In this article we will discuss about:- 1. Aroma Compounds in Food 2. Food Flavors 3. Sugar Substitutes 4. Sorbitol 5. Food Colors 6. Browning Reactions 7. Antinutritional Factors 8. Chemical Changes during the Processing of Volatile Compounds.
- Aroma Compounds in Food
- Food Flavors
- Sugar Substitutes in Food
- Sorbitol in Food
- Food Colors
- Browning Reactions of Food
- Antinutritional Factors in Food
- Chemical Changes during the Processing of Volatile Compounds in Food
1. Aroma Compounds in Food:
When food is consumed the interaction of taste, odor and textural feeling provides overall sensations which are commonly referred to as flavor. Flavor compounds classified into two groups: Those responsible for taste and those responsible for odor. The compounds which are responsible for the odor of food items are called as aroma substances. Aroma substance is otherwise called odorant, a chemical compound that has a smell or odor.
Aroma substances are volatile compounds which are perceived by the odor receptor sites of the smell organ (the olfactory tissue of the nasal cavity). They reach the receptor when drawn in through the nose by orthonasal detection and via the throat after being released by chewing by retro nasal detection. Key odorants are the most important types of aroma compound which provide a characteristic aroma to a particular food.
Aroma compounds can be found in food, wine, spices, fragrance oils, and essential oils. For example – many form biochemically during ripening of fruits and other crops. Aroma compounds play a significant role in the production of flavorants, which are used in the food service industry to flavor, improve and increase the appeal of their products.
Classification of Aroma Compounds in Food:
Depending on the functional groups, aroma compounds are classified into various groups:
The most important types of aroma alcohol are benzyl alcohol (oxidizes to benzaldehyde, almond), ethyl maltol (cooked fruit), furaneol (strawberry), menthol (peppermint), etc.
Various types of aldehydes constitute the aroma effect in food are acetaldehyde (pungent), benzaldehyde (marzipan, almond), hexanal (green, grassy), cinnamaldehyde (cinnamon, citral (lemongrass, lemon oil), hexenal (green tomatoes), neral (citrus, lemongrass), vanillin (vanilla).
Amines such as cadaverine (rotting flesh), Indole (jasmine flowery), putrescine (rotting flesh), pyridine (very unpleasant), trimethylamine (fish) are the major aroma compounds in food.
Esters includes ethyl acetate (fruity), ethyl butanoate (fruity), fructone (fruity, apple-like), octyl acetate (orange), isoamyl acetate (banana), pentyl pentanoate (apple, pineapple), etc.
Ketones are also giving characteristic aroma effects. Octenone gives blood, metallic, mushroom-like aroma effect, acetyl pyrroline create fresh bread and jasmine odour, and acetyl tetrahydropyridine also create fresh bread, popcorn odor.
Lactones produce sweet coconut odor.
It gives limonene (orange) and nerol (sweet rose) odor.
The lowest concentration of a compound that is responsible for its odor is called odor threshold (recognition threshold). The threshold value is determined by smelling (orthonasal value) or tasting the food sample (retronasal value). Threshold value of food helps to find out the intensity or potency of odorous substances in food. The threshold value is directly depends on the vapor pressure, which is affected by both temperature and medium.
The aroma substance consists of highly diversified classes of compounds; some of them are highly reactive and are present in food in very less concentration. The elucidation of their chemical structure and characterization of sensory properties are very complex process. So, the analysis of aroma content is a tedious task in food processing. The aroma analysis is one of the prime objectives of food processing, because it helps to determine the quality of raw materials, intermediated and end products.
The elucidation of aroma of any food is a stepwise process that is listed below:
(i) Isolation of the volatile compounds.
(ii) Differentiation of the aroma substances from the remaining components of volatile fraction by dilution analysis.
(iii) Concentration and identification.
(iv) Quantification and calculation of aroma values (threshold values).
(v) Simulation of the aroma on the basis of the analytical results.
Flavor is the sensory impression of a food and is determined by the chemical senses of taste and smell. Flavorant is defined as a substance that gives flavor, altering the characteristics of the solute and causing it to become sweet, sour, tangy, etc. It is also referred to as the edible chemicals and extracts that alter the flavor of food and food products through the sense of smell.
Smell is the main determinant of a food item’s flavor. The taste of food is limited to sweet, sour, bitter, salty, and savory (umami) but the smell of a food is limitless. So, a food flavor can be easily altered by changing its smell while keeping its taste similar.
