Unit 1: Introduction to Food Chemistry PDF

# UNIT I ## INTRODUCTION TO FOOD CHEMISTRY ### INTRODUCTION TO VARIOUS FOOD GROUPS Food is a substance required for growth development nutrition repair and reproduction of human body Foods are composed of various chemical constituents. The three major constituents of foods are carbohydrates protei...

# UNIT I ## INTRODUCTION TO FOOD CHEMISTRY ### INTRODUCTION TO VARIOUS FOOD GROUPS Food is a substance required for growth development nutrition repair and reproduction of human body Foods are composed of various chemical constituents. The three major constituents of foods are carbohydrates proteins and lipids In addition foods contain small amounts of vitamins minerals pigments flavouring substances and enzymes. Another important constituent of food is water which is present in small or large quantities. These constituents give foods their structure texture colour flavour and nutritive value. ### I. Cereals Cereals are the seeds of grass family, Gramineae. The term ‘cereal’ is derived from ‘ceres’ the Roman goddess of grain. Cereals are mostly the bulk of the food consumed by humans and form a staple food for various populations. They are rich source of carbohydrates and constitute a high percentage of calories intake by man particularly in developing countries. The principal cereal crops include: * Rice (Oryza sativa) * Wheat (Triticum aestivum), * Maize (Zea mays), * Barley (Hordeum vulgare), * Oats (Avena sativa), * Sorghum (Sorghum vulgare), * Millets (eg. Finger millet or ragi-Eleusine coracana), * Rye (Secale cereale) etc. ### General characteristics of cereals: * Cereals are monocotyledonous. * All the cereal grains possess endosperm (the largest part) germ and bran. * Carbohydrates are the major constituents of cereals, comprising of 80% of the dry matter. * Cereals are often subjected to milling before being consumed as food. Milling of cereals involves removal of outer husk (lemma and palea) germ and bran. * Wheat is called as king of cereals. It is subjected to milling to produce flour which is used to make bakery products. * Cereals are deficient in amino acid lysine and rich in methionine. ### II. Legumes or pulses Legume is the alternate name for pulses. Pulses are the edible fruits or seeds of pod bearing plants. These belong to the family of Leguminosae. They have high protein content ranging from 20 - 40% which increases their nutritional significance. Due to this pulses are widely consumed in developing countries to overcome the protein-energy malnutrition. In addition these act as an important source of protein for the people following vegetarian diet. In countries like India pulses are also called as poor man’s meat. Major pulses which find an important place in our diet include: * Pea (Pisum sativu) * Kidney bean (Phaseolus vulgaris), * Green gram (Phaseolus aureus), * Black gram (Phaseolus mungo), * Lentil (Lens esculenta) * Chickpea (or Bengal gram or channa-Cicer arietinum) * Red gram (or arhar-Cajanus cajan) ### General characteristics of pulses: * The pulse seeds are dicotyledonous. * India is the largest producer as well as largest consumer of pulses * The protein content of pulses is high (20-40%) which is commonly twice that of cereal grains. * Pulses are deficient in amino acid methionine and rich in lysine. * Pulses contain several anti nutritional factors such as enzyme inhibitors lathyrogens favic agents haemagglutinins saponins goitrogens etc. Some of these are heat stable while others can be destroyed by heat. ### III. Fruits Fruits are produced from flowers of plants. These are ripened ovary or ovaries of plant together with adjacent tissue. Fruits are generally fleshy or pulpy in character often juicy and usually sweet. These often have wide range of flavours colours and textures. Fruits are of several types: * Soft fruits: Strawberry (Frageria virginiana) blackberry (Rubus ulmifolius) raspberry (Rubus idaeus) etc. * Pomes or hard fruits: Apple (Malus pumila) pears (Pyrus communis), * Stone fruits or drupes: Apricot (Prunus armeniaca) cherry (Prunus avium) peach (Prunus persica) plum (Prunus domestica) etc. * Citrus fruits: Sweet oranges (Citrus sinensis) sour oranges (Citrus aurantium) Lemon (Citrus limon) etc. * Tropical fruits: Mango (Mangifera indica) Banana (Musa paradisica) avocado (Persea americana) pineapple (Ananas comosus) melon (Cucmis melo) guava (Psidium guajava) papaya (Carica papaya) etc. * Others: Grapes (Vitis vinifera) figs (Ficus carica) kiwi (Artnidia deliciosa) etc. ### General characteristics of fruits: * Fruits generally have very low amount of protein and fat. * The moisture content of fruits is very high 80-90%. * Fruits are rich in vitamins. * These are excellent source of dietary fiber. * These are rich source of antioxidants. ### IV. Vegetables