Institute of Food Science and Biotechnology, University of Hohenheim, Garbenstr. 21, 70593 Stuttgart, Germany.
Department of Chemical and Biological Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstr. 4, 91058 Erlangen, Germany.
J Dairy Sci. 2018 May;101(5):3866-3877. doi: 10.3168/jds.2017-13905. Epub 2018 Feb 22.
Machinery such as pumps used for the commercial production of fermented milk products cause vibrations that can spread to the fermentation tanks. During fermentation, such vibrations can disturb the gelation of milk proteins by causing texture defects including lumpiness and syneresis. To study the effect of vibrations on yogurt structure systematically, an experimental setup was developed consisting of a vibration exciter to generate defined vibrational states and accelerometers for monitoring. During the fermentation of skim milk, vibrations (frequency sweep: 25 to 1,005 Hz) were introduced at different pH (5.7 to 5.1, step width 0.1 units) for 200 s. Physical properties of set gels (syneresis, firmness) and resultant stirred yogurts (visible particles, rheology, laser diffraction) were analyzed. Vibrational treatments at pH 5.5 to 5.2 increased syneresis, gel firmness, and the number of large particles (d > 0.9 mm); hence, this period was considered critical. The particle number increased from 34 ± 5 to 242 ± 16 particles per 100 g of yogurt due to vibrations at pH 5.4. In further experiments, yogurts were excited with fixed frequencies (30, 300, and 1,000 Hz). All treatments increased syneresis, firmness, and particle formation. As the strongest effect was observed by applying 30 Hz, the amplitude was set to vibration accelerations of a = 5, 10, 15, 20, and 25 m/s in the final experiments. The number of large particles was increased due to each treatment and a positive correlation with the amplitude was found. We concluded that vibrations during gelation increase the collision probability of aggregating milk proteins, resulting in a compressed set gel with syneresis. Resultant stirred yogurts exhibit large particles with a compact structure leading to a reduced water-holding capacity and product viscosity.
用于商业生产发酵乳制品的泵等设备会产生振动，这些振动会传播到发酵罐中。在发酵过程中，这种振动会通过导致质地缺陷（包括块状和离析）来干扰牛奶蛋白的胶凝。为了系统地研究振动对酸奶结构的影响，开发了一种实验装置，该装置由一个振动激励器组成，用于产生定义的振动状态，以及用于监测的加速度计。在脱脂牛奶的发酵过程中，在不同的 pH 值（5.7 至 5.1，步宽 0.1 个单位）下引入振动（频率扫描：25 至 1,005 Hz），持续 200 s。对凝固凝胶的物理特性（离析、硬度）和所得搅拌酸奶（可见颗粒、流变学、激光衍射）进行了分析。在 pH 值为 5.5 至 5.2 时的振动处理增加了离析率、凝胶硬度和大颗粒数量（d > 0.9 mm）；因此，这一时期被认为是关键时期。由于 pH 值为 5.4 时的振动，酸奶中的颗粒数量从 34 ± 5 个增加到 242 ± 16 个/100 g。在进一步的实验中，将酸奶以固定频率（30、300 和 1,000 Hz）进行激励。所有处理都增加了离析率、硬度和颗粒形成。由于在施加 30 Hz 时观察到最强的效果，因此在最终实验中，将振幅设置为振动加速度 a = 5、10、15、20 和 25 m/s。由于每种处理都会增加大颗粒的数量，并且发现与振幅呈正相关。我们得出结论，凝胶化过程中的振动会增加聚集牛奶蛋白的碰撞概率，从而导致离析的压缩凝固凝胶。所得搅拌酸奶具有紧凑结构的大颗粒，导致保水能力和产品粘度降低。
