Starch is the major storage carbohydrate in plants. It’s also perhaps the most important agricultural commodity in food and is a basic source of energy for most of the world’s population. Without it much of the global populace would suffer severe energy shortages as well as a notable loss in food and paper. Gelatinizing starch is one way many of us are able to make this important carbohydrate palatable. The mechanisms of starch gelatinization are slowly being revealed but there is still plenty of research ongoing into how this process occurs and how it can be managed in a food processing context. High pressure processing for example produced some interesting effects which reveal new insights into this molecule.
Starch is classified into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS), according to the rate of glucose release and its absorption in the gastrointestinal tract.
Analytical Methods Of Examining Starch Gelatinization
- Viscometry including use of the Brabender Viscoamylograph.
- Optical microscopy
- Electron microscopy
- Differential scanning calorimetry (DSC)
- X-ray diffraction
- Nuclear magnetic resonance (NMR) spectroscopy
- Fourier Transform infrared (FTIR) spectroscopy
- Simultaneous X-ray scattering.
- Enzyme assays for digestability and hydrolysis using alpha-amylase
The Brabender Viscoamylograph is often used to assess the swelling capacity, pasting temperature, measures of shear and thermal stability and to some extent retrogradation. The specific measures are the starting temperature for gelatinization, its maximum level, actual gelatinization temperature, viscosity during holding, and at the end of cooling. The technique for measurement involves heating the starch in distilled water at a constant heating rate in a rotating bowl. The mixture is cooled. A sensor measures the change in viscosity as torque over time against temperature.
Optical microscopy with staining is used to observe the degree of swelling of a starch granule and how long this process actually takes. It’s also used to check the integrity and physical size of the granules before and after swelling.
The thermal analytical techniques are used to examine heat flow changes which occur in both first- and second-order transitions in starch polymers. A technique like DSC for example is ideal for delving into gelatinization. X-ray diffraction reveals changes in crystallinity and helps characterise the changes in crystal structure.
Both FTIR and NMR offer insights into the molecular structure of starch during gelatinization. FTIR is often employed to detect the absorption of different bond vibrations in starch molecules. It can also help in observations involved in other forms of structural changes such as the alteration in crystallinity versus amorphous structural formation. NMR is used to measure loss of detectable structural change in granules especially with respect to the ordering of water as gelatinization progresses. The extent and commencement of gelatinization depends on the type of starch especially in the ratio of amylose to amylopectin present. It also depends on the availability and amount of water associated with the starch. The swelling of starch granules during any heating processes causes cell disruption. This swelling along with starch gelatinization produces texture softening. The consequence is improved palatability as the food is softer to eat (Pithers, 2003).
In canning, the amylopectin and amylose behave differently when they are cooked. Amylose produces an opaque solution that sets into a firm gel when it cools following heating. The amylopectin portion forms a translucent paste that remains fluid when it is cooled – both very different behaviours.
Water Absorption Index
The water absorption index (WAI) is usually based on a method of Anderson and Griffith (1969). The WAI measures the volume occupied by a starch granule or the polymer after swelling in an excess amount of water. Usually the starch granules are suspended in distilled water at room temperature for 30 minutes. These are gently stirred to avoid breaking the granules and then centrifuged at 3000 rpm for 15 minutes. The supernatant is poured into a tared evaporating dish. The remaining gel is weighed and the WAI calculated as grams of gel obtained per gram of solid.
WAI = (weight of supernatant)/Weight of dry solid.
Water Solubility Index
The water solubility index (WSI) is the amount of polysaccharides or polysaccharide released from the granule on the addition of excess of water. The WSI is the weight of dry solids in the supernatant from the water absorption index test expressed as a percentage of the original weight of the sample.
WSI (%) = (weight of dissolved solids in supernatant)*100/(weight of dry solids)
Gelatinization starts at various temperatures depending on the type of starch, especially its ratio of amylose to amylopectin. The gelatinization temperature (Tp) is a measure of the perfection of starch crystallites or rather how well the starch is structured and ordered.
The enthalpy of gelatinization which is measured using DSC is a measure of the degree of crystallinity (Tester and Morrison, 1990). In other circles (Cooke and Gidley, 1992), the enthalpy measure reflects loss of molecular or double helical order rather than loss of crystalline register. The latter aspect is monitored using X-ray crystallography.