Flavorings are focused on altering or enhancing the flavors of natural food product such as meats and vegetables, or creating flavor for food products that do not have the desired flavors such as candies and other snacks. Most types of flavorings are focused on smell and taste.
There are three principal types of flavorings used in foods listed below:
1. Natural Flavoring Substances:
Flavoring substances obtained from plant or animal raw materials, by physical, microbiological or enzymatic processes. They can be used in their natural state or processed for human consumption, but cannot contain any nature-identical or artificial flavoring substances.
Due to the high cost or unavailability of natural flavor extracts, most commercial flavorants are nature- identical, which means that they synthesized by chemically rather than extracted from natural plants or animal sources. To produce natural flavors, the flavorant first extracted from the source substance. The methods of extraction may be solvent extraction, distillation, or other physical forces. The extracts are then purified and subsequently added to food products.
Flavoring substances that are produced by chemical synthesis which are chemically identical to natural flavoring substances present in food products intended for human consumption. They lack any artificial flavoring substances.
Flavoring substances that are not identified in a natural product intended for human consumption. To produce the artificial flavors, flavor manufacturers must both find out the individual naturally occurring aroma chemicals and mix them appropriately to produce a desired flavor or create a novel non-toxic artificial compound that gives a specific flavor.
Most artificial flavors are specific and complex mixtures of single naturally occurring flavor compounds combined together to enhance a natural flavor. The compounds used to produce artificial flavors are almost identical to those that occur naturally. Artificial flavors may be safer to consume than natural flavors due to the standards of purity and mixture consistency that are enforced by the law for food manufacturing and processing.
Natural flavors may contain toxins from their sources while artificial flavors are typically more pure and undergo more testing before being sold for consumption. Many artificial flavorants are esters.
Flavor enhancers are amino acid or nucleotide derivatives that capable of enhancing the odor of food. Most flavor enhancers are called as savory flavorants or umami. These are manufactured as sodium or calcium salts.
The important types of flavor enhancing substances are:
1. Glutamic acid salts – Sodium salt of glutamic acid is called monosodium glutamate (MSG), one of the most commonly used flavor enhancers in food processing.
2. Glycine salts – A simple amino acid that is usually used in conjunction with glutamic acid as flavor enhancers.
3. Guanylic acid salts – Nucleotide salts that are usually used in conjunction with glutamic acid as flavor enhancers.
4. Inosinic acid salts – Nucleotide salts created from the breakdown of AMP. Due to high costs of production, it is usually used in conjunction with glutamic acid as flavor enhancer.
5. Organic acids – Organic acid are usually not considered and regulated as flavorants by law.
But they can impart different sour or taste that alters the flavor of a food –
(i) Acetic acid – It gives vinegar sour taste and distinctive smell
(ii) Citric acid – It is found in citrus fruits and gives them their sour taste
(iii) Lactic acid – It is found in various milk products and give them a rich tartness
(iv) Malic acid – It is found in apples and gives them their sour or tart taste
(v) Tartaric acid – It is present in grapes and wines and gives them a tart taste
Small molecules such as ethanol, propanol, butanol (alcohols), acetaldehyde, propanaldehyde (aldehydes), acetic acid, propionic acid and butyric acid (acids) are highly volatile and exhibit pungent ethereal, diffusive, harsh, or chemical odour characteristics. Compounds containing -OH, -CHO, -CO, and -COOH play important roles for the development of odor in different foods. Acid is sour, aldehyde is fresh and ester is fruity.
Long alkyl groups and ketone derivatives enhances fatty or oily note. Lactones are cyclic compounds with ester functional groups which enhances fruity, oily, sweet notes. The molecular weight and boiling point of compounds are directly related with the flavor development, e.g., esters and aldehyde have higher molecular weight but lower boiling point but lactones have relatively high boiling point.
3. Sugar Substitutes in Food:
A sugar substitute is a food additive that duplicates the effects of sugar in taste, but it provide less amount food energy. Sugar and its substituents impart sweetness, tenderness, browning, hygroscopy (water retaining); and functions in various other ways in food systems.
