热稳定性直接影响牛乳的加工与感官特性, 准确判断牛乳的热稳定性对优化液态乳制品的加工条件具有重要意义。 然而, 当前液态奶的稳定性评价主要是通过加速实验后肉眼观察分层、 沉淀等情况, 以及动态光散射等手段, 尚无快速可靠以及量化的评价标准, 严重制约了热处理工艺的选择效率。 Turbiscan多重光技术在测试流体的稳定性时, 无需对样品进行前处理, 可实时检测样品的背向散射光和透射光的强度, 计算出体系内部颗粒的迁移速率、 沉淀层的厚度、 体系的不稳定指数等参数, 因此是评价流体物理特性的有效手段。 研究以动态光散射测试结果为对照, 利用Turbiscan多重光技术测定了脱脂乳经过80 ℃条件下加热30 min后, 分别在pH值6.3, 6.5, 6.7和6.9, CaCl2的浓度在0, 20, 40, 60和80 mmol·L-1条件下的热稳定性。 结果表明, 当CaCl2添加量为20, 40, 60和80 mmol·L-1时, 脱脂乳的z-平均直径从152.7增加到1 284.4 nm, 20 h后的不稳定指数值从0.98增加到17.04, 样品顶端的背散射光强变化值从-4.3降低到-37.4, 底端的背散射光强变化值从2.2增加到14.7; 当样品的pH值分别为6.3, 6.5, 6.7和6.9时, 脱脂乳的z-平均直径分别为148.1, 152.7, 132.4和122.4 nm, 20 h后的不稳定指数值分别为1.32, 1.02, 0.98和1.41, 样品顶端的背散射光强变化值分别为-3.1, -4.7, -4.2和-5.6, 底端的背散射光强变化值分别为5.7, 3.4, 4.1和6.8。 结果表明, 随着CaCl2添加量的增加, 脱脂乳的热稳定性显著降低, pH值对脱脂乳热稳定性的影响较小。 同时发现, 与动态光散射技术相比, Turbiscan多重光散射技术可更精确、 方便、 快捷地获得热处理后的牛乳背散射光强值和体系不稳定指数等牛乳稳定性指标, 从而明确热处理导致的乳蛋白分散体系的不稳定性发生的机理。 Turbiscan多重光散射技术比动态光散射技术测定更方便快捷, 该研究对优化乳制品加工工艺具有重要指导意义。

Thermal stability could significantly affect the processing and sensory properties of milk. Accurately determining the thermal stability of milk is of great significance for optimizing the processing conditions of liquid dairy products. However, the current stability evaluation method on liquid milk is mainly through the observation of stratification, precipitation, or dynamic light scattering technology. There is no fast and reliable and quantitative evaluation standard, which seriously restricts the selection efficiency of the heat treatment process of liquid milk. Turbiscan multiple light technology can test the stability of the fluid without pre-processing the samples. It can monitor the backscattered light and transmitted light intensity of the samples timely, and calculate the migration rate of the internal particles and the thickness of the precipitate layer. In this study, the dynamic light scattering test results were used as control. The Turbiscan multiplex light technique was used to measure the heat stability of skim milk treated with different pre-heating pH values, including pH 6.3, 6.5, 6.7 and 6.9 at 80 ℃ for 30 min. The concentration of CaCl2 before heating was 0, 20, 40, 60 and 80 mmol·L-1, respectively. The results showed that when the CaCl2 increased from 20 to 80 mmol·L-1, the z-average diameter of skim milk changed from 152.7 to 1 284 nm, and the instability index values (after 20 h) increased from 0.98 to 17.04; the backscattered light intensity change value decreased from -4.3 to -37.4, and the backscattered light intensity change value from the bottom end increased from 2.2 to 14.7. When the pH values were 6.3, 6.5, 6.7 and 6.9, respectively, the z-average diameters of skim milk were 148.1, 152.7, 132.4 and 122.4 nm, respectively, and the instability index values 1.20, 1.32, 1.02, 0.98 and 1.41, respectively; the backscattered light intensity values at the top of the sample were -3.1, -4.7, -4.2 and -5.6, the backscattered light intensity changes at the bottom were 5.7, 3.4, 4.1, and 6.8, respectively. The results showed the heat stability of milk can be significantly influenced by the CaCl2, while the pH value has little effect on the thermal stability of skim milk. At the same time, it is also found that compared with dynamic light scattering technology, Turbiscan multiple light scattering technology was more accurately, conveniently and quickly to obtain the stability index, such as milk backscattered light intensity value and instability index. This research has important guiding significance for optimizing dairy processing technology. The Turbiscan multiplex light technology is more convenient and faster than the dynamic light scattering technique.