Post-processing of concentrated fermented milk: influence of temperature and holding time on the formation of particle clusters.

摘要

Graininess is a common structural defect in microgel suspensions, e.g., yogurt or fresh cheese. The aim of this study was to investigate the impact of the temperature (23-54 degrees C) and the holding time (1-300 min) on the formation of particle clusters in concentrated, fermented milk (protein content 8.2% (w/w)) during post-processing. Since graininess is correlated with the presence of large protein aggregates, temperature treatment during post-processing was varied systematically to promote particle growth and the particle size d_75.3 was measured. The samples revealed a polydisperse particle size distribution present at all temperatures and holding times. With longer holding times, rearrangement led to larger particle clusters while the number of smaller particles decreased. The increase in the d_75.3 was fitted with a power law function while higher temperatures promoted both the aggregation rate and the particle size. By using the Arrhenius equation, the activation energy, E_A, was calculated (26 kJ.mol--1), which was in agreement with the aggregation kinetics occurring at smaller scales, e.g., the acid-induced aggregation of casein micelles and the temperature-induced aggregation of casein submicelles. According to the activation energy, the particle growth in microgel suspensions was proposed to be predominantly diffusion-limited. The results of this study imply that particle size during post-processing is adjustable. Furthermore, the particle size increases with increasing temperature load, thus, rapid cooling reduces the aggregation rate and stops particle growth. The results give manufacturers a useful tool to control the graininess that affects the structure and the sensory properties. copyright INRA and Springer Science+Business Media B.V. 2011.