Sugar substituents or sweeteners are broadly classified in to two:
Natural sweeteners (nutritive sweeteners) are sugar substituents which are naturally present in some types of food like berries, fruits, vegetables, and mushrooms and which can provide energy. But these substituents are very difficult to extract from fruits and vegetables, so they are produced by catalytic hydrogenation of the appropriate reducing sugar. The important types of natural sugar substituents are sucrose, fructose, sorbitol, xylitol, lactitol, etc. nutritive sweeteners, (e.g. sucrose, fructose) are generally recognized as safe (GRAS) by the Food and Drug Administration (FDA).
A type of sugar substitute with a distinct classification from artificial sweeteners is called sugar alcohol. Sugar alcohols are caloric (slightly less calories than sugar), chemically reduced carbohydrates that provide sweetness to foods. The sugar alcohols are similar in chemical structure to glucose, but an alcohol group replaces the aldehyde group of glucose. Sugar alcohols are less sweet than sucrose, but have similar bulk properties and can be used in a wide range of food products.
The important types of sugar alcohol include:
(a) Sorbitol, which is commercially produced from glucose and contains 2.6 cal /g.
(b) Mannitol, which contains 1.6 cal /g.
(c) Xylitol, isomalt, hydrogenated starch hydrolysate (HSH), and hydrogenated glucose syrup (HGS) are other sugar alcohols.
Sugar alcohol contains little or negligible sugar content and our body do not metabolize sugar alcohols, so persons with diabetes may use sugar alcohols without a rise in their blood sugar. Large amount of sugar alcohols may cause intestinal diarrhea, therefore, they are not recommended for use in significant amounts.
2. Non-Nutritive Sweeteners:
Artificial sweeteners or high-intensity sweeteners are one category of sugar substitute. They are non-caloric, non-nutritive, intense sugar substitutes, whose use has grown in response to increased consumer demand. They must be FDA approved before use. The food and beverage industry is increasingly replacing sugar or corn syrup with artificial sweeteners in a range of products traditionally containing sugar.
The most important characteristics of an ideal non-nutritive sweetener are:
(a) A sweetness quality and profile identical to that of sucrose.
(b) Sensory and chemical stability under the relevant food processing and storage conditions.
(c) Compatibility with other food ingredients and stability toward other constituents in the food.
(d) Complete safety, shown as freedom from toxic, allergenic and other physiological properties.
(e) Complete freedom from metabolism in the body.
(f) High specific sweetness intensity.
The important types of artificial sweeteners are – saccharin, aspartame, sucralose, neotame, and ace-sulfame potassium (ace-sulfame K) and cyclamate.
Sorbitol, also known as glucitol, is a sugar alcohol that the body metabolizes slowly. It is a natural sweetener as well as sugar alcohol. It may be listed under the inactive ingredients. It also occurs naturally in many stone fruits, berries and vegetables. It is produced by reduction of glucose changing the aldehyde group to an additional hydroxyl group.
Sorbitol is a sugar substitute often used in diet foods (including diet drinks and ice cream) and sugar-free chewing gum, mints and cough syrups. Sorbitols provide half the sweetness of sucrose and may be used as humectants, because they increase the water- holding capacity of the food. Sorbitol also used as a bulking agent.
In combination with aspartame and saccharin, it provides the volume, texture and thick consistency of sugar. Sorbitol is referred to as a nutritive sweetener because it provides dietary energy 2.6 kilocalories per gram versus the average 4 kilocalories (17 kilojoules) for carbohydrates.
Saccharin is an artificial sweetener. Saccharin is a non-caloric substance produced from methyl anthranilate, a substance naturally found in grapes. It is 300-700 times as sweet as sucrose, produces no glycemic response in humans, synergizes the sweetening power of nutritive and non-nutritive sweeteners, and its sweetness is thermo stable. It occurs as a white crystalline powder with a molecular formula of C7H5NO3S and molecular weight of 183.18.
It is used in products such as soft drinks, tabletop sweeteners, baked goods, jams, chewing gum, canned fruits, candies, dessert toppings and salad dressings. It is also used in cosmetic products, vitamins and pharmaceuticals. Calcium or sodium saccharin, combined with dextrose (nutritive, glucose) and an anticaking agent, may be used in table top sweeteners. Saccharin may be used in combination with aspartame.
Saccharin is unstable when heated but it does not react chemically with other food ingredients. Blends of saccharin with other sweeteners are often used to compensate for each sweetener’s weaknesses. Saccharin is often used together with aspartame in diet. Saccharin is believed to be an important discovery, especially for diabetics, as it goes directly through the human digestive system without being digested.