Vegetables are plants or parts of plants that are used as food in raw or cooked form along with the main course of meal. These have diverse colours flavours and nutritional value. Vegetables add variety to our diet. These are essential items of food for both rich as well as poor. The majority of Indian population is vegetarian. However the overall intake of vegetables is still low in India. It is mainly because of their high cost long preparation time and also the ignorance of people about their health benefits. Vegetables are classified on the basis of the parts of plant being consumed. The examples of this type of classification can be given below: * Roots: Carrot beet root radish turnip etc. * Tubers: Potato sweet potato tapioca etc. * Bulb: Onion garlic leek * Leaves: Cabbage lettuce spinach fenugreek leaves mint leaves coriander leaves * Flowers: Plaintain flower cauliflower neem flower broccoli * Fruits: Tomato brinjal pumpkin cucumber capsicum plaintain * Stems: Plaintain stem ginger amaranth stem lotus stem. * Fungi: Mushrooms ### General characteristics of vegetables: * Vegetables generally have very low amount of fat and protein content. * Vegetables from roots and tubers are a good source of carbohydrate. * The moisture content of vegetables is very high 80-90%. * Vegetables are rich in vitamins and minerals. * These have the non volatile acids such as citric acid malic acid oxalic acid etc. * These contain various pigments that provide wide range of colours to vegetables. ### V. Milk Milk is a liquid food that comes from the normal secretion of mammary glands of mammals. It is a complex mixture of carbohydrates proteins lipids organic compounds and many organic salts dispersed in water. Cow is the principal source of milk for human consumption in many parts of world. However humans also consume the milk from other species like buffalo goat sheep camel etc. In India more milk is obtained from buffalo than cow. The composition of milk varies with species breed diet lactation period and interval between milking. There are individual variations between animals also. The most variable component of milk is fat followed by protein. ### General characteristics of milk: * Milk is a low acid food. * The moisture content of milk from any species is above 80%. * Milk and dairy products are the major source of calcium in our diet. * Milk is a good source of niacin and tryptophan. * It is a poor source of iron. * It is a poor source of vitamin C. ### VI. Meat Meat is defined as the flesh of animals used as food. There are about 3000 mammalian species of animals that are used as a source of meat for human consumption all over the world. It includes flesh of animals as well as the organs such as liver kidney brains and other edible tissues. Different categories of meat consumed by humans are: * Mutton: Meat of sheep and goat. * Beef: Meat of cows buffaloes and bulls. * Veal: Meat of calves. * Chicken: Meat of poultry birds. * Pork: Meat of pigs. * Game meat: Meat from wild animals. ### General characteristics of meat: * Meat is a low acid food. * It has 15-20% proteins of high biological value. * The fat content of meat varies from 5-40%. The accumulation of fat between muscle fibers and bundles is called marbling. * Meat is an excellent source of B-complex vitamins. * Red colour of meat is due to the presence of myoglobin. ### VII. Spices Spices are aromatic compounds produced by plants that enhance the flavour of foods. Flavour is one of the chief characteristics that determine the palatability and consumer acceptance of a food. A food rich in nutrition but unsavoury in flavor is not preferred by the consumers. Spices enhance and improve the flavour of foods. Different spices are produced from different parts of plants like roots leaves seeds barks flowers. Examples of spices include: * Pungent spices: Chillies (Capsicum annum) pepper (Pepper nigrum) Ginger (Zingiber officinale) * Aromatic fruits: Cardamom (Elettaria cardamom) nutmeg mace * Aromatic seeds: Cumin coriander seed fenugreek seeds poppy seeds. * Aromatic leaves: Bay leaves mint (Mentha arvensis) * Aromatic bark: Cinnamon (Cinnamomum zeylanicum) * Aromatic flowers: Clove * Bulbs: Garlic (Allium sativum) * Coloured spices: Saffron (Crocus sativus) turmeric (Curcuma longa). ### General characteristics of spices: * Spices enhance the flavour of foods. * Spices enhance the colour of foods. * These stimulate salivation acid secretion and digestive enzymes secretion in body. * These help to add variety in the diet. * Some spices have antioxidant anti inflammatory (eg. Ginger) and antibacterial properties (eg. Garlic and asafeotida). * Some spices are anticancerous (eg. Coriander and coriander seeds) and antimutagenic (eg. Clove). * Some spices reduce cholesterol level in blood (eg. Garlic). * Some spices