Heat-Induced Coagulation of Milk
In this chapter, methods for assessing the heat stability of unconcentrated and concentrated milk are discussed. The effects of compositional factors, processing conditions and additives on the heat stability of milk are reviewed and the changes that occu
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O'CONNELL AND
19.1
P.F. Fox
ABSTRACT
In this chapter, methods for assessing the heat stability of unconcentrated and concentrated milk are discussed. The effects of compositional factors, processing conditions and additives on the heat stability of milk are reviewed and the changes that occur on heating are considered in relation to the heat stability of milk. Three possible explanations for the pH-dependence and mechanism of thermal coagulation of milk, which incorporate the research on the heat stability of milk over the last century, are presented.
19.2 19.2.1
HEAT STABILITY OF MILK
Introduction
The utilization of milk by humans as a readily digestible source of proteins, lipids and carbohydrate, dates back to antiquity. Historically, milk was preserved by fermentation in the form of fermented milk or cheese or the lipid phase was conserved as butter or ghee. However, with developments in thermal processing, milk can now be stored in liquid form or as powders. The first heat treatment of milk with a specific objective has been attributed to Louis Pasteur, ca. 1860. Today, the majority of milk, regardless of its final use, is subjected to at least one heat treatment. Common heating regimes and their specific objectives are listed in Table 19.1. The effect of heat-treating milk on its nutritional and sensory quality, inactivation of enzymes, the technology of thermal processing and methods for assessing the intensity of heat treatments have been reviewed comprehensively (Burton, 1984; Calvo and La Hoz, 1992; Andersson and Advanced Dairy Chemistry Volume 1: Proteins. 3rd edn. Edited by P.F. Fox and P.L.H. McSweeney. Kluwer AcademiciPlenum Publishers, 2003.
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HEAT-INDUCED COAGULATION OF MILK
TABLE 19.1 Common heat treatments applied in the dairy industry, their parameters and specific objectives
Heating regime Thermization Pasteurization LTLTI HTST 2 Forewarming Sterilization UHT 3 In-container Production of specific products
Conditions
Objective
65°C x 15 min
Killing of spoilage microbes
63°C x 30 min x 15 sec 90°C x 2-10 min 120°C x 20 sec
Killing of pathogenic microbes
noc
Preparatory step for sterilizaton
Sterilization 130-140°C x 3-5 sec 1l0-1l5°C x 10-20 min Y oghurts and protein 85-90°C x 5-15 min coprecipitates
1Low temperature long time. 2High temperature short time. 3Ultra-high temperature.
Oste, 1995a,b; Hinrichs and Kessler, 1995; Farkye and Imafidon, 1995; Nursten, 1995; Pellegrino et at., 1995; Stepaniak and S0rhaug, 1995; Recio et al., 1997).
Relative to other food systems, milk is extremely heat stable and can tolerate the conditions applied in most thermal processes. Indeed, if the pH of milk is readjusted periodically to its original value, it may be heated at 140°C for at least 3 h without coagulating (Fox, 1981a). The very high thermal stability of milk is due to the loose, ill-defined three-dimensional structure of its principal proteins, the caseins. Conversely, the whey proteins, which have compact globular structures with unique native conformations, are quite heat
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