Dairy and Food Engineering

Lesson 17. Composition of food products

17.1 Carbohydrates

17.1.1 Fiber

Carbohydrates are the structural and storage organelles of plants. The structural carbohydrates in plants are complex compounds of repeating five carbon sugars called pentosans or repeating six carbon sugars called glucans. Hemicellulose and cellulose are non-digestible carbohydrates and together with another complex non-carbohydrate compound known as lignin which forms dietary fiber that can’t be softened appreciably by hydration or heating, thus they generally impart a rough mouth feel to food products.

 Another plant component is pectin which is soluble in hot water but non-digestible, forms a component of cell walls of plants and causes the softening of vegetables when cooked due to change in the pectin molecular structure. Commercial pectin preparations are used for manufacturing jams, jellies and preserves as they are very good gelling agents.

Some plant seeds, secretions from the bark, or the stem and leaves of aquatic plants contain soluble complex carbohydrates called gums. Gums can be dissolved in water to produce viscous solutions and they forms gels at specific concentrations, or in the presence of specific ions or sugar. Due to wide applicability of gums, they provide tools to food technologists to modify and customize the body, mouth feel and texture of the modern foods.

17.1.2 Sugars

The simple sugars are the primary products of photosynthesis as well as the finished product of the complete digestion of carbohydrates. Simple sugars are sweet soluble carbohydrates and cannot be further broken down. The end products of digestion of complex carbohydrates in the digestive tract, are quickly absorbed from the intestines into the blood circulatory system, that provide rapid energy to some individuals, but it could also be unfavourable to diabetics who are susceptible to hyper/hypoglycemic fluctuations with intakes of  simple sugars. Refined sugars are the common carbohydrate sweeteners such as cane sugar, corn syrup and high fructose corn syrup. The consumption of these sugars in excess with minimal intake of the other dietary nutrients increase the susceptibility of tooth decay by promoting the growth of certain bacteria. These health issues must be taken into consideration by a food technologist before choosing the carbohydrate for food formulations.

17.1.3 Starch

Starch, a complex molecule consisting of a long chain of repeating units of the simple sugar, glucose is a storage carbohydrate in grains and root crops. As plants also utilize starch for respiration, it is easily digestible by humans. It is also recommended to provide the majority of dietary calories. It plays a significant role in determining the texture of foods in addition to supplying calories.

Starch, in native form, is available as granules in a cellular matrix of grains and storage roots or tubers of plants. A purified starch can be easily separated from other components by macerating the root or milling the grain releases these granules. Wheat flour is a mixture of starch along with the other components of the grain, whereas corn starch is a pure starch. Within the starch granule, there are two types of polymers, amylose, a straight chain polymer and amylopectin, a branched chain polymer. These large complex molecules are tightly coiled within the granule. Heating of starch in presence of water allows the granules to absorb water, swell and release the amylase and amylopectin into solution. These hydrated polymer molecules in the solution forms the gelatinized starch, where intact starch granules are not visible. The gelatinization process is manifested by an increase in viscosity. Different starches will have different gelatinization temperature and different viscosities at different concentrations. When starch is gelatinized at an appropriate concentration and allowed to cool, it forms a gel that possesses the ability of starch to bind water and to impart firmness to food products.

Amylose and amylopectin are present in different starches in different proportions and provide different characteristics on gelatinization. Gelatinized amylose solutions are clear and form firm gels with a tendency to release free water from the gel matrix on cooling, whereas amylopectin on gelatinization gives opaque solutions that do not form firm gels on cooling. Amylose has a higher gelatinization temperature than amylopectin.

Most natural starches consist of one or more of the other polymer. For example, waxy maize starch contains practically all amylopectin, whereas regular corn starch has more amylose than amylopectin.

