Effects of monoglycerides on pasting properties of wheat starch after repeated heating and cooling

Abstract

The effects of repeated heating and cooling on the properties of pastes prepared from a commercial wheat starch (Triticum aestivum L.) with added monoglycerides were studied using a Rapid Visco Analyser (RVA). The nanostructure of the freeze-dried pastes was determined by X-ray diffraction and small-angle X-ray scattering. Pastes prepared from the wheat starch alone, or from the starch mixed with tripalmitin, which does not form complexes with starch, produced regular viscosity profiles in the RVA when subjected to multiple heat-cool cycles. In comparison, the effects of adding monoglycerides (or monoacylglycerols) depended on the chain length and saturation of the fatty acid of the monoglyceride. Repeated heat-cool cycles in the RVA of the starch with different monoglycerides induced the formation of complexes of varying stability that influenced the viscosity trace of the paste during multiple heating and cooling cycles. Small-angle X-ray scattering in combination with X-ray diffraction proved useful in describing the nanostructural changes in the RVA pastes induced by monoglycerides and temperature cycling. The results indicate that the functional properties of starch pastes may be manipulated through the strategic selection of an added monoglyceride.

Introduction

Starch is a major source of dietary carbohydrate and is used in many food applications as a thickener, texture enhancer, stabilizer and for controlling consistency and moisture retention (Ellis et al., 1998). The characteristics of many foods result from the specific pasting, gelatinization and retrogradation properties of starch, which can be influenced significantly by additives. One of the most important interactions of starch is between amylose and lipids, which can alter both the functional and nutritional characteristics of starchy foods (Hibi et al., 1990, Riisom et al., 1984). Complexation with lipids reduces the solubility of starch in water, alters the rheological properties of pastes, decreases swelling capacity, increases gelatinization temperature and reduces gel rigidity (reviewed by Copeland et al., 2009). From a nutritional perspective, complexation with lipids affects the susceptibility of starch to enzymatic breakdown. Lipids slow down amylose retrogradation, thereby retarding the formation of resistant starch, but conversely, in vitro studies have shown that starch complexed with lipids is attacked more slowly by amylases (Crowe et al., 2000, Eerlingen et al., 1994, Jeong and Lim, 2003).

With rising consumer demand for functional foods, methods to evaluate gelatinization and retrogradation behavior of starches are of interest. Instruments such as the Rapid Visco Analyser (RVA, Newport Scientific) are used extensively in empirical studies to characterize the viscosity of starch throughout the gelatinization and retrogradation processes. The RVA has been used to form and characterize starch–lipid complexes (Mira et al., 2005, Tang and Copeland, 2007a), and to investigate the varietal influences on complex formation between lipids and wheat starch (Blazek and Copeland, 2009, Salman and Copeland, 2010). These and most other RVA studies reported in the literature have used a standardized single heat-cool profile, with only a few reports of modified RVA protocols (Batey, 2007, Blakeney and Booth, 2001, Corke et al., 1996, Nelles et al., 2000).

RVA and similar techniques provide valuable information on process adequacy, and can be coupled with diffraction, scattering, thermal and microscopic techniques to enable the food microstructure to be related to functionality. Salman and Copeland (2010) used a novel extended RVA profile, which included repeated heating and cooling to investigate the properties of starch and starch–lipid pastes. The rationale behind using repeated heating and cooling is that various processed foods, such as pasteurized foods, convenience, restaurant and mass catering meals, and precooked foods, undergo repeated heating and cooling or are held at elevated temperatures for a period before consumption. Information on the behavior of starch in such systems is very limited. The objectives of the present study were to increase our understanding of the interactions between starch and lipids by exploring the behavior of wheat starch pastes with added monoglycerides (also referred to as monoacylgycerols) during repeated heating and cooling in the RVA. Although food processing is unlikely to include such complex thermal treatments, we chose to extend the number of heat/cool cycles to observe the thermal stability of the starch–lipid complexes. In order to explore the relationship between viscosity and structure of the pastes, RVA measurements were supported by XRD and SAXS analyses of the resulting freeze-dried pastes.