reduce blood glucose level and help to mitigate diabetes (eg. Fenugreek seeds). ### VIII. Plantation crops Plantation crops may be defined as a group of commercial cultivated crops that are perennial in nature. Such crops are extensively cultivated in tropical and subtropical regions. The examples include tea coffee cocoa sugar cane etc. * **Tea (Camellia sinensis):** Tea is one of the most widely consumed beverages all over the world. Its plant is an evergreen shrub and tea is obtained from its leaves. Tea leaves are usually plucked by hand. Near about 5-6 pluckings are made in a season with an average interval of about a week. The leaves are further processed to obtain three types of tea: * **Black tea:** The tea leaves are withered rolled fermented dried and packed. Withering is done by spreading tea leaves thinly on racks or shelves to dry the leaves partially. It is the most popular form of tea. * **Green tea:** The tea leaves are only heated rolled and dried. Withered and fermention steps are not carried out in green tea. Drying is done to inactivate the enzymes. The colour flavour and aroma of green tea is significantly different than black tea. * **Oolong tea:** Oolong tea is intermediate between black tea and green tea. The leaves of the tea plant are slightly withered and light fermented to obtain oolong tea. * **Coffee:** Coffee is an important beverage used all over the world. There are many species of coffee. There are many species of coffee but three species are of commercial importance. * **Coffee arabica:** It has the largest and best quality coffee beans. It was indigenous to Ethopia and was introduced in India through Arabia. * **Coffee robusta:** It has the beans of lower quality, however, it is still grown in India because its plant can be grown at lower elevation, is longer living and disease resistant. * **Coffee liberica:** It has the beans of very lower quality with low flavour. Its plant is also susceptible to diseases. Due to these reasons this coffee is not grown in India. * **Cocoa (Theobroma cocoa):** Cocoa tree is grown to obtain cocoa powder and chocolate. These products are prepared from cocoa pods. ### CARBOHYDRATES Carbohydrates are organic compounds containing carbon hydrogen and oxygen Historically the term "carbohydrate" has been used to represent 'hydrates of carbon' with the general formula Cn(H2O)n These are among the most abundant compounds on earth. Examples of carbohydrates include simple sugars starches celluloses pectins and gums. They are an important source of energy in our diet. However some polysaccharides act as dietary fiber with very important functional properties. #### Sources of carbohydrates: Carbohydrates are abundant in naturally occurring foods. These include table sugar (99%) cereals (60-80%) pulses (50-60%) roots and tubers (20-40%) #### Classification of carbohydrates: Carbohydrates are generally classified on basis of the number of sugar molecules contained by them. On this basis carbohydrates are classified as: * **Monosaccharides:** The word monosaccharide is derived from mono meaning "one" and saccharide meaning "sugar". These are the simplest carbohydrates with the general formula of CnH2nOn. Monosaccharides contain three to eight carbon atoms but only those with five or six carbon atoms are common. Two of the most important ones in foods are the six-carbon sugars namely glucose and fructose. These have the general formula C6H12O6. * **Glucose** is known as an aldose sugar because it contains an aldehyde group (CHO) located on the first carbon atom of the chain. It is the main sugar metabolized by the body for energy. However, our body is capable of metabolizing only the D-isomer of glucose. * **Fructose** is a structural isomer of glucose. It is a six-carbon sugar like glucose but it is a ketose sugar not an aldose. This is because it contains a ketone group rather than an aldehyde group in its molecular chain. In other words fructose has the same chemical formula but a completely different three-dimensional structure. While the glucose forms a six-membered ring fructose forms a five-membered ring structure. * **Oligosaccharides:** Oligosaccharides is a name given to a class of carbohydrates that contain two to ten single sugar units. This group can be further studied under the subheadings as; disaccharides - the carbohydrates containing two sugar units trisachharides - the carbohydrates containing three sugar units and so on. * **Disaccharides:** Disaccharides contain two monosaccharides joined together through a glycosidic bond. Some common examples of diasaccharides are: * **Sucrose:** Sucrose is also called as table sugar. Sucrose is a disaccharide containing two sugar units - glucose and fructose. * **Lactose:** Lactose is also known as milk sugar because it is the sugar found