Dairy Products Technology Center, California Polytechnic State University, San Luis Obispo 93407, USA.
J Dairy Sci. 2010 Dec;93(12):5561-71. doi: 10.3168/jds.2010-3128.
We determined the effects of standardization material, protein content, and pH on the heat stability of reconstituted milk made from low-heat (LH) and medium-heat (MH) nonfat dry milk (NDM). Low-heat and MH NDM were standardized downward from 35.5% to 34, 32, and 30% protein by adding either edible lactose powder (ELP) or permeate powder (PP) from skim milk ultrafiltration. These powders were called standardized skim milk powders (SSMP). The LH and MH NDM and SSMP were reconstituted to 9% total solids. Furthermore, subsamples of reconstituted NDM and SSMP samples were set aside to measure heat stability at native (unadjusted) pH, and the rest were adjusted to pH 6.3 to 7.0. Heat stability is defined as heat coagulation time at 140°C of the reconstituted LH or MH NDM and SSMP samples. The entire experiment was replicated 3 times at unadjusted pH values and 2 times at adjusted pH values. At an unadjusted pH, powder type, standardization material, and protein content influenced the heat stability of the samples. Heat stability for reconstituted LH NDM and SSMP was higher than reconstituted MH NDM and SSMP. Generally, decreased heat stability was observed in reconstituted LH or MH SSMP as protein content was decreased by standardization. However, adding ELP to MH SSMP did not significantly change its heat stability. When pH was adjusted to values between 6.3 and 7.0, powder type, standardization material, and pH had a significant effect on heat stability, whereas protein content did not. Maximum heat stability was noted at pH 6.7 for both reconstituted LH NDM and SSMP samples, and at pH 6.6 for both reconstituted MH NDM and SSMP samples. Furthermore, for samples with adjusted pH, higher heat stability was observed for reconstituted LH SSMP containing PP compared with reconstituted milk from LH SSMP containing ELP. However, no statistical difference was observed in the heat stability of reconstituted milk from MH NDM and MH SSMP samples. We conclude that powder type (LH or MH) and effect of standardization material (ELP or PP) can help explain differences in heat stability. The difference in the heat stability of powder type may be associated with the difference in the pH of maximum heat stability and compositional differences in the standardization material (ELP or PP).
我们研究了标准化物质、蛋白质含量和 pH 值对低热处理（LH）和中热处理（MH）脱脂奶粉（NDM）再制奶热稳定性的影响。通过添加食用乳糖粉（ELP）或脱脂乳超滤渗透粉（PP），将 LH 和 MH NDM 从 35.5%标准化至 34%、32%和 30%的蛋白质。这些粉末被称为标准化脱脂奶粉（SSMP）。LH 和 MH NDM 和 SSMP 被重新配制为 9%总固体。此外，将部分再制的 NDM 和 SSMP 样品分离出来，以测量在天然（未调整）pH 值下的热稳定性，其余样品则调整至 pH 值 6.3 至 7.0。热稳定性定义为再制 LH 或 MH NDM 和 SSMP 样品在 140°C 下的热凝固时间。整个实验在未调整 pH 值的情况下重复了 3 次，在调整 pH 值的情况下重复了 2 次。在未调整 pH 值的情况下，粉末类型、标准化物质和蛋白质含量影响了样品的热稳定性。再制的 LH NDM 和 SSMP 的热稳定性高于再制的 MH NDM 和 SSMP。通常，随着标准化过程中蛋白质含量的降低，再制的 LH 或 MH SSMP 的热稳定性降低。然而，向 MH SSMP 中添加 ELP 并未显著改变其热稳定性。当 pH 值调整至 6.3 至 7.0 之间时，粉末类型、标准化物质和 pH 值对热稳定性有显著影响，而蛋白质含量没有影响。对于再制的 LH NDM 和 SSMP 样品，pH 值为 6.7 时观察到最大热稳定性，对于再制的 MH NDM 和 SSMP 样品，pH 值为 6.6 时观察到最大热稳定性。此外，对于调整 pH 值的样品，与再制的 LH SSMP 相比，含有 PP 的 LH SSMP 的再制牛奶具有更高的热稳定性。然而，MH NDM 和 MH SSMP 样品的再制牛奶的热稳定性没有统计学差异。我们得出的结论是，粉末类型（LH 或 MH）和标准化物质（ELP 或 PP）的作用可以帮助解释热稳定性的差异。粉末类型的热稳定性差异可能与最大热稳定性的 pH 值差异以及标准化物质（ELP 或 PP）的组成差异有关。