Although saccharin has no food energy, it can trigger the release of insulin in rats, apparently as a result of its taste. In its acid form, saccharin is not water-soluble. The form used as an artificial sweetener is usually its sodium salt. The calcium salt is also sometimes used, especially by people restricting their dietary sodium intake.
Cyclamate is an artificial sweetener that was discovered in 1937 at the University of Illinois by graduate student Michael Sveda. In the US in 1958 it was designated GRAS (Generally Recognized as Safe). Cyclamate was marketed in tablet form for use by diabetics as an alternative tabletop sweetener, as well as in a liquid form. Since 1969, its sale and use has been banned by the Food and Drug Administration in the United States because of some health problem.
Cyclamate is the sodium or calcium salt of cyclamic acid (cyclohexanesulfamic acid). It is prepared by the sulfonation of cyclo- hexylamine; this can be accomplished by reacting cyclohexylamine with either sulfamic acid or sulfur trioxide.
Cyclamate is 30-50 times sweeter than sugar, making it the least potent of the commercially used artificial sweeteners. Some people find it to have an unpleasant aftertaste, but generally less than saccharin or acesulfame potassium. But the mixture of 10 parts cyclamate to 1 part saccharin is common and masks the off-tastes of both sweeteners. It is less expensive than most sweeteners, including sucralose, and is stable under heating.
Food coloring is any substance that is added to food or drink to change its color. Food coloring is used both in commercial food production and in domestic cooking. Due to its safety and general availability, food coloring is also used in a variety of non-food applications. From its color, we can often tell whether food is fresh or stale, of good or poor flavor, and whether it contains particular ingredients.
Because color is important to the consumer, colors are added to food to give an attractive appearance, when it has been lost during processing or storage, or to overcome natural color variation to ensure a consistent product. Food colors and flavors are closely associated. When we see a food we anticipate certain flavors.
We learn to reject foods that are not colored in a familiar way. We expect that a red apple will be sweet, a green plum will be sour and a brown ice cream will have a chocolate flavor. Tests with testing panels showed, that when children were asked to identify the flavors of red and yellow jellies, the majority identified the red jellies as strawberry flavored and the yellow ones as lemon flavored, regardless of the flavor actually present.
A primary reason for food coloring includes:
(i) Offsetting color loss due to light, air, extremes of temperature, moisture, and storage conditions.
(ii) Masking natural variations in color.
(iii) Enhancing naturally occurring colors.
(iv) Providing identity to foods.
(v) Protecting flavors and vitamins from damage by light.
(vi) Decorative or artistic purposes such as cake icing.
Natural food color is any dye obtained from any vegetable, animal or mineral, that is capable of coloring food, drugs, cosmetics or any part of human body. These natural colors come from variety of sources such as seeds, fruits and vegetables, leaves, algae and insects. According to the application, a suitable natural color related factors such as pH, heat, light, storage and the other ingredients.
Natural colors are not required to be tested by a number of regulatory bodies throughout the world, including the United States FDA.
The important types of natural food colors are explained below:
Anthocyanins are water soluble pigments responsible for the attractive red, purple and blue colors of many flowers, fruits and vegetables. They are sensitive to pH change, being reddest in strongly acidic conditions and become bluer as the pH rises. It is used in drinks, jams and sugar confectionery.
The color of beetroot is water soluble and has limited stability when exposed to light, heat and oxygen. It is particularly suited to frozen, dried and short shelf-life products such as ice creams and yoghurt.
The water soluble red colored pigment carminic acid (carmine) is derived from the female cochineal insect. Uses include alcoholic beverages and processed meat products.
Chlorophyll is the most widely distributed natural plant pigment, present in all green leafy vegetables. It is a green, oil soluble color. Chlorophyllins are water soluble and relatively stable when exposed to heat and light. Uses include sugar confectionery and dairy products.
Over 400 different carotenoids have been identified in red/ orange/yellow fruits, vegetables and plants. Nature produces carotenoids at the rate of 1000 million tons per year. Most are oil soluble, heat stable and are not affected by pH change. The uses of carotenoids include margarine, dairy products and soft drinks.
Turmeric is a well-known spice, used widely in cookery. Its pigment, curcumin, is oil soluble and tends to fade in light, but has good heat stability. It gives a lemon yellow shade in food systems. Its applications include curry, soups and confectionery.
Riboflavin, Vitamin B2, is used for fortification and coloring. It is water soluble, heat stable and is used in dairy products, cereals and dessert mixes.