in milk. Lactose is a disaccharide containing two sugar units - glucose and galactose. It is the least sweet and least soluble of all the sugars. * **Maltose:** Maltose is the building block for starch. It is a disaccharide containing two glucose units. * **Other oligosaccharides:** Other oligosaccharides include raffinose and stachyose. Raffinose is a trisaccharide containing galactose glucose and fructose. Stachyose is a tetrasaccharide containing glucose fructose and two galactose units. Both occur in legumes such as dry beans and peas. They are not hydrolyzed or digested by the human digestive system and become food for bacteria in the large intestine. The bacteria metabolize these carbohydrates and produce gas causing varying degrees of discomfort. * **Polysaccharides:** Polysaccharides are complex polymers of carbohydrates containing more than ten glucose units in their molecular chain. Mostly polysaccharides have as many as several thousand sugar units linked together to form a molecule. The most important food polysaccharides are the starches pectins and gums. All of these are complex carbohydrate polymers with different properties depending upon the number of sugar units that make up the molecule the type of glycosidic linkages and the degree of branching of the molecules. #### STARCH Starch is a polymer of glucose. It is made up of two types of molecular chains namely amylose and amylopectin. Both of these long chains are made up of glucose molecules linked through a-1,4-glycosidic linkages. The difference between amylase and amylopectin is in the degree of branching. Amylose is a linear chain with little or no branching whereas amylopectin chains have extensive branching throughout the molecule. The branches on the main chain of amylopectin are joined through a -1, 6-glycosidic linkages. The degree of branching has a considerable effect on the properties of these glucose chains eg. due to branching, amylopectin is less soluble in water as compared to amylose. Some of the common sources of starch in human diet include wheat potatoes maize (corn) rice and cassava. Generally the starch obtained from different sources contains 20 to 25% amylose and 75 to 80% amylopectin by weight. However, some cultivated plant varieties have only amylopectin in the starch without amylase. Such starch is known as waxy starch. #### Properties of starch: * Pure starch is a white tasteless and odorless powder. * It is insoluble in cold water or alcohol. * When heated in abundant water the granules of native starch swell and burst disrupting the crystalline structure of granules. With this the smaller amylose molecules start leaching out of the granule and increase the viscosity of the mixture. This process is called starch gelatinization. * The gelatinization temperature of starch varies depending on starch cultivar amylose/amylopectin ratio and water content. * When the hot viscous solution of starch is cooled down the amylase and amylopectin chains attempt to rearrange and realign their chains again to form a crystalline structure. This process is called as retrogradation. * The process of retrogradation is responsible for the hardening of bread or staling. #### Other Polysaccharides: Other polysaccharides mostly include pectin and gums. These are also of the considerable importance in food technology. Some of these are used as stabilizers and thickeners in food preparations while some are used for the preparation of food products. Pectin is obtained from plants. Structurally pectin is a long-chain polymer of a-d-galacturonic acid which is an acid derived from the simple sugar galactose. Pectin is widely found in fruits and vegetables. It is soluble in water and under appropriate conditions it forms gel. Pectin is considered as a rich source of dietary fiber. It is widely used as gelling agents in jams and jellies. ### PROTEINS Proteins are the principal components of all living cells making up 50% or more of their dry weight. These are large biomolecules made up of carbon (C) hydrogen (H) oxygen (O) and nitrogen (N). Some proteins also contain the elements like sulphur phosphorus and iron. Chiefly proteins play a role in forming the complex muscle system and the connective tissue network in our body. Amino acids are the basic building blocks of proteins. A linear chain of amino acid residues is called a polypeptide. When a polypeptide chain of more than 50 amino acids folds to form a well defined conformation, it is called as a protein. Every protein has a unique structure and conformation or shape which enables it to carry out a specific function in a living cell. Once formed proteins only exist for a certain period and are then degraded and recycled by the cell’s machinery through the process of protein turnover. #### Essential Amino Acids: * Arginine * Isoleucine * Histidine * Leucine * Methionine * Lysine * Phenylalanine * Tryptophan * Threonine * Valine #### Non-Essential Amino Acids: * Alanine * Artjinine * Asparagine * Aspartic Acid * Cysteine * Glutamic Acid * Glutamine * Glycine * Proline * Serine * Tyrosine There are about 21 primary amino acids found in nature, some of which are designated as essential amino acids while others are designated as non-essential amino acids. The amino acids that cannot be synthesized by the body and need to be acquired through food are called as essential amino acids. On the other hand the amino acids that can be synthesized by the body are called non essential amino acids. All of the amino acids have different properties depending on their structure and composition. Proteins are very important in foods both nutritionally and as functional ingredients. They play an important role in determining the texture of a food. #### Sources of proteins: * Meat poultry fish * Eggs * Dairy products * Legumes * Nuts and seeds #### Classification of proteins: Proteins are classified on the basis of shape constitution nature of molecules and solubility as discussed below. * Classification of proteins on basis of structure: * **Fibrous protein:** These are the proteins that have an elongated or fiber like structure. Their structure is very simple. They have less biological functions. These proteins are insoluble in water and resistant to proteolytic enzymes. These are mostly present in animals. Examples include scleroproteins like keratine elastin collagen fibroin etc. * **Globular proteins:** These are the proteins that are formed by coiling of polypeptide chains. Due to this globular proteins have a spherical or globular shape. These are dynamic in nature and their structure is very complex. These are mostly soluble in water. They have variety of biological functions for example albumin globulin insulin and hormones like oxytocin etc. * Classification of proteins on basis of constitution: * **Simple proteins:** These proteins are made up of amino acids only. eg. albumins globulins prolamins etc. * **Conjugated proteins:** These are complex proteins that contain a non amino acid substance called as a prosthetic group in amino-acid chain. These are of following types:- * **Glycoproteins:** Combination of proteins and carbohydrates. * **Lipoproteins:** Combination of proteins and lipids. * **Nucleoproteins:** Combination of protein and nucleic acid. * **Mucoproteins:** Combination of proteins and carbohydrates. * **Chromoproteins:** Combination of proteins and coloured pigments. * **Metalloprotein:** Combination of proteins and metal ions. * **Phosphoprotein:** Combination of proteins and phosphate group. * **Derived proteins:** When proteins are hydrolyzed by acids alkalies or enzymes the degradation products obtained from them are called derived proteins. * Classification of proteins on basis of the nature of Molecules: * **Acidic proteins:** Acidic amino acids are those amino acids that have acidic side chains at neutral pH. The proteins that contain such acidic amino acids are called as acidic proteins. All such proteins are negatively charged (anions) eg. blood groups. Examples of acidic amino acids: Aspartic acid Glutamic acid. * **Basic proteins:** Basic amino acids are those amino acids that have basic side chains at neutral pH. The proteins that contain such basic amino acids are called as basic proteins. All such proteins are positively charged (cations). Examples of basic amino acids: Lysine Arginine. * Classification of proteins on basis of solubility * Proteins are divided into following classes on basis of their solubility * Albumin: soluble in water * Globulin: soluble in dilute salt (sodium chloride) * Prolamin: soluble in ethanol/water solvent * Glutelin: soluble/in dilute alkali solution (sodium hydroxide) #### Properties of proteins: * **Shape:** Proteins exist in wide range of shapes. Some of these are globular in shape while some others have long fibre like shapes. * **Amphoteric nature:** Proteins are amphoteric in nature. This is because these have acidic carboxylic groups (-COOH) as well as basic amine groups (-NH2). Therefore these may act like acids as well as like bases depending upon the pH. * **Isoelectric point:** An isoelectric point is the pH at which the protein is electrically neutral. This is because at this pH the number of positive charges on the protein is equal to the number of negative charges making the overall charge equal to zero. At isoelectric point the protein molecules usually precipitate because there is no charge on molecules to create force of repulsion and keep them suspended in the solvent. The value of isoelectric pH is different for different proteins. * **Denaturation:** Denaturation refers to the change in highly ordered structure