Depending on the sources, food starches are classified into three: 1. Natural Starches – Amylose and Amylopectins 2. Modified Starches 3. Waxy Starches.
1. Natural Starches – Amylose and Amylopectins:
These are naturally present starches in various types of plant foods. The starch granules from various grains differ in size, ranging from 2 to 150 microns. Their shape may be round or polygonal in corn, wheat, waxy maize, etc. Starch is made up of two molecules, amylose and amylopectin, whose parts are connected by glycosidic linkages.
Amylose molecules typically make up approximately one-quarter of starch. Amylose is a long linear chain composed of thousands of glucose units with attachment of the carbon 1 and carbon 4 of glucose units, and therefore contains α -1, 4 glycosidic linkages. It forms a three-dimensional network when molecules associate upon cooling, and is responsible for the gelation of cooked, cooled starch pastes. The amylose content of various starch sources includes – cereal grains – 26-28% and roots and tubers – 17-23%.
Amylose chain Amylopectin molecules constitute approximately three quarters of the polymers in a starch granule. The glucose chain of amylopectin contains α -1, 4 glycosidic linkages, similar to amylose, also with α -1, 6 branching at every 15-30 glucose units of the chain. There is a linkage between the carbon 1 of the glucose and carbon 6 of the branch.
The chains are highly branched and bushy. The branches make amylopectin less soluble in water than amylose. Starches with a high percentage of amylopectin will thicken a mixture but do not form a gel. The greater the amylopectin, the more viscous the starch paste and the greater the amount of amylose the stronger the gel.
2. Modified Starches:
Natural starches may be modified chemically to produce physical changes that contribute to shelf stability, appearance, convenience, and performance in food preparation.
The important types of modified starches used in food biotechnology are:
(i) Pregelatinised starches
(ii) Cold water-swelling (CWS) starches
(iii) Cross-linked starches
(iv) Stabilized (substituted) starches, and
(v) Acid modified starches.
(i) Pregelatinized Starches:
Pregelatinized starch is an instant starch that has been gelatinized and then dried. It subsequently swells in liquid without the application of heat. Pregelatinised starch appears in many foods, including instant pudding mixes. The main properties of pregelatinized starches are it will dispersible in cold water, it can be cooked and dried and it undergoes irreversible change and cannot return to its original ungelatinized condition.
(ii) Cold Water-Swelling (CWS) Starches:
Cold water-swelling (CWS) starch is an instant starch that remains as an intact granule. It offers convenience, stability, clarity, and texture. Cold water-swelling starches maybe gelling or non-gelling. They may be used in cold-process salad dressings, providing the thick, creamy mouth feel in no-fat salad dressings.
Cross-linked starches are those that undergo a molecular reaction at selected hydroxyl (-OH) groups of two adjoining, intact, starch molecules. The purpose of cross-linking is to enable the starch to withstand such conditions as low pH, high shear, or high temperatures.
The cross-linked starch becomes less fragile and more resistant to rupture than the original unmodified starch. These starches are used in many foods, especially acid food products such as pizza sauce or barbecue sauce because the modified starch is more acid resistant than an unmodified starch.
Stabilized (substituted) starches are used in frozen foods and other foods stored at cold temperature in order to prevent gelling. The stabilized starch produces pastes able to withstand several times in freeze condition and also foods such as sauces and gravies stored at cold temperatures.
Acid modified starch is starch that is subject to treatment in acid slurry by heat treatment. Once the starch is mixed into a food product and farms a strong gel upon cooling.
3. Waxy Starches:
Waxy starches are derived from some natural strains of barley, corn, rice, and sorghum. They do not contain amylose, become less thick and do not produce gels, e.g., waxy cornstarch, which does not have the same gel forming properties as ordinary cornstarch. It contains no gel producing amylose, but consists of amylopectin.
Pectins, gums and mucilages are the important soluble fiber like polysaccharides in foods. They are widely used as gelling agents, thickeners, and stabilizers. They are constituents of plant tissue and are large, complex molecules whose exact nature is not certain. Because of their functional properties it is helpful to produce convenience and special texture foods.
Pectic substances including protopectin, pectinic acid, and pectic acid are an important constituent of plant tissue and are found mainly in the primary cell wall. They also occur between cell walls, where they act as intercellular cement. Pectic substances may be grouped into one of the three categories depending on the number of methyl ester groups attached to the polymer.
They are listed follows:
It is found in immature fruits and is a high-molecular-weight methylated galacturonic acid polymer. It is insoluble in water but can be converted to water dispersible pectin by heating in boiling water. It cannot form gels.
It is a methylated form of galacturonic acid that is formed by enzymatic hydrolysis of protopectin as a fruit ripens. The high- molecular-weight pectinic acids are known as pectins. Pectinic acids are dispersible in water and can form gels.
It is a shorter-chain derivative of pectinic acid that is formed as fruit over ripens.
Pectins are high-molecular-weight pectinic acids and are dispersible in water. It is a linear polymers of D-galacturonic acid joined by α -1, 4-glycosidic linkages. Some of the carboxyl groups along the galacturonic acid chain are esterified with methanol. The degree of esterification in unmodified pectins ranges from about 60% in apple pulp to about 10% in strawberries. According to the degree of esterification, pectins are classified as high methoxyl or low methoxyl pectins.
Low-methoxyl pectins contain mostly free carboxyl groups and only 20-40% of the carboxyl groups are esterified. So, most of them are cross-links with divalent ions such as calcium and form gels
High-methoxyl pectins contain a high proportion (usually 50- 58%) of esterified carboxyl groups. It will not produce cross-links with divalent ions, so these pectins do not form gels. But they can be made to gel with the addition of sugar and acid. It is the high- methoxyl pectins that are commonly used to form pectin jellies.
Pectins are commonly seen in jams, jellies, apple, citrus fruits and some desserts. Pectins with a high-molecular weight and a high proportion of methyl ester groups have the best jelly-forming ability. The pectin content of fruits is variable and depends not only on the type of fruit but also on its maturity or ripeness. If jellies or jams are made at home, it is best to add commercial pectin to ensure that there is sufficient pectin to form a gel. Purified pectin is made from apple cores and skins (apple pomace) and from the white inner skin of citrus fruits.
A pectin gel consists of water held in a three-dimensional network of pectin molecules. Pectin is dispersible in water and forms a sol (solid dispersed in liquid continuous phase), but under the appropriate conditions, it can be converted into a gel (liquid dispersed in solid continuous phase). It is not easy to form pectin gels; it requires a delicate balance of pectin, water, sugar, and acid.
Pectin is hydrophilic (water loving) due to the large number of polar hydroxyl groups and charged carboxyl groups on the molecule. When pectin is dispersed in water, some of the acid groups ionize and water binds to both the charged and polar groups on the molecules. The negative charge on the pectin molecules, coupled with their attraction for water, keeps them apart so that they form a stable sol.