Reversibility of pH-induced changes in the calcium distribution and melting characteristics of mozzarella cheese

Previous studies that used a post-manufacture method to increase or decrease the pH of mozzarella cheese demonstrated that cheese melting characteristics and calcium distributon are profoundly influenced by pH. The study objective was to evaluate whether these pH-induced changes are reversible. Two experiments were conducted. In the first, representative shredded samples of cultured low-moisture part-skim mozzarella cheese were exposed to acetic acid vapour to decrease the pH by ca. 0.6 pH units in three increments. After equilibration at 4degreesC for 24 h, the samples were analysed for apparent viscosity and water-soluble calcium. The sample with the lowest pH was divided into representative subsamples and exposed to ammonia to increase the pH by ca. 0.6 pH units in three increments. After equilibration at 4degreesC for 24 h, the subsamples were analysed for apparent viscosity and water-soluble calcium. This experiment was replicated three times with different batches of cheese. Decreasing the cheese pH from ca. 5.3 to 4.8 resulted in nonlinear (quadratic) increases in apparent viscosity and water-soluble calcium, and cheeses lost the ability to melt and stretch below pH 5.0. Upon reversal of cheese pH back to ca. 5.1, apparent viscosity and water-soluble calcium decreased in nonlinear (quadratic) manner and cheeses regained the ability to melt and stretch. The design of the second experiment was similar to that of the first, except that the pH was first increased in three increments to ca. pH 6.6 and then decreased. Increasing the cheese pH from ca. 5.2 to 6.6 resulted in a linear increase in apparent viscosity and a more fibrous and elastic melted consistency. Upon reversal of cheese pH back to ca. 5.2, apparent viscosity decreased in a linear manner, showing a high degree of reversibility. Water-soluble calcium showed a linear decrease with increasing pH, and then increased in like manner when the pH was reversed. Results suggest that the reversible association and dissociation of calcium from the casein matrix contributed to structural changes that were responsible for the observed reversible pH-induced changes in melted consistency.