Vegetable carbon black is a heat and light insoluble pigment, used primarily in sugar confectionery. Metals such as gold, silver and aluminum are used for surface coloring, mainly in confectionary. There are 26 colors permitted to be used in food and 28 to be used in cosmetics and pharmaceuticals. A few commonly used natural colors are annatto (seed), turmeric, beet juice (root), red cabbage (vegetable) and spinach (leaf).
Color additives are available for use in food as either “dyes” or “lakes”. Dyes dissolve in water, but are not soluble in oil. Dyes are manufactured as powders, granules, liquids or other special purpose forms. They can be used in beverages, dry mixes, baked goods, confections, dairy products, pet foods and a variety of other products. Dyes also have side effects which lakes do not, including the fact that large amounts of dyes ingested can color stools. Lakes are the combination of dyes and insoluble material.
Lakes are not oil soluble, but are oil dispersible. Lakes are more stable than dyes and are ideal for coloring products containing fats and oils or items lacking sufficient moisture to dissolve dyes. Typical uses include coated tablets, cake and doughnut mixes, hard candies and chewing gums, lipsticks, soaps, shampoos, talc, etc.
When proteins or amino acids are heated in the presence of carbohydrates, the process is called a ‘browning reaction’. This reaction may yield dark brown color to the food item after its treatment, hence named as browning reaction. It occurs in a variety of common foods, including toasted bread, dried fruits, gravy mixes, beef stew, franks with beans, etc. These reactions are responsible for the characteristic flavors, aromas, and brown color of many cooked foods.
Classification of Browning Reactions:
Based on the involvement of enzymes, browning reactions are commonly classified into two:
1. Enzymatic browning
2. Non-enzymatic browning –
(a) Maillard reaction (Maillard browning).
1. Maillard Browning Reaction:
The Maillard reaction is the reaction that is responsible for the brown color of baked products. A free carbonyl group of a reducing sugar reacts with a free amino group on a protein by heating and the result is a brown color. It is vitally important in the preparation or presentation of many types of food and it is a non-enzymatic browning.
The reaction is named after the French Chemist Louis-Camille Maillard who discovered it in the 1910s while attempting to reproduce biological protein synthesis. The reactive carbonyl group of the sugar reacts with the nucleophilic amino group of the amino acid, and forms variety molecules responsible for a range of odors and flavors.
Maillard reactions are important in baking, frying or other heating techniques of all foods. Maillard reactions are responsible for the flavor of bread, cookies, cakes, meat, beer, chocolate, popcorn and cooked rice. In many cases, such as in coffee, the flavor is a combination of Maillard reactions and caramelization. However, caramelization only takes place above 120-150°C, whereas Maillard reactions already occur at room temperature.
Mechanism of Maillard Browning:
The Maillard reaction required an initiation, a reducing sugar (such as glucose or lactose) and an amino compound. The initial step of the Maillard reaction between glucose and an amino acid (RNH2), in which R is the amino acid side group. When the food is heated sugar and amino group will react with each other in a multistage process to form an amadori compound.
In the next stage the amino group from the amadori compound is removed, which results in reactive compounds that are finally degraded to the important flavor components furfural and hydroxymethyl furfural (HMF). The other reaction is the so- called amadori-rearrangement, which is the starting point of the main browning reactions.
Furfural and hydroxymethylfurfural are characteristic flavor compounds of the Maillard reaction. Furfural is the result of a reaction with a pentose sugar (such as ribose); HMF is the result of a reaction with a hexose (glucose, saccharose).
Factors Influencing Maillard Reaction:
The most important factors which influence Maillard reactions are:
(i) Types of amino acid or sugar
(ii) Concentration of protein and sugar content in food
(iii) High temperature
(iv) High pH
(v) Time interval
(vi) Water activity (aw) – low water content, and
(vii) Presence of oxygen.
Detrimental Effect of Maillard Reaction:
Maillard browning is responsible for the discoloration of food products such as powdered milk and powdered egg. The reaction causes loss of essential amino acids such as lysine, tryptophan, histidine, etc., as these are the amino acid with free amino groups that are able to react with reducing sugar. Because of Maillard reaction large number of food items loses its nutritional quality.
Control of Maillard Reaction:
Because of the above potential problems food manufacturer will attempt to control the Maillard reaction in food by different methods.
The important methods are:
(i) Reduce the initial concentration of sugar and protein content in food.