This process of conversion of gelatinized starch from a soluble to an insoluble form is called retrogradation. It is a reversible process that may be reversed by heating. In low moisture cooked starchy products, for example, retrogradation in bread leads to stiffening of the structure or staling, making the product hard and dry. With decreasing moisture content and reduced temperature, the rate of starch retrogradation increases. In frozen foods, water is immobilized by freezing and so there is little movement from entrapment within the starch molecular network. The retrogradation in frozen foods can be minimized if both freezing and thawing processes are carried out rapidly. When starchy foods are stored under refrigeration, they are most susceptible to retrogradation due to the time of holding at low temperature just above the freezing point is the critical period for development of retrograded starch. When starch is used in a food formulation its retrogradation can irreversibly alter the desirable product quality attributes. Amylose due to its straight molecular structure is more susceptible to retrogradation compared to amylopectin. Nowadays commercially available modified starches, which are designed to alter the gelatinization temperature, viscosity enhancing properties and retrogradation tendency of natural starches are boon for the food technologists to formulate newer products with improved functionalities.

17.2 Fats and oils

Fats or oils consist of three long chain fatty acids associated with a glycerol molecule and are also called triglycerides. They have a similar chemical structure, but different types of fatty acids in the molecule that alters its physical state at room temperature. When the fatty acids are highly saturated, each carbon atom in the molecule has its full complement of hydrogen atoms. Fats made up of saturated fatty acids are stable against reacting with oxygen and hence are less prone to oxidative rancidity. On the other hand, oils are made up of unsaturated fatty acids, i.e., they do not have their full complement of hydrogen atoms in the molecule which may easily react with oxygen to produce compounds which impart a rancid odour or flavor. Physiologically, in animals, saturated fats tend to be present in leaf fat or adipose tissue while unsaturated fats form a component of cellular membranes. The degree of saturation of such fats depends upon the source animal. Amongst all fats, red meat contains the highest level of saturated fatty acids while fish has the least amount of saturated fats followed by poultry. Plant-derived oils, such as peanut, canola, corn, soybean and cottonseed are highly unsaturated, whereas, tropical plant oils such as cocoa, coconut and palm oil contain highly saturated fats.

In the human body, saturated fats cannot be dehydrogenated into unsaturated compounds, hence the necessity for highly unsaturated fats to form cellular membranes must be met by dietary intake of such compounds. These highly unsaturated fats are known as polyunsaturated fats and considered as essential dietary nutrients. No major physiological function is performed by saturated fats except to serve as a source of dietary calories. Fats have nearly double the caloric equivalent of the same weight of carbohydrates or proteins. Thus, the products having high fat are also high in calories, which can be reduced by formulating low fat products. The melting point of fat used in a food formulation affects the texture, appearance and flavour of the product depending on how the components blend, how the product holds water and fat as it undergoes the mechanical and thermal processes. As fat is an excellent carrier for colour and flavours, the success of a food formulation mainly depends upon the choice of fat. The type of fat in the product formulation will also decide the packaging options for adequate storage to prevent development of oxidative rancidity.

17.3 Proteins

Proteins are the structural component of animal tissue. The muscles which contract and relax for body movement are composed of the proteins namely, actin and myosin. The connective tissue that separates muscle bundles is collagen which also forms the matrix so that calcium and phosphorus are deposited in bones.

Proteins consist of a long chain of amino acids or nitrogen-containing organic compounds. There are 22 known amino acids in proteins. The type and number of amino acids in a protein decides its protein type. Plant and animal proteins diverge in the distribution of different types of amino acids in the molecule. Animal proteins most closely resemble the amino acid profile of human tissue protein, so they are best for human growth and maintenance. Plant proteins, may contain a low level of some of the essential amino acids and so more of it must be consumed in the diet to fulfil the requirement for growth and maintenance of healthy tissue. Some proteins such as gelatin have no nutritive value as they lack one or more of the other essential amino acids.

Proteins get metabolized, for energy and thus contribute to dietary calories. Proteins on digestion produce free amino acids. Commercial proteolytic enzymes from microbial sources may also digest proteins in vitro to alter their solubility, gelled product textural attributes, or to impart a characteristic flavour. Proteins are generally bland in flavour. However, with the decrease in the amino acid chain length, the flavour becomes more distinct. Hydrolysed proteins with a high fraction of free amino acids have a very strong flavour and odour, while some may have even a bitter flavour. A careful formulation and processing of foods containing animal muscle and plant based protein foods is required to get a stable solid matrix with the ability of proteins to bind water and fat. Proteins and carbohydrates have the same caloric equivalent; therefore, they may be exchanged in the food formulations to get reduced cost. A number of commercial protein products and protein hydrolysates are available to food scientists for different product formulations.

