
enzymes • Enzymes are biological catalyst. • They help in carrying out a reaction at lower activation energy. • All enzymes are proteins in nature except ribozymes
ENYMES IN FRUITS AND VEGETABLES • These enzymes are present in different foods too, like in fruits, vegetables, etc • Post-harvesting, the fruits and vegetabes keep on respiration. As a result their metabolic activity goes on un-interrupted even after harvesting from trees. • Such metabolic activities may be undesirable from food preservation point of view.
Coating materials for preservation • Because the fruits and vegatbles may keep on their metabolic activity, which may result in over ripening etc. • Therefore to control its respiration and other metabolic activities, raw fruits and vegatbes are coated with some coating material. • These coating materilas may be different biopolymers or synthetic polymers.
ENZYMATIC BROWNING • Enzymatic browning is a chemical process which occurs in fruits and vegetables by the enzyme polyphenoloxidase, which results in brown pigments. Enzymatic browning can be observed in fruits (apricots, pears, bananas, grapes), vegetables (potatoes, mushrooms, lettuce) and also in seafood (shrimps, spiny lobsters and crabs). • Enzymatic browning is detrimental to quality, particularly in postharvest storage of fresh fruits, juices and some shellfish. Enzymatic browning may be responsible for up to 50% of all losses during fruit and vegetables production.
POLYPHENOLS ROLE IN BROWNING • Polyphenols – main components in enzymatic browning • Polyphenols, also called phenolic compounds, are group of chemical substances present in plants (fruits, vegetables) which play an important role during enzymatic browning, because they are substrates for the browning-enzymes. • Phenolic compounds are responsible for the colour of many plants, such as apples, they are part of the taste and flavour of beverages (apple juice, tea), and are important anti-oxidants in plants.
POLYPHENOLS ROLE IN BROWNING • During food processing and storage many polyphenols are unstable due to the fact that they undergo chemical and biochemical reactions. The most important is enzymatic oxidation causing browning of vegetables, fruits. This reaction mostly occurs after cutting or other mechanical treatment of product due to breaking cells.
PREVENTION OF ENZYMATIC BROWNING • Prevention of enzymatic browning • The control of browning is one of the most important issues in thefood industry, as colour is a significant attribute of food which influences consumer decision and brown foods (especially fruits) are seen as spoiled. • Several methods can be applied to avoid enzymatic browning, based on inactivating the enzyme (heat) or by removing essential components (most often oxygen) from the product.
PREVENTION OF ENZYMATIC BROWNING • Blanching is a short heat treatment to destroy or inactivate enzymes before freezing of products (mainly vegetables). Enzyme activity may discolour or toughen vegetables during freezing, which results in quality loss. Blanching brightens the colour, softens the texture, but has little effect on nutrient content or flavour as it is a relatively short process. • The blanching temperature depends on the type of enzyme which occurs in the product, but is generally between 70 and 100 °C, sometimes higher when more resistant enzymes are to be inactivated. Table 2 below gives an indication of the temperature needed to inactivate some important enzymes.
PREVENTION OF ENZYMATIC BROWNING • Refrigeration and chilling are used to prevent spoilage of vegetables and fruits during distribution and retailing. Chilling is applied often for broccoli, berries, spinach, peas, bananas, mangoes, avocados, tomatoes. At temperatures below 7 °C the polyphenoloxidase enzyme activity is inhibited, but the enzyme is not inactivated. Therefore the temperature should be well controlled. • Freezing • Like refrigeration, freezing inhibits, but not inactivates the enzyme. After thawing, the enzyme activity will resume. • Change p. H • The enzyme activity is p. H dependent. Lowering of the p. H to 4. 0 by the addition of citric, ascorbic or other acids inhibits the enzyme activity. During homepreparation of vegetables or fruits lemon juice or vinegar is often sprinkled on the fruit to prevent browning.
PREVENTION OF ENZYMATIC BROWNING • Dehydratation is caused by the removing water molecules from the product. The PPO enzyme needs sufficient water to be active. By drying the enzyme is inhibited, but not destroyed. • To avoid flavour and quality loss, dehydration should not involve heat. • Common methods for dehydration are: • Freezing-drying when moisture is removed by sublimation (the change from solid to gas). Products are frozen and slowly dehydrated under vacuum. • Lowering water activity by adding water-binding chemicals. The most commonly used substances are salt (sodium chloride), sucrose, and other sugars, glycerol, propylene glycol and syrups or honey.
PREVENTION OF ENZYMATIC BROWNING • Irradiation, or as it is sometimes called "cold pasteurization", is a process in which food is submitted to ionized radiation in order to kill bacteria and reduce the enzyme activity. Irradiation is often applied in meats, seafood, fruits, vegetables, and cereal grains for long-term preservation. • Several types of irradiation methods are used in food processing: gamma rays, X-rays and accelerated electrons (electron beams). • Disadvantages of radiation are loss of nutrients and (low) consumer acceptance. Irradiation is thus rarely used.
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