The temperature at which initiation begins is the T-onset. The T-peak occurs where the endothermic reaction occurs at the maximum point of gelatinization. The T-conclusion occurs when all starch granules are fully gelatinized and the viscosity curve remains constant.
Pasting is a good measure of how gelatinization is influenced by starch behaviour. It is measured by preparing a dispersion of the starch in water – about 10% w/v. The starch dispersion is heated from 25 to 95ºC at a specified heating rate. A typical rate is set at 7ºC/min, held for 5 minutes at the highest temperature and then cooled to somewhere around 50ºC, all at the same rate.
Mechanism Of Starch Gelatinization
Starch gelatinization is a complex process as we outlined briefly in the introduction. Most review articles describe three main processes happening to starch granule. It involves the breaking down of intermolecular bonds between starch molecules when both water and heat is present. It is thus a process that is dependent on the actions of heat and moisture on hydrogen bonding within the packed amylose and amylopectin chains of this granule.
Gelatinization involves the uncoiling of external chains of amylopectin that are packed together as double helices in a cluster. These double helices are stabilised by hydrogen bonding. Such clusters incidentally create crystalline zones in the native starch.
When water is added to powder starch, there is swelling of the granule. The hydrogen bonding in the less ordered amorphous regions of the granule is disrupted. The hydrogen bonds in the starch are normally between a hydroxyl hydrogen and an oxygen and when these are broken internally, the hydrogen bonding sites are replaced by water. The starch double helices break down as the water molecules begin to associate with the exposed hydroxyl groups on the starch molecules. In this case water has behaved like a plasticizer.
When the hydrogen bonds are broken during the uncoiling process, the starch can begin to gelatinise. The next phase is the crystal structure disappearing, followed by amylose leaching.
The process of gelatinization is one of the key features in the extrusion of snack foods and should be thoroughly understood if this technology is to be successful in product development. The creation of meat-free burgers and other vegan foods relies heavily on this type of technology.
Starch Gelatinization In An Extrusion Process
It has been long accepted that when it comes to starch gelatinization in an extrusion process, the mechanisms occurring may well be very different to those elsewhere. Most studies consider dilute mixtures of starch in water which is heated in a shear-absent environment. Extrusion of food will be very different.
One of the main factors to consider is the moisture content. Chaing and Johnson (1977) investigated the gelatinization of wheat flour. When the temperature was above 80ºC, starch gelatinization rose sharply. The higher the moisture content the higher degree of gelatinization. Extrusion temperature was more important however than moisture levels however. If the residence time dropped by increasing the screw speed so did the degree of gelatinization.
There are contradictory results when it comes to moisture content with other flours. Gomez and Aguilera (1983, 1984) noticed that when the moisture content dropped there was a higher degree of gelatinization in ground white corn and corn starch.
The starch gelatinization temperature changes when other ingredients are added during extrusion. Some ingredients will accelerate the disruption of hydrogen bonds which improves gelatinization whilst others disrupt this process. The addition of sugar increases the degree of gelatinization which implies a water activity effect when wheat flour is extruded.
Bhattacharya and Hanna (1987) examined the kinetics of gelatinization during the extrusion of ordinary corn (30% amylose) and waxy corn (1% amylose). In their model system the degree of gelatinization decreased with increasing moisture content. It increased however with extruder barrel temperature. In terms of reaction kinetics, gelatinization showed pseudo-zero-order with higher rate constants for waxy corn compared to high amylose corn. These rate constants decreased as the extrusion temperature dropped.
A number of great reviews examine the changes in gelatinization during extrusion. See Camire et al., 1990, Lai & Kokini (1991).
Retrogradation In Starch Gelatinization
When starch has gelatinised and then allowed to cool, it will start to undergo retrogradation. Here, the starch begins to thicken and moves from an amorphous structure to a more crystalline form. Eventually, the starch aggregates sufficiently to form a gel.
Various starch polymer aggregates form during retrogradation. The amylose molecules tend to form weak associations with each other through hydrogen bonding. This produces the gel structure. Indeed starch with a high amylose content will form a strong gel and the degree of strength depends on the amylose content. High levels of amylopectin produce weaker gels than amylose ones. We often see amylopectin loosely binding with each other as is the case with the amylose fraction. These are weak associations with water being retained and bound into the matrix.