of proteins due to which protein losses its biological activity. It happens due to the cleavage of some weak bonds in the protein molecule which are responsible for holding its structure in a particular orientation. * **Water binding capacity:** Protein molecules have a tendency to bind with water molecules. Different proteins can bind with varying amounts of water which depends upon the number of hydrophilic groups in protein molecule. More the hydrophilic groups more is the water binding capacity of proteins. * **Solubility:** Solubility is an important property of proteins. Solubility of proteins varies with the pH of solution. At acidic and alkaline pH protein molecules are electrically charged called anions and cations respectively which increases their solubility However at their isoelectric pH protein molecules are electrically neutral due to which their solubility is reduced. * **Hydrolysis:** Protein hydrolysis means breaking down of protein molecule due to the cleavage of peptide bond between amino acids. This is also called as proteolysis. Hydrolysis of proteins can be done by two methods: * **Acid hydrolysis:** It means the hydrolysis of protein molecules using concentrated acids like sulfuric acid hydrochloric acid etc. * **Enzymatic hydrolysis:** It means the hydrolysis of protein molecules using proteolytic enzymes like ficin papain bromelain trypsin rennet etc. ### FATS Fats or lipids is a name given to a group of diverse compounds which are insoluble in water but soluble in non-polar organic solvents like chloroform benzene ether etc. Food lipids are generally referred to as fats in case these are in solid state at room temperature or oils in case these are in liquid state at room temperature. Some of these lipids are non-polar in nature eg. triacyl glycerol and cholesterol while some others are polar in nature eg. phospholipids. The polar and non-polar lipids have different solubility and functional properties. Fat is a principal component of the diet. It is enjoyed in the diet due to its flavor mouthfeel texture and aroma. Fats also carry the fat-soluble vitamins A, D, E and K. #### Sources of fats: The total lipid content and the lipid composition of foods can vary tremendously. Different sources of fats and oils include: * Mustard seeds (mustard oil) * Sunflower seeds (sunflower oil) * Coconut (coconut oil) * Olives (olive oil) * Groundnuts (groundnut oil) * Fatty fish * Egg yolk * Avocado * Beef and mutton * Cheese #### Properties of fats and oils: * **Crystal formation or crystallization:** Crystallization is a process in which fat molecules arrange themselves into an ordered and well defined structure to minimize their energy state. It begins with decreasing the temperature of fats which reduces the movement of fat molecules. Gradually the molecules are attracted to each other through Vander Waals forces and align themselves in a particular order forming crystals. * **Polymorphism:** Fats show polymorphism which means that these can exist in different crystalline forms. A fat may crystallize in one of the three different crystal forms depending upon the conditions during crystallization and on the composition of the fat. These are alpha (α) beta príme (β) and beta forms (β). Among them the smallest and least stable (meta stable) crystals are called alpha (α) crystals. These are formed if fats are chilled rapidly. The alpha crystals change readily to beta prime (β) crystals which have intermediate stability. These crystals further convert to coarse beta (β) crystals which have highest stability. * **Melting points:** Melting is a process during which solid lipids convert to liquid form. It happens when the lipid molecules absorb heat energy which helps these molecules to overcome the forces of attraction with other molecules and move more freely in liquid state. The greater the attractive forces between molecules higher will be the heat energy required to melt them. Fats do not have a sharp melting point. These melt over a range of temperature. This is because fats are a mixture of triglycerides that melt at different temperatures. Melting point of the fats depends upon the following factors: * **Stability of crystals:** Melting point depends upon the stability of fat crystals. More the stability of crystals higher is their melting point. Thus beta (β) crystals have highest melting point and alpha (α) crystals have lowest melting point. * **Chain length:** Long-chain fatty acids have a higher melting point than short chain fatty acids because there is more attraction between long chains than between short chains. * **Number of double bonds:** Melting point is determined by the number of double bonds. As the number of double bonds increase the melting point decreases. Double bonds introduce kinks into the chain which makes it harder