Introduction
Starch is a form of carbohydrate that is present in plant tissues, particularly cereals, tubers, and legumes (Vamadevan & Bertoft 2015). The biological role of starch is to act as a food reserve in plants in the form of crystalline granules. Starch is a polysaccharide, which is a sugar made of repeating units of disaccharides. The building blocks of starch are D-glucose molecules. Starch comprises a blend of two polymers, which have the glucose monosaccharides arranged in different configurations. The first polymer is amylose, a straight-chain compound that consists of approximately 500 glucose molecules joined together by alpha 1,4 glycosidic linkages (Sun et al. 2015). The other polysaccharide is amylopectin, which is a branched sugar polymer made up of repeating units of glucose joined together by alpha 1,4 and alpha 1,6 glycosidic bonds.
The branching occurs after every eight to ten glucose units and consists of alpha 1,6 linkages at the branching points. These two compounds have different physical properties, which affect the overall characteristics of starch. For example, amylose has a low water holding capacity as opposed to amylopectin. Another notable difference is the digestibility of the two polymers. Amylose is partially digestible while amylopectin is entirely digestible. Even though native starch has a relatively simple chemical composition, the structure, and functionality of starch exhibit extensive variations between as well as within plant species. It is even possible to have such variations in starch obtained from the same plant (Wang & Copeland 2015). This variability is apparent in the morphology of starch granules regarding size and shape, the amount of amylose, the structure of amylopectin concerning the length of chains, and the position of branching. Another notable difference is the arrangement of amylose and amylopectin into crystalline and shapeless regions within starch granules.
The addition of water to starch leads to the formation of a suspension that is referred to as sol. During cooking, starch undergoes various processes one of which is gelatinization. Gelatinization occurs when starch granules are heated to a liquid in moist surroundings. This process destabilizes the bonds holding the starch together, which leads to swelling. Other changes that occur include an increase in volume, thickness, and translucency. In this state, the starch is referred to as a gel. Starch undergoes partial gelatinization at temperatures ranging from 72 oC to 87 oC depending on the type of starch (Wang & Copeland 2013). At temperatures of 100 oC, complete gelatinization of starch occurs. When the temperatures drop below 100 oC, gelation occurs. Cooling of gelatinized starch also causes retrogradation, which can be described as the formation of starch molecules in a different pattern from that observed before gelatinization.
The quantities of amylose determine the gelatinization of starch. The higher the levels of amylose, the greater the extent of gelatinization. This observation is attributed to the creation of a three-dimensional network between amylose molecules. This network entraps water molecules and increases the firmness of the starch mass. Amylopectin does not form a gel. The amount of gel present in starch-containing food is measured as the slowly available glucose (SAG) value (Vamadevan & Bertoft 2015). The gel strength is proportional to the SAG value, which is an indicator of the extent of starch gelatinization. Another useful parameter is the (rapidly available glucose) RAG value of starch. Studies show that RAG and SAG values point toward the speed of glucose liberation from food and absorption by the small intestines, which influence the glycemic indices of food (Waterschoot et al. 2015). Slow-release carbohydrates elicit metabolic responses that favor optimal health.
To produce cooked starch with good consistency, one should mix starch in cold liquids before heating to prevent the formation of lumps. There should be continuous stirring during heating for similar reasons. Without stirring, starch is likely to settle at the bottom of the pan and form lumps. However, the stirring process can stop once the starch starts boiling.
Well-cooked starch should be clear, have fully swollen granules, and be highly viscous (Waterschoot et al. 2015). The original form of starch as obtained from various plant sources has several shortcomings that affect the outcome following cooking. Native starch disintegrates rapidly when cooked in an acidic environment, shearing forces, and heat. It also has poor shelf stability. To avoid such problems, starch may be altered by chemical or physical modification. Chemical modification means include cross-linking, stabilization, and conversion. On the other hand, physical modification strategies include pre-gelatinization or heat treatment. Additionally, the handling of starch influences the quality of the resultant product. For example, blending and stirring introduce shearing forces that contribute to the gelatinization (Wang et al. 2015)
Acid hydrolysis leads to substantial structural and functional features of starch without interfering with its granular properties. Acid-hydrolyzed starch has extensive applications in the food and non-food industries. Examples of uses of this form of starch in the food industry include as a coagulating agent in the production of gum sweeties and cheese loaves, fat substitutes in low-fat versions of margarine, mayonnaise, ice-cream, and milk products (Wang & Copeland 2015). On the other hand, non-food industry applications of acid-hydrolyzed starches include the manufacture of cationic and amphoteric starches, strengthening of threads during weaving in the textile industry, and as glue to adhere plaster and paper in the production of gypsum boards in the construction industry among other uses (Wang & Copeland 2015). Therefore, it is crucial to have an in-depth understanding of the impact of acids on the structure and function of starch to improve the industrial applications of starch. The purpose of this experiment is to determine the effect of acids on starch gels.
Results
Overall, the addition of vinegar increased the SAG values of the flour mixture. It was observed that the percentage of SAG increased with an increase in the amount of vinegar that was added during blending. On the other hand, the SAG value was inversely proportional to the amount of vinegar added after blending and stirring.