The retrogradation of native and cross-linked starch pastes, increased significantly during refrigerated storage. Syneresis in the stored gels is due to the increased molecular association between the starch chains at reduced temperature, excluding water from the gel structure. Retrogradation properties of starch are indirectly influenced by structural arrangements of starch chains within the amorphous and crystalline regions of the ungelatinized granule, which in turn, influence the extent of granule breakdown during gelatinization and the interactions that occur between starch chains during gel storage (Kaur et al., 2006). Furthermore, the increase in syneresis during storage has been attributed to the interaction between leached out amylose and amylopectin chains.
Amylose aggregation and crystallization were completed within the first few hours of storage while amylopectin aggregation and crystallization occurred during later stages (Sodhi & Singh, 2005).
Cross-linking resulted in an ordered structure of the starch pastes thus resulting in higher degree of retrogradation. As the level of cross-linking increased, the paste became more unstable in low temperature conditions. In contrast, acetylation decreased syneresis due to the presence of acetyl groups on the starch molecules, that are able to increase water retention capacity of refrigerated stored gels (Sodhi & Singh, 2005).
Cross-Linking Of Starch
Cross-linking reinforces the hydrogen bonds in the granule with chemical bonds that act as a bridge between the starch molecules (Jyothi et al., 2006). It is one of the ways in which starch lends itself to modification. There are many different approaches that can be taken here.
Important factors in the cross-linking reaction include chemical composition of reagent, reagent concentration, pH, reaction time and temperature. Because the degree of cross-linking for food starch is very low, the extent of reaction and yield of cross-linked starch are difficult to measure chemically; hence there is a need for physical property measurement. When phosphorus oxy chloride (phosphoryl chloride, POCl3, MW153.3) is added to starch slurry under alkaline conditions (pH 8–12), the hydrophilic phosphorus group immediately reacts with the starch hydroxyls, forming a distarch phosphate (Hirsch & Kokini, 2002).
Cross-linking alters, not only the physical properties, but also the thermal transition characteristics of starch, although the effect of cross-linking depends on the botanical source of the starch and the cross-linking agent. Decrease in retrogradation rate and increase in gelatinization temperature have been observed with cross-linked starch, and these phenomena are related to the reduced mobility of amorphous chains in the starch granule as a result of intermolecular bridges (Singh, Kaur, & McCarthy, 2007). However, Jyothi et al. (2006) showed that cross-linked starch has more pronounced syneresis than has native starch because of ordered structure in the starch paste, thus resulting in a higher degree of retrogradation.
Acetylation of starch is an important substitution method that has been applied to starch that imparts the thickening needed in food application. Acetylated starch is a granular starch ester with the CH3CO group introduced at low temperature. Acetylated starch has improved properties over its native form and has been used for its stability and resistance to retrogradation (Singh, Chawla, & Singh, 2004). It increases viscosity, solubility, swelling factor, hardness, cohesiveness, adhesiveness and translucency of the gels while it decreases initial gelatinization temperature (González & Perez, 2002).
Cross-linking starch from oats using POCl3 at two different levels of 0.5 and 1.0 g/kg alongside two different levels of acetylation using acetic anhydride using 6 and 78 per cent w/w has been examined (Mirmoghtadaie et al., 2009). Cross-linking reduced the swelling factor and did not improve gelatinization temperature. It did however increase the level of syneresis in comparison with native starch.
Acetylation increased swelling factor but reduced gelatinization temperature and syneresis of oat starch.
A number of techniques deserve closer investigation. For example, differential scanning calorimetry (DSC), 13C-NMR spectrometry, and X-ray diffraction have been used to examine wheat, maize, potato, and tapioca starches following defined thermal pretreatments to provide samples exhibiting degrees of structure loss (Cooke and Gidley 1992). As 13C-NMR spectrometry is a short-distance range probe, detected order by this technique corresponds to double-helix content in contrast to X-ray diffraction, which detects only those double helices that are packed in regular arrays.
The data reported from DSC studies as applied to oat starch has great potential. Paton reported in 1987 on the application of DSC with oat starch. A few studies looked at the changes in thermal characteristics of starch gels as a result of storage over time. For example Nakazawa et al., (1985) looked at potato starch and its retrogradation.
