1Faculty of Agricultural Food and Natural Resources, University of Sydney, NSW 2006
2Value Added Wheat CRC, North Ryde, NSW, 1670.
Starch and lipids are major food ingredients that have important functional interactions in food systems. Formation of complexes with lipids modifies the properties of starch, for example reducing starch solubility in water, delaying starch retrogradation and slowing starch hydrolysis by enzymes (Holm et al, 1983, Eliasson & Krog, 1985, Szczodrak & Pomeranz, 1992, Guraya et al, 1997, Crowe et al, 2000). Factors that affect the formation of starch-lipid complexes, such as starch and lipid type, ratio of starch to lipid, temperature and pH, have been considered extensively. However, less information is available on the influence of lipid concentration on complex formation. In this study, we have used the Rapid Visco Analyser (RVA) as convenient method to investigate the effect of type and concentration of fatty acids on the formation of starch–lipid complexes. The effect of fatty acids on starch paste viscosities was compared with the reduction in iodine-binding capacity due to formation of starch-lipid complexes.
Wheat starch was obtained from Penford Pty Ltd and was used as supplied. According to the supplier's specifications, it contained 9.9% moisture, 0.25-0.30% lipid, and 25.5% amylose. Caprylic acid (C8:0), lauric acid (C12:0), palmitic acid (C16:0), stearic acid (C18:0) were from Sigma-Aldrich (St. Louis, MO, U.S.A.)
Starch paste viscosities were determined using an RVA-4 from Newport Scientific as described previously (Tang & Copeland, 2004). Fatty acids were weighed accurately into a test canister and 25 ± 0.01ml of distilled water was added, followed by 2.5 ± 0.01 g of starch. The mixture was agitated by raising and lowering the plastic paddle through the canister for 10 times. Newport Scientific Standard Method 1 (STD1) was used for the tests. Starch-only reference pasting curves were recorded in each set of tests. Changes in final viscosity (ΔFV) due to the addition of fatty acids were calculated as follows:
ΔFV = (FV Starch-lipid - FV Starch-only)/ FV Starch-only × 100
Starch paste (5.0 g) was removed from the RVA canister at the conclusion of the run and mixed with 25 ml of distilled water at 40-50°C in a 50-ml capped tube. The tube was vortexed for 2 min, and 100 μl of the resulting dispersion was mixed with 15 ml of distilled water followed by the addition of 2 ml of iodine solution (2.0% KI and 1.3% I2 in distilled water). The absorbance was measured at 690nm. A starch-only paste was used as a reference. To avoid starch retrogradation, the tests were performed within 60 mins. Complexing index (CI) was calculated as follows:
CI = [(Abs reference -Abs starch-lipid )/Abs reference] x 100
The formation of starch-fatty acid complexes was monitored by the increase in final viscosity (ΔFV) of the starch paste and by the loss of iodine binding capacity (complexing index, CI) of the starch. Starch mixed with low concentrations of caprylic acids (C8:0) showed little complex formation, as indicated by ΔFV and CI (Fig. 1). Significant complex formation was noted only when amounts of caprylic acid greater than 40.5 mg were mixed with 2.5 g of wheat starch Fig. 1).
Increases in ΔFV and CI indicated that complex formation between wheat starch and lauric acid (C12:0), palmitic acid (C16:0) and stearic acid (C18:0) increased as the amount of fatty acids was increased (Fig. 1). The ΔFV and CI with lauric acid increased to maximum values when about 22.5 mg of the fatty acid was mixed with 2.5 g of starch, and these parameters did not change significantly as the amount of lauric acid in the mixtures was increased further. In contrast, both the ΔFV and CI for mixtures of starch and palmitic acid increased to a maximum with about 13.5 mg of the fatty acid, and then decreased to a value close to that of the starch only reference as the amount of fatty acid was increased beyond this amount. Similar results were observed with starch and stearic acid mixtures. Thus, with these long chain saturated fatty acids, increasing the concentration of lipid led firstly to an increase in amount of starch-lipid complexes, as indicated by increased ΔFV and CI. Further increases in the concentration of the fatty acids above a critical concentration led to a decrease in the extent of complex formation (Fig. 1).
X-ray diffraction analysis of the starch-lipid pastes formed with stearic acid indicated that at high concentrations of stearic acid self association of the lipid into micellar structures was occurring (results not shown). This suggests that when the concentration of water-insoluble lipids reaches a certain level, formation of starch-lipid complexes decreases due to the lipids self associating in preference to binding with amylose. Starch lipid complex formation did not decrease at the highest concentrations of caprylic and lauric acids used in these experiments as these fatty acids are much more water soluble than palmitic and stearic acids.
Figure 1: Comparison of final viscosity (FV) and complexing index (CI) of 2.5 g of wheat starch in 25 ml of water mixed with different amounts of lauric acid (a), palmitic acid (b), caprylic acid (c) and stearic acid (d). The data are representative of one of duplicate experiments.
In a starch, lipid and water system of the type that occured in the RVA mixtures, the lipid molecules can dissolve in the water, form complexes with amylose in starch, or self-associate into micellar structures. Formation of complexes of the lipids with amylose is therefore likely to be influenced by the solubility of the lipid in water and the critical micellar concentration. This means that the optimal concentration for the formation of complexes between starch and lipids is likely to vary depending on the type of the lipid and its water solubility.
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