17.4 Other food components

Food components though present in low concentrations, define a characteristic colour and flavour of foods.

17.4.1 Acids

After the basic chief nutrients, acids exist at the next higher level. They are intermediates in plant metabolism; and even certain plants have the capacity to accumulate specific acids at enough high levels to impart a distinctive flavour and mouth feel.

Acids impart a sour flavour and its intensity depends upon the pH and the fraction of un-dissociated acid. Generally, strong acids give a harsh mouth feel on swallowing. The different interactions of the un-dissociated acid molecules with other food components such as carbohydrates may control or enhance the sourness perception. Commonly, sugars tone down sourness, whereas mineral salts exaggerate the sensation. Gums tend to minimize the harshness of the acid, by coating the linings of the throat, but they also prolong the sourness sensation after swallowing. Lactic acid has a slightly bitter after taste, while tartaric acid leaves a slight scratchy feeling in the throat after swallowing the product. Citric acid is best used in a formulation with a small amount of sugar to give a smooth non-tenacious sourness sensation after swallowing.

Acids may be naturally present in the product or they may be an added ingredient. Citric acid is the most common plant acid, primarily available in citrus fruits viz.; lactic acid in fermented dairy products, malic acid in apples and tartaric acid in grapes. Most of these acids are produced commercially by fermentation and are available in pure form for use in food formulations.

17.4.2 Pigments

Plants synthesize pigments to produce the various colours of plant products. The most widely distributed pigment is chlorophyll, the green pigment in plants. It is water soluble and is unstable with heat, particularly at low pH. Degradation of chlorophyll leads to bleaching out of green colour and transformation into brown colour. Hence during processing of products care must be taken or else green colour may change to brown.

The next most widely distributed water-soluble pigments are the anthocyanins, whose colours ranges from pink to deep blue. It is pH dependent, being red at the low end and as pH increases, it intensifies to blue. Anthocyanins also change colour to brown when they degrade during storage or during heat treatment of the product. The degraded pigments tend to agglomerate with surrounding degradation products to precipitate slightly at the bottom of the container. The presence of oxygen along with the elevated storage temperature tends to accelerate the degradation of anthocyanins, while the presence of sugar tends to slow down its rate. Anthocyanins do not have an attractive colour at near neutral pH and so they are mostly used as a colourant in frozen sliced fruit and shelf stable juices or beverages.

The natural fat-soluble colours are available in yellow, orange and red. These compounds include carotene, which is a precursor of vitamin A and a group of similar compounds, called carotenoids. These fat soluble colours are heat stable, although during storage slow loss of colour can occur in conditions which promote oxidation. Natural plant derived water and fat soluble colour pigments are commercially available to be used as food colorants. Meat pigments are normally not intensified by addition of artificial colorants although on some processed meat products, addition of coloured spices such as paprika, turmeric and annatto enhances the colour. Myoglobin, a meat pigment varies in concentration in the meat with the age and species of the animal. Raw meats, when stored in absence of O₂ changes colour to dull red with a purplish hue and when exposed to O₂ turns deep red. Oxidized pigments turn to brown met-myoglobin, when cooked it becomes grey and addition of nitrite produces a stable pink colour, which is the characteristic colour of cured meats.

17.4.3 Micronutrients

The micronutrients needed for meeting dietary requirements are normally not taken into consideration in food product development, except when formulating analogues using ingredients that do not contain the micronutrients. Flavour compounds when volatile, are responsible for the aroma of food and when non-volatile they contribute to the taste sensation. Non-volatile compounds are high molecular weight alcohols (e.g. tannins). Whereas volatile compounds have low molecular weight and may be lost during processing. A group of compounds called carbonyls is an important component which participates in flavour development during heating which usually result in roasted flavours. The flavour of, roasted nuts, roasted beef and coffee and cocoa may be attributed to the reactions involving these carbonyls. Although these compounds may be naturally present, formulations may be developed where these compounds are added to intensify the desired flavour or colour effect.