Published studies on the pasting behavior of oats have been confined to few cultivars, with little consideration of the effects of environment or environment by genotype interactions. For example, the composition of oats grown in Australia differ substantially from those from the Northern Hemisphere. Characteristically, Australian oats are higher in lipid and lower in protein than those from Canada (V. D. Burrows, personal communication). It should be remembered that oats are grown in Australia under conditions of rising temperatures, increasing day-length but falling moisture, in contrast to most other oat-growing countries, where harvest is approached with falling temperatures, shortening day-length, and stable if not increasing moisture availability. The effects of these differences in agronomic practice on oat quality need to be determined if breeders and processors are to be guided in selection of progeny or parcels of grain suitable for human food use.
Åman, P. (1987). The variation in chemical composition of Swedish oats. Acta Agric. Scand. 37 pp. 347-52.
Biliaderis, C. G., Maurice, T. J., and Vose, J. R. 1980. Starch gelatinization phenomena studied by differential scanning calorimetry. J. Food Sci. 45:1669-1674. https://doi.org/10.1111/j.1365-2621.1980.tb07586.x
Biliaderis, C. G., Page, C. M., Maurice, T. J., and Juliano, B. O. 1986. Thermal characterisation of rice starches: A polymeric approach to phase transitions of granular starch. J. Agric. Food Chem. 34:6-14
Caldwell, E.F. and Pomeranz, Y. (1973) Industrial uses of oats, in Industrial Uses of Cereals, (ed. Y. Pomeranz), American Association of Cereal Chemists, St Paul, MN, pp. 393–411.
Camire, M. E., Camire, A., & Krumhar, K. (1990). Chemical and nutritional changes in foods during extrusion. Critical Reviews in Food Science & Nutrition, 29(1), pp. 35-57.
Chiang, B.Y. and Johnson, J.A. (1977). Gelatinization of starch in extruded products. Cereal Chem. 54(3): pp. 436.
Collison, R. (1968). Swelling and gelation of starch. Page 168-193 in: Starch and Its Derivatives. J. A. Radley, ed. 4th ed. Chapman and Hall: London
Doublier, J. L., Paton, D., and Llamas, G. (1987). A rheological investigation of oat starch pastes. Cereal Chemistry 64 pp. 21-26
Fast RB & Caldwell EF (2000) Breakfast Cereals and How They Are Made, 2nd ed. St Paul, MN: American Association of Cereal Chemists.
Ganßmann W., Vorwerck K. (1995) Oat milling, processing and storage. In: Welch R.W. (eds) The Oat Crop. World Crop Series. Springer, Dordrecht. Germany. DOI https://doi.org/10.1007/978-94-011-0015-1_12
Giradet, N. & Webster, F.H. (2011) Oat milling: specifications, storage, and processing. In Oats: Chemistry and Technology, 2nd ed., pp. 301–302 [FH Webster and PJ Wood, editors]. St Paul, MN: American Association of Cereal Chemists.
Gomez, M.H. and Aguilera, J.M. (1983). Changes in the starch fraction during extrusion cooling of corn. J. Food Sci. 48 pp. 378.
Gudmundsson, M., and Eliasson, A. C. 1989. Some physico-chemical properties of oat starches extracted from varieties with different oil content. Acta Agric. Scand. 39 pp.101-111.
Gudmundsson, M., and Eliasson, A. C. 1991. Thermal and viscous properties of rye starch extracted from different varieties. Cereal Chem. 68 pp. 172-177.
Hartunian-Sowa, M., and White, P. J. 1992. Characterisation of starch isolated from oat groats with different amount of lipid. Cereal Chem. 69 pp. 521-527
Lai, L. S., & Kokini, J. L. (1991). Physicochemical changes and rheological properties of starch during extrusion (a review). Biotechnology Progress, 7(3), pp. 251-266.
Lasztity, R. (1998). Oat grain-A wonderful reservoir of natural nutrients and biologically active substances. Food Rev. Int. 14, pp. 99–119
Lineback, D. R. (1984). The starch granule organization and properties. Bakers Digest, 1984.
