Foaming of milk

Voderbet, Sapna Kamath (2007). Foaming of milk PhD Thesis, School of Land, Crop and Food Sciences, University of Queensland.

       
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Author Voderbet, Sapna Kamath
Thesis Title Foaming of milk
School, Centre or Institute School of Land, Crop and Food Sciences
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor A/Prof Hilton Deeth
Total pages 215
Language eng
Subjects 290104 Other Food Sciences
Abstract/Summary Foaming can be beneficial or undesirable depending on the temperature at/to which milk is foamed. For instance, foaming of milk is beneficial when making Italian style coffee such as cappuccino where steam from a steam generator is injected into milk to create a foam and to simultaneously heat the milk to ~ 65 – 70°C. Foaming of milk is undesirable during operations such as pumping of cold (5°C) milk in a dairy processing plant. The main objective of this study was to develop an understanding of the foaming patterns of milk and the factors that affect the foaming properties in the temperature range 5 – 85°C and use this understanding to devise treatments and additives to improve/reduce the foaming of milk depending on the end stage application. A review of literature showed that very little information was available on the foaming properties of milk as a function of temperature. Moreover a clear gap existed between an understanding of the foaming properties of milk proteins in single or binary systems and the foaming properties of milk per se. Foaming properties of milk over a broad temperature range were studied using a specially designed foaming apparatus, which enabled foams to be produced under standardised conditions. Foaming patterns of a range of milks which included raw whole milk, pasteurised homogenised full-cream milk, pasteurised skim milk, UHT homogenised full-cream milk, UHT skim milk and reconstituted milks made from low-, medium- and high-heat skim milk powder, were established as a function of temperature (5 – 85°C). The foaming properties of a given milk were largely determined by the temperature at which milk was foamed, its fat content, and the processing conditions used during its manufacture. Skim milks had a pronounced foam stability peak at 45°C when foamed over a temperature range of 5 – 85°C. Image analysis of skim milk foams, as observed under a light microscope, showed that the surface of skim milk foams coarsened over time. This occurred most rapidly in foams produced at 5°C and least rapidly in those produced at 45°C. The foaming properties of full-cream milks were largely determined by the physical state of milk fat. Milk fat was detrimental to foam formation and stability of full-cream milks when milk fat globules contained both solid and liquid fat i.e. in the temperature range 5 – 40°C. The extent of whey protein denaturation had a positive effect on the foam stability of reconstituted milks at lower temperatures (< 45°C) and a negative correlation with foam stability at higher temperatures When skim milk was separated into its micellar, ultracentrifugal and defatted ultracentrifugal fractions, the foaming pattern of the parent milk as a function of temperature (5 – 85°C), resembled that of the micellar fraction, with a pronounced foam stability peak at 45°C. The foaming pattern of the serum phase of milk could not be determined due to precipitation of calcium, when the defatted ultracentrifugal supernatant was heated to temperatures G 45°C. The sedimentable interfacial material isolated by low-speed centrifugation of collapsed skim milk foams was made up of large-sized casein micelles; particle size measurements and electrophoretic analysis of this material showed that the stability of skim milk foams was mainly determined by the ability of micellar caseins to adsorb and form a stabilising network at the air– serum interface of the foams. Addition of calcium-chelating agents, which increased the non-micellar casein content of milk, decreased foam stability, while addition of calcium salts, which increased the micellar casein content of milk, increased foam stability, reinforcing the observation that adsorption of micellar caseins is required for the stability of skim milk foams. The role of whey proteins in the stabilisation of skim milk foams could not be determined from a study of the sedimentable interfacial material, as they were lost in the supernatant during low speed centrifugation of skim milk foams. Lipolysis as measured by the FFA content of milk had a negative effect on the steam foaming properties of milk, with foamability and foam stability decreasing and the coarseness of foam increasing with an increase in the FFA content. The destabilising effect of the products of lipolysis was higher when milk was foamed at lower temperatures. A good correlation was observed between surface tension, FFA content and foam stability of milk. However, natural variations in other surface active components of milk such as fat, protein and phospholipids, tend to mask the effect of changing FFA content on the surface tension value of milk and as a result it was difficult to arrive at an absolute surface tension value of milk below which foaming properties of milk were adversely affected. Increasing the total solids content of milk decreased foamability and increased foam stability due to an increase in the viscosity coupled with an increase in the amount of protein available for adsorption at the air–serum interface of milk foams. As in the case of increasing total solids content, addition of H-carrageenan decreased foamability and increased the foam stability of milk, due to an increase in viscosity of the milk. Emulsification of oils such as olive, canola and sunflower oils in skim milk using a microfluidiser resulted in a pronounced increase in the foam stability of milk. The foaming patterns of milks containing oil instead of milk fat resembled that of skim milk with a pronounced foam stability peak at 45°C. This suggested that lipids which are fully liquid above 0°C are not detrimental to the foaming properties of milk foamed in the temperature range 5 – 85°C. The higher foam stability of milks containing added oil, when compared to the parent skim milk, was attributed to an increase in the viscosity of milk as a result of the added oil. Pre-heat treatment of milk to temperatures G 65°C altered the foaming properties of milk. The foamability of milk increased with an increase in the temperature of preheat treatment from 65 to 85°C, while the foam stability was highest when milk was subjected to a pre-heat treatment of 65°C. Lowering of the measurable FFA content of milk, by the likely binding of FFA to BSA and I-lactoglobulin, and the lack of whey protein denaturation, could be responsible for the higher foam stability of milk subjected to a pre-heat treatment of 65°C. The present study represents a significant step forward in understanding the foaming behavior of milk, which is determined by the complex interplay between temperature, the structure, conformation and distribution of milk proteins, and the nature of other components of milk such as fat, mineral salts and FFAs. An understanding of the importance of caseins in the stabilisation of milk foams and the effect of the physical state of milk fat on the foaming properties of milk, achieved in the present study, opens up exciting avenues for the manipulation of the foaming behavior of milk, especially to produce specialty milks with improved foaming properties.
Keyword Milk -- Composition
Milk -- Heat treatment

 
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Created: Fri, 21 Nov 2008, 16:18:39 EST