Characterisation of volatile compounds in model milk systems containing different ratios of caseins, whey proteins and fat as affected by low frequency sonication
Creators: name: BUI, Thi Hong Anh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300602
subject: Food chemistry and food sensory science

description: High-intensity low-frequency ultrasound causes physical effects such as shear forces, micro jet, shock waves, turbulence due to acoustic cavitation and leads to the disruption and/or denaturation of milk components. In addition, acoustic cavitation generates free radicals in liquid milk, which in turn can lead to the formation of undesirable volatile compounds. Flavour is one of the most significant contributors to the acceptability of consumer towards milk and milk products. Thus, creating off flavours within milk systems with sonication can hinder or limit its application to some extent although significant improvement of products' properties and processing efficiencies can be attained. 

The overall objective of this research was to characterise the volatile compounds produced in milk systems containing different milk proteins (caseins and whey proteins) and fat at varying concentrations. The research simultaneously investigated the corresponding changes of milk fat and proteins towards the production of volatile compounds as the function of low-frequency ultrasound treatment.

First, the research aimed to investigate the effect of low-frequency ultrasound and fat concentration on the production of volatile compounds and corresponding changes of fatty acids in whey protein systems (WF).  High-intensity low-frequency ultrasound at 20 kHz up to 10 min with energy densities ranging from 9.54 to 190.8 J mL-1 was applied to WF systems containing various fat contents of 2%, 4%, and 6% (w/w). The volatile compounds produced in WF mixtures were mainly categorised into groups of hydrocarbons, aldehydes, ketones, esters, and alcohols. Most of these volatile compounds originated from milk fat as products of lipid oxidation or cleavage of fatty acids, whereas some carbonyl compounds such as aldehydes, ethyl acetate, and hexanol were found in both milk fat and protein systems as products of lipid oxidation and/or protein degradation. It was found that the production of volatile compounds was negligibly affected by the fat content. However, there were significant changes in the amounts of some typical fatty acids (myristic acid, palmitic acid, oleic acid, and stearic acid) in WF systems consisting 4% and 6% fat, whereas no significant changes of these fatty acids were observed in WF systems with 2% fat. On the other hand, sonication time prominently affected the production of volatile compounds irrespective of the fat content due to the function of physical and chemical effects generated by acoustic cavitation from ultrasound treatment.

Secondly, the research aimed to examine the effects of low-frequency ultrasound and fat content on the production of volatile compounds and corresponding changes of fatty acids in model casein systems (CF). Similar ultrasound treatment conditions that were applied previously to WF systems were given to CF systems containing varying fat contents (2, 4, and 6%). The results showed that similar volatile compounds were detected both in fat and CF systems. These volatiles belonged to the groups of aldehydes, ketones, esters, alcohols, and hydrocarbons, which were the products of oxidation of lipids or protein degradation due to acoustic cavitation. The fat content had a minor effect on the production of volatiles, whereas ultrasound treatment significantly influenced both the production of volatile compounds and fatty acid changes as analysed by chemometric (using principal component analysis). Short sonication times less than 5 min generated similar volatile profiles to the untreated samples. In contrast, prolonged sonication for 5 and 10 min remarkably increased the production of volatile compounds and the amounts of fatty acids. Therefore, the application of low-frequency ultrasound for short periods should be considered to minimize the production of volatile compounds which can ultimately affect the taste.

Thirdly, the effects of ultrasound and fat content on the structure of milk fat and proteins were examined using Fourier transform mid-infrared spectrometry (FT-MIR). Low-frequency ultrasound (20 kHz) was applied to model casein (CF) and whey protein (WF) systems with varying fat contents of 2%, 4%, and 6% (w/w) under different energy densities ranging from 9.54 to 190.8 J ml-1. It was found that the fat content and sonication time influenced the intensities of fatty chains and ester groups for both CF and WF systems. Sonication caused changes to the structure of fat, denaturation of milk proteins forming more extensive structures, and enhanced the interaction between protein-protein, and protein-lipids. Furthermore, the intensities of ester groups, which are assigned to triacylglycerol and lipid-protein complexes, increased due to the interaction between milk fat and proteins. Milk protein denaturation and disruption of milk fat globules/fat clusters have contributed significantly to the build-up of lipid-protein interactions.

Finally, the research investigated whether ultrasound and the ratio of casein to whey proteins affected the production of volatile compounds when they are present in mixtures and concurrently examined the correlated changes in the structures of milk components. Ultrasound treatment (20 kHz, up to 10 min, &lt;10oC) was applied to mixtures of fat and milk proteins with varying ratios of casein to whey protein (50:50, 60:40, 80:20). The results showed that sonication time had significant effects on the production of volatile compounds, while the variation of casein to whey protein ratios were independent of the production of volatile compounds. The concentration of some aldehydes, ketones and esters were found to significantly increase with sonication. Ultrasound was found to cause the concurrent increase in the amounts of fatty acids as well as an increase in the intensity of ester groups assigned to fat and complexes of lipid-protein. Accordingly, ultrasound led to the denaturation of milk proteins (whey protein and casein) with a decrease in their native structure (side chain, alpha-helix, beta-turn, beta-sheet) to form more extensive structures (random coil). Moreover, the increase in intermolecular beta-sheet suggested the formation of milk protein interactions among whey proteins and whey/caseins or between milk proteins and fat forming lipid-protein complexes. 

In summary, the findings of volatile profiles, their origins in different sonicated milk systems and corresponding changes in milk components will advance the applications of ultrasound by controlling and minimizing the production of undesirable volatile compounds. Consequently, this will reduce the generation of off-flavours, determine the acceptability of the products to the consumers, and promote the applications of low-frequency ultrasound in the dairy industry.