(ii) Control of pH – maintain lower pH.
(iii) Addition of food additives such as SO2 and sodium metabisulphate as these inhibits Maillard reaction.
Caramelization is one of the most important types of browning processes in foods; it gives desirable color and flavor in bakery goods, coffee, beverages, beer and peanuts. Caramelization causes important changes in foods, not only in color but also in flavor. As no enzymes are involved in the caramelization process, it is a non-enzymatic browning reaction.
Caramelization occurs during dry heating and roasting of foods with a high concentration of carbohydrates (sugars). Caramelization is the oxidation of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color. Due to this process large numbers of volatile chemicals are released, producing the characteristic caramel flavor.
The process of caramelization starts with the melting of the sugar at high temperatures, followed by foaming (boiling). At this stage saccharose (sugar) decomposes into glucose and fructose. This is followed by a condensation step, in which the individual sugars lose water and react with each other to form difructose- anhydride. The next step is the isomerization of aldoses to ketoses and further dehydration reactions. The last series of reactions include both fragmentation reactions (flavor production) and polymerization reactions (color production).
Mechanism of Caramelization:
Caramelization is a complex process that produces hundreds of chemical products and includes the following steps:
(i) Equilibration of anomeric and ring forms
(ii) Sucrose inversion to fructose and glucose
(iii) Condensation reactions – loss of water
(iv) Intramolecular bonding – Formation of di-fructose anhydride.
(v) Isomerization of aldoses to ketoses
(vi) Dehydration reactions
(vii) Fragmentation reactions (Flavor production), and
(viii) Unsaturated polymer formation (Color formation).
During caramelization several flavor components as well as polymeric caramels are produced. Caramels are complex mixture of various high molecular weight components.
They can be classified into three groups:
1. Caramelans (C24H36O18).
2. Caramelens (C36H50O25).
3. Caramelins (C125H188O80).
These polymers are often used as colors in commercial food products, from colas to soya sauce, confectionary and ice-cream.
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 (pears, bananas, and grapes), vegetables (potatoes, mushrooms, lettuce) and also in seafood (shrimps, spiny lobsters and crabs).
Enzymatic browning is detrimental to quality, particularly in post-harvest 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. But enzymatic browning is essential for the color and taste of tea, coffee and chocolate.
The important components responsible for enzymatic browning are:
(ii) Polyphenoloxidase (PPO).
Polyphenols are main components in enzymatic browning polyphenols, also called phenolic compounds, and 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 color of many plants, such as apples, they are part of the taste and flavor of beverages (apple juice, tea), and are important anti-oxidants in plants.
Polyphenols are normally complex organic substances, which contain more than one phenol group (carbolic acid). Polyphenols can be divided into many different sub categories, such as anthocyans (colors in fruits), flavonoids (catechins, tannins in tea and wine) and non- flavonoids components (gallic acid in tea leaves). Flavonoids are formed in plants from the aromatic amino acids phenylalanine and tyrosine.
Polyphenoloxidases are a class of enzymes that were first discovered in mushrooms and are widely distributed in nature.
They appear to reside in the plastids and chloroplasts of plants, although freely existing in the cytoplasm of senescing or ripening plants. Polyphenoloxidase is thought to play an important role in the resistance of plants to microbial and viral infections and to adverse climatic conditions. Polyphenoloxidase also occurs in animals and is thought to increase disease resistance in insects and crustaceans.
In the presence of oxygen from air, the enzyme catalyzes the first steps in the biochemical conversion of phenolics to produce quinones, which undergo further polymerization to yield dark, insoluble polymers referred to as melanins. Polyphenoloxidase catalyses two basic reactions – hydroxylation and oxidation. Both reactions utilize molecular oxygen (air) as a co-substrate. The reaction is not only dependent on the presence of air, but also on the pH (acidity). The reaction does not occur at acid (pH <5) or alkaline (pH >8) conditions.
7. Antinutritional Factors in Food:
Antinutritional factor is a substance which is sometimes present in human or animal foods and reduces growth. Examples are – phytate, protease inhibitors (notably soyabean trypsin inhibitor) and excessive dietary fiber. Antinutrients are natural or synthetic compounds that interfere with the absorption of nutrients. Nutrition studies focus on those antinutrients commonly found in food sources and beverages.