for the fat molecules to fit together to form crystals. Thus the attractive forces between the molecules are weaker. * **Isomeric configuration:** Cis isomers of fats have lower melting point while trans isomers have higher melting point. This is because cis isomers pack poorly and trans isomers can be stalked over each other more efficiently. ### RANCIDITY Rancidity may be defined as a process of partial or complete oxidation or hydrolysis of fats due to the exposure to air light moisture or bacterial action producing short chain aldehydes ketones and free fatty acids. Such processes lead to the generation of off flavours and off aromas in foods. Rancidity is of three types as discussed below: * **Hydrolytic rancidity:** It is a kind of rancidity which is brought about by the activity of the enzymes like lipase. These enzymes catalyze the reactions between fats and water producing free fatty acids and glycerol. Among the free fatty acids those having short chains produce more off aromas. For example butyric and caproic acids produce off flavour and off aroma in rancid butter. Hydrolytic rancidity can be prevented by heating the fats sufficient enough to destroy lipase enzyme. * **Oxidative rancidity:** It is a kind of rancidity which occurs due to the reaction of fats with oxygen. In these reactions oxygen causes the cleavage of double bonds in unsaturated fatty acids generating volatile aldehydes and ketones that have a very bad odour. These reactions are catalyzed by sunlight and can’t be stopped even by the frozen storage of foods. Oxidative rancidity can be prevented by light-proof packing oxygen-free packing and addition of antioxidants. * **Microbial rancidity:** It is a kind of rancidity which is caused by the growth and activity of microorganisms like bacteria and moulds that can use their enzymes to breakdown the fat. These microorganisms can produce the enzymes like lipase which causes the rancidity of fats, however, their growth is dependent on the availability of water in foods. Processes like pasteurization or addition of antioxidants such as vitamin E can reduce the microbial rancidity in foods. ### WATER IN FOODS Water is an important constituent of foods. It is present in small quantities in some foods such as cereals and pulses while in others like milk meat fruits and vegetables it is present in large quantities. In water the HOH molecule has a V-like shape and the O-H bond is polarized due to an asymmetric charge distribution within the molecule. One of the important properties of water molecules is that there exist strong attractive forces among polarized water molecules. These intermolecular attractive forces explain hydrogen bonding between the water molecules. #### Water activity: There exists a relationship between the moisture content of foods and their shelf life. Earlier it was believed that more the moisture content less is the shelf life of foods. However soon it was observed that various types of food with the same water content differ significantly in their shelf life. Thus, moisture content is not the only factor that determines the shelf-life of foods. It is because some of this moisture content is bound with the ionic dipolar and other types of hydrophilic sites within the food. This proportion of water which is bound has restricted mobility. It remains unfrozen at ordinary freezing temperatures of water isn’t removed during dehydration, remains unavailable as a solvent for more solutes and has the properties different from those of "bulk" water in the same food. This water most oftenly called as 'bound water' does not participate in spoilage causing reactions or microbial activity. As a result, it can be said that bound water does not affect the shelf life of foods. In other words, it is only the free water that participates in spoilage causing reactions microbial activity and ultimately the shelf life of foods. More the proportion of free water in any food, lower is the expected shelf life of that food under ordinary conditions. In this context the term "water activity" (aw) was developed to reflect the intensity with which water associates with various non aqueous constituents within the food and exist as bound water. Water activity (aw) may be defined as a ratio of the vapour pressure of water in a solution (Ps) to the vapour pressure of pure water (Pw): $a_w = \frac{P_s}{P_w}$ The values of water activity range from 0 to 1. Higher the water activity, higher is the proportion of water available for spoilage causing reactions and microbial activity within the foods. The spoilage causing microorganisms such as bacteria, mould, and yeast multiply at a high aw (above 0.90). In order to control their growth, preservation techniques involving decrease in water activity such as dehydration and freezing may be employed.