Table 1: Effect of Acid on Starch Gels.
Discussion
It was observed that the addition of vinegar increased the SAG values of the flour mixtures when compared to the control. This observation was in line with those noted by Waterschoot et al. (2015) that the addition of small quantities of acids prior to gelatinization increases the viscosity of cornstarch pastes. The addition of an acid to starch causes it to undergo hydrolysis leading to the formation of simple sugars. The acid donates protons to the glycosidic oxygen and makes it positively charged. Consequently, the positively charged oxygen attracts electrons from the carbon atom, which neutralizes the oxygen’s positive charge leading to the formation of a carbocation. A water molecule attacks the carbocation resulting in the transfer of the positive charge from carbon to the oxygen present in water. Subsequently, the oxygen in the water attracts electrons from one of the hydrogen atoms, which liberates a proton thereby regenerating the acid catalyst. This reaction mechanism is known as the nucleophilic substitution (SN1) reaction. Consequently, the higher the concentration of acid, the higher the rate of the SN1 reaction, which corresponds to the extent of gelatinization.
It was noted that the percentage of SAG increased with an increase in the amount of vinegar that was added during blending. The mechanical action of stirring also contributes to the breakdown of starch granules. The orientation of amylopectin molecules within the starch granule is usually radial with the non-reducing ends of the chains facing toward the exterior surface (Fu et al. 2015). Studies show that starches containing high quantities of amylose are less susceptible to acid hydrolysis while starches containing low quantities of amylose are easily hydrolyzed by acids. The reduced sensitivity of high-amylose starches to acid hydrolysis is because of an extensive network between chains, which forms a dense arrangement in the amorphous region of starch (Hernández-Jaimes et al. 2013). Additionally, it is suggested that constrained swelling of starch with high levels of amylose slows down the permeation of hydrogen ions from the acid into the granules. Conversely, slackly arranged double helices within starch crystals are responsible for the high susceptibility of low-amylose starch to acid hydrolysis (Utrilla-Coello et al. 2014).
It was also observed that the SAG value was inversely proportional to the amount of vinegar added after blending and stirring. This observation was attributed to the pH of the mixture. Adding small quantities (one tablespoon) of acid after gelatinization induced by blending and stirring increased the SAG to 100% as indicated in Table 1. However, when three tablespoons of the same acid were added to the mixture following stirring and blending, there was a decline in the SAG from 100% to 11.4%. The stirring and blending introduced shearing forces to the starch (Wang et al. 2015). The initial increase in SAG is attributed to the leakage of glucose chains from starch granules, which prompted the enmeshing of the glucose chains. However, adding large quantities of acid leads to an additional reduction in the pH, which causes the starch granules to collapse hence the reduction in viscosity.
The content of amylose content is reported to reduce substantially in the initial phases of acid hydrolysis. This decrease is because of the preferential hydrolysis of the formless portions of starch granules, which supports the supposition that large quantities of amylose are located in the amorphous sections of starch granules (Wang & Copeland 2013). Previous studies have investigated other changes in the structure of starch granules during acid hydrolysis by nuclear magnetic resonance spectra (Wang & Copeland 2013). The impact of acid hydrolysis on the morphology of the granules varies based on the source of starch and the extent of acid hydrolysis. In this experiment, the source of starch was the same in all the setups.
A small extent of acid hydrolysis does not lead to substantial changes in the morphology of starch granules. The outer surface of the granules attains a roughened appearance even though the granules stay undamaged (Wang et al., 2012). However, widespread pitting on the internal surface occurs. This pitting represents the formation of apertures. Extensive hydrolysis destroys the inner parts of the starch granules leading to the formation of noticeable marginal interchanging growth rings and filamentous sections. Further hydrolysis breaks down the starch granules to form platelet nanocrystals (Sun et al. 2015).
Acid hydrolysis has a significant effect on the swelling capacity of starch granules. However, this effect is not harmonious in different sources of starch. For example, the swelling capacity of waxy maize and rice, sorghum, peas, and arrowroot is lost completely following 24 hours of hydrolysis. The undamaged structure of amylopectin is thought to influence the swelling of starch granules and water-retention capabilities. The disruption of the amylopectin structure hinders the formation of a complete network because the broken chains have a high likelihood of dissolving. As a result, they fail to trap water. This observation suggests that acid-hydrolyzed starch granules are delicate and disintegrate instead of enlarging when heated in the presence of water.
The temperature-related properties and gelatinization traits of starch are influenced by acid hydrolysis to a great extent. The changes in these features are a reflection of a rise in the gelatinization temperature because of the enhanced molecular organization in acid hydrolyzed starch (Utrilla-Coello et al. 2014). Additionally, the preferential hydrolysis of unstructured sections of the starch granules lessens the weakening effect of swelling in these regions. Another supposition is that longer amylopectin chains form following the elimination of branch points.
Food processing controls the physical and certain biological properties of starch. The consequences of changing the rate and extent of starch digestion are useful for food and industrial applications. In conclusion, the hydrolysis of starch by acids alters the granular organization of starch, which leads to the exhibition of different behaviors when heated in water. The resultant pastes have reduced viscosity, enhanced gel strength, and higher solubility in water.
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