Effect of protein and calcium chloride concentration, pH, and temperature on almond protein gelation

Abstract

Almond (Prunus dulcis) seeds are good sources of protein that contain healthy fatty acids and dietary fiber. Protein gelation has been traditionally achieved by heating, but other factors can be affected in different gel structures in protein gelation. However, most studies were focused on the heat-induced gel formation of soymilk proteins, there was limited research on the gelling property of almond milk proteins. Therefore, this study aimed to investigate the effects of protein, and calcium chloride concentration, pH, and temperature on almond protein gel-forming and to analyze the physicochemical properties of the almond protein gel. Heat-induced gel formation of almond milk was studied using different ratios of almond seeds to distilled water (1:2, 1:3, 1:4, and 1:5 (w/v)), calcium chloride concentration (0.3, 0.5, and 1%), pH (3, 4, 5, 6, and 7) and temperature (90, 95, and 100º C). Textural profile analysis (TPA), water-holding capacity (WHC), and gel strength of full-fat almond milk (FFAM) gels were evaluated. In addition, the combined factors of FFAM gels were analyzed for the TPA, WHC, gel strength, and rheological measurement. The results showed that the FFAM gel ratio of 1:2 showed the highest hardness (43.03 g), gel strength (0.18 N), and WHC (95.44 %) than other ratios of FFAM gels due to the higher protein, fat, and total solid contents. FFAM (1:2) gel had a significantly different (p˂0.05) with FFAM (1:4) gel on texture properties, gel strength, and WHC. The addition of 0.3% calcium chloride in 1:2 FFAM gels yields the highest gel hardness (44.76 g), strength (0.22 N), and WHC (99.2 %). The 0.3% calcium chloride was significantly different (p˂0.05) between 0.5% and 1% calcium chloride in each FFAM gels on gel strength. The maximum hardness, gel strength, and WHC values obtained the pH 6 in each FFAM gels. However, only a high protein concentration (5.37%) of FFAM (1:2) could be obtained at any pH value when heating. The FFAM gels induced with a temperature of 90º C results in the maximum hardness, gel strength, and WHC in each ratio of FFAM. FFAM gels decreased in gel hardness, strength, and WHC when higher temperatures (95 and 100º C) were applied. High-temperature heating of FFAM leads to higher denaturation of protein which does not set into gels upon cooling. In addition, when the factors were combined to form FFAM gel (1:3+0.3% CaCl2+pH 6+90ºC), the gel strength was significantly different (P˂0.05) from that of FFAM gels for each factor. Moreover, storage modulus (G’- 4359.4 Pa) was higher than loss modulus (G”- 751 Pa) indicating that the gel had formed a continuous network structure. The data obtained from this study provide basic information on the factors and properties of almond milk gel and useful for the application of almond milk gel in vegetarian products.