Heat stability of reconstituted, protein-standardized skim milk powders V. Sikand,* P. S. Tong,* 1 and J. Walker† *Dairy Products Technology Center, and †Department of Statistics, California Polytechnic State University, San Luis Obispo 93407 ABSTRACT We determined the effects of standardization mate- rial, protein content, and pH on the heat stability of reconstituted milk made from low-heat (LH) and medi- um-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 stan- dardized 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 co- agulation 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 sta- bility 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 signifi- cant 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 stabil- ity and compositional differences in the standardization material (ELP or PP). Key words: heat stability, nonfat dry milk, protein standardization INTRODUCTION Milk and milk products are heat-treated to increase shelf life and ensure their safety for human consumption. Therefore, it is important that milk and milk powders used in various food applications be heat stable. Heat stability is defined as the ability of milk to withstand high temperatures without flocculation, gelation, or protein separation (Fox, 1982). Heat stability of milk is mainly a function of its milk protein stability (Fox and Morrissey, 1977; O’Connell and Fox, 2003; Singh, 2004) and may be affected by protein content. Natural fluctuations in milk protein content (Harland et al., 1955; Ng-Kwai-Hang et al., 1987; Heck et al., 2009) have been reported throughout the seasons. Thus, protein standardization can be used to achieve more consistent protein content in dairy products (Jensen, 1988; Rattray and Jelen, 1996; Hard- ham, 1998; Sikand et al., 2008) and perhaps improve heat stability. Milk protein standardization refers to the adjustment of protein content by the addition of small quantities of either retentate or permeate obtained from milk ultrafiltration (UF; Kieseker and Healey, 1996). Per Codex Alimentarius 207 (Codex Alimentarius, 1999), milk-derived permeate or lactose can be used to stan- dardize protein content in the manufacturing of skim milk powder (SMP) for various food applications. Milk-derived permeate contains minerals, nonprotein nitrogen, and lactose, whereas lactose contains 99% lactose and trace minerals. Green et al. (1984) found that low-molecular-weight components comprising nonprotein nitrogen appeared to be accounted for primarily by urea, AA, and ammonia. A positive cor- J. Dairy Sci. 93:5561–5571 doi:10.3168/jds.2010-3128 © American Dairy Science Association ® , 2010. 5561 Received February 1, 2010. Accepted September 3, 2010. 1 Corresponding author: ptong@calpoly.edu