Physical, rheological, and microstructural properties of whey protein enriched yogurt influenced by heating the milk at different pH values

Here, we investigate the effects of pH adjustment before heating milk on the textural properties of whey protein enriched yogurt. Whey protein enriched skim milk was adjusted to pH of 6.3, 6.7, and 7.1 followed by heating at 85 °C for 30 min. Yogurts prepared from milk heated at pH 6.7 showed remarkably higher G′ (storage modulus) values, water holding capacity and firmness compared to those prepared from unheated milk, or milk heated at pH 6.3 or 7.1. A relatively more compact protein network was observed in the yogurt made from milk heated at pH 6.7. Heating milk at pH 6.7 formed both whey protein-associated casein micelles (WPA-CM) and soluble whey protein/κ-casein complexes (SWP-CC), resulting in the presence of numerous aggregating particles, which was responsible for the firmer yogurt gel. Heating milk at pH 7.1 (mostly SWP-CC) resulted in higher yogurt firmness than heating at pH 6.3 (mostly WPA-CM). Practical applications Whey protein fortification and heating milk improve yogurt texture. However, different pH values of the milk before heating and whey protein fortification affect the properties of the yogurt. Variation of milk pH before heat treatment affects the degree of association between denatured whey proteins and casein micelles. As a result, inconsistencies occur in yogurt gel texture. This study showed that yogurt made from milk heated at pH 6.7 had increased firmness and water holding capacity compared with that when the milk was heated at pH 6.3 or 7.1. The pH of milk naturally changes slightly throughout the year, which in turn might affect yogurt firmness. Therefore, milk pH is an important parameter which needs to be adjusted to produce uniform quality yogurt to obtain a well-accepted commercial product.