One common example is phytic acid, which forms insoluble complexes with calcium, zinc, iron and copper. Proteins can also be antinutrients, such as the trypsin inhibitors and lectins found in legumes. These enzyme inhibitors interfere with digestion. Other important types of antinutrients are the flavonoids, which are a group of polyphenolic compounds that include tannins.
These compounds chelate metals such as iron and zinc and reduce the absorption of these nutrients, but they also inhibit digestive enzymes and may also precipitate proteins. However, polyphenols such as tannins have anticancer properties, so foods such as green tea that contain large amounts of these compounds might be good for the health of some people despite their antinutrient properties.
Antinutrients are found at some level in almost all foods for a variety of reasons. However, their levels are reduced in modern crops, probably as an outcome of the process of domestication. The possibility now exists to eliminate antinutrients entirely using genetic engineering, but, since these compounds may also have beneficial effects (such polyphenols reduce the risk of cancer, heart disease or diabetes), such genetic modifications could make the foods more nutritious but not improve people’s health.
Many traditional methods of food preparation such as fermentation, cooking, and malting increase the nutritive quality of plant foods through reducing certain antinutrients such as phytic acid, polyphenols, and oxalic acid. Such processing methods are widely-used in societies where cereals and legumes form a major part of the diet. An important example of such processing is the fermentation of cassava to produce cassava flour – this fermentation reduces the levels of both toxins and antinutrients in the tuber.
8. Chemical Changes during the Processing of Volatile Compounds in Food:
During deep fat frying the deteriorative chemical process of hydrolysis, oxidation and polymerization occur. The oil break down to form volatile products and nonvolatile monomeric and polymeric compounds. A small amount of oxidation is necessary to form volatile compounds such as 2, 4- decadienal that are responsible for deep fried flavor. The amount of the compounds that are formed and their chemical structures depend on so many factors such as oil and food types, frying conditions, polymerization, hydrolysis, oxidation, etc.
The individual process of hydrolysis, oxidation and polymerization are briefly outlined below:
As food is placed in oil at frying temperature, air and water initiate a series of interrelated reactions. Water and steam hydrolyze triglycerides, which produce mono and diglycerides, then free fatty acids and glycerol. Glycerol partially evaporates because it volatilizes above 150°C and reaction equilibrium is shifted in flavor of other hydrolysis products.
The extent of hydrolysis depends on factors such as oil temperature, interface area between the oil and the aqueous phase, and amount of water and steam because water hydrolyzes oil more quickly than steam. Free fatty acids and low molecular weight acidic products arising from fat oxidation enhance the hydrolysis in the presence of steam during frying.
Oxygen activates a series of reaction involving formation of many compounds indicating free radicals, hydroperoxides, aldehydes, ketones and conjugated acids. The chemical reaction that occurs during the oxidation process contributes to the formation of both volatile and nonvolatile decomposition products For example – ethyl linoleate oxidation leads to the formation of conjugated hydroperoxides that can form long chain products or they can cyclize and form peroxide polymers.
Secondary oxidation products such as aldehydes, that are volatile, significantly contribute to the odour of the oil and flavor of the fried food. If the secondary oxidation products are unsaturated aldehydes such as 2, 4- decadienal, 2, 4- nonadienal, 2, 4-octadienal, 2- heptanal, 2- octanal, etc. contribute to the characteristic deep fried flavor in oils.
Saturated and unsaturated aldehyde such as hexanal, heptanal, octanal, nonanal, and 2- decanal contribute undesirable off-odour and off-flavors. Oils that contain linoleic acid and linoleic acids such as soya, sunflower and canola have significant off flavors formation under frying conditions.
During frying oils with polyunsaturated fatty acids, linoleic acid, have a distinct induction period of hydroperoxides followed by a rapid increase in peroxide values, then a rapid destruction of peroxides. Measurement of PUSFA can help determine extent of thermal oxidation. Volatile degradation products are usually saturated and monosaturated hydroxyl, aldehyde, keto and dicarboxylic acids, hydrocarbons, alcohols, aldehydes, ketones and other aromatic compounds.
Polymerization of frying results in the formation of compounds with high molecular weight and polarity. Polymers can form free radicals or triglycerides by a particular reaction called Diels-Alder reactions. Cyclic fatty acids can form within one fatty acid; dimeric fatty acid can form between two fatty acids or between triglycerides and polymers with high molecular weight are obtained as these molecules continue to cross link. As polymerized products increases in the frying oil, viscosity of the oil also increases and the color of the oil darkness.