MacMasters, M. M., Wolf, M. J., and Seckinger, H. L. 1947. The possible use of oat and other cereal grains for starch production. Am. Miller Process. 75 pp. 82-83
Miller, S.S (2011) Microstructure and chemistry of the oat kernel. In: Oats: Chemistry and Technology, 2nd ed., pp. 77–94 [FH Webster and PJ Wood, editors]. St Paul, MN: American Association of Cereal Chemists
Mirmoghtadaie, L., Kadivar, M., & Shahedi, M. (2009). Effects of cross-linking and acetylation on oat starch properties. Food Chemistry, 116(3), pp. 709-713 (Article).
Paton, D. 1986. Oat starch: Physical, chemical and structural properties. Pp. 93-120 in: Oats, Chemistry and Technology. F. H. Webster, ed. Am. Assoc. Cereal Chem.: St. Paul, MN.
Paton, D. A. V. I. D. (1987). Differential scanning calorimetry of oat starch pastes. Cereal Chem, 64(6), pp. 394-399
Pomeranz, Y. (1995). Industrial uses of oats. In: The Oat Crop (pp. 480-503). Springer, Dordrecht.
Shamekh, S., Forssell, P., and Poutanen, K. (1994). Solubility pattern and recrystallization behavior of oat starch. Starch/Staerke 46 pp. 129-133
Singh, N., Chawla, D., & Singh, J. (2004). Influence of acetic anhydride on physicochemical morphological and thermal properties of corn and potato starch. Food Chemistry, 86, pp. 601–608.
Singh, J., Kaur, L., & McCarthy, O. J. (2007). Factors influencing the physicochemical morphological thermal and rheological properties of some chemically modified starches for food application – A review. Food Hydrocolloids, 21, 1–22
Sodhi, N. S., & Singh, N. (2005). Characteristics of acetylated starches prepared using starches separated from different rice starch. Journal of Food Engineering, 70, pp. 117–127
Sowa, S. M. H., & White, P. J. (1992). Characterization of starch isolated from oat groats with different amounts of lipid. Cereal Chemistry, 69, pp. 521–527
Tester, R. F., and Karkalas, J. 1996. Swelling and gelatinization of oat starches. Cereal Chemistry 73 pp. 271-277
Tosh SM (2013) Review of human studies investigating the post-prandial blood-glucose lowering ability of oat and barley food products. Eur. J. Clin. Nutr. 67, pp. 310–317
Tosh, S. M., & Chu, Y. (2015). Systematic review of the effect of processing of whole-grain oat cereals on glycaemic response. British Journal of Nutrition, 114(8), pp. 1256-1262 DOI: https://doi.org/10.1017/S0007114515002895
Van Hung, P., & Morita, N. (2005). Physicochemical properties of hydroxypropylated and cross linked starches from A- type and B- type wheat starch granules. Carbohydrate Polymers, 59, pp. 239–246.
Virtanen, T., Autio, K., Suortti, K., and Poutanen, K. (1993) Heat-induced changes in native and acid-modified oat starch pastes. J. Cereal Sci. 17 pp. 137-145
Webster FH (2011) Oat utilization: past, present and future. In: Oats: Chemistry and Technology, 2nd ed., pp. 347–361 [FH Webster and PJ Wood, editors]. St Paul, MN: AACC International Inc
Webster, F. H., & Wood, P. J. (2011). Oat utilization: past, present and future. Oats: chemistry and technology, pp. 347-361
Welch RW (2011) Nutrient composition and nutritional quality of oats and comparisons with other cereals. In Oats: Chemistry and Technology, pp. 95–107 [FH Webster and PJ Wood, editors]. St Paul, MN: AACC International Inc.
Xu, J., Kuang, Q., Wang, K., Zhou, S., Wang, S., Liu, X., & Wang, S. (2017). Insights into molecular structure and digestion rate of oat starch. Food chemistry, 220, 25-30.
Yiu SH, Weisz J & Wood PJ (1991) Comparison of the effects of microwave and conventional cooking on starch and beta-glucan in rolled oats. Cereal Chem 68, pp. 372–379
Zhou, M., Robards, K., Glennie‐Holmes, M., & Helliwell, S. (1998). Structure and pasting properties of oat starch. Cereal Chemistry, 75(3), pp. 273-281 https://doi.org/10.1094/CCHEM.19188.8.131.523