Elucidating the formation of cookable milk gels

Research output: Book/ReportPh.D. thesisResearch

Heat and acid-induced milk gels do not melt or flow upon cooking or frying and can therefore be considered a non-melting cheese, making them interesting as a dairy-based meat alternative. Due to a lack of knowledge of why milk composition and acidification temperature affect the properties of the gel formed, the present thesis aims to elucidate the formation of directly acidified milk gels at temperatures ≥60 °C. The thesis is based on four scientific papers elucidating the effect of acidification temperature and milk composition on the structure and physicochemical properties of the gels formed. Acidification temperature was found to greatly impact the speed of coagulum formation and chemical interactions within the protein network. The coagulum was mainly governed by calcium bonds (30-63 %), which increased by 71-187 %, depending, on pH with the increase of temperature from 60 to 90 °C. After the coagulum was pressed into a coherent gel the aggregate density dictated the hardness of the gels. Altering the milk composition, either by changing the milk sources, i.e. cow milk (CM) or buffalo milk (BM), or by the addition of whey protein or casein micelles, showed that both concentration and protein type influenced the gel formed. BM has a higher casein content (3.4-3.8 % w/w) compared to CM (2.8-3.0 % w/w), resulting in a less moist, denser, and harder gel. Correlating well with the dense and hard gel formed when the casein content of CM was increased to 4.8 % w/w. Increasing whey protein content from just 0.7 to 0.9 % w/w resulted in a significantly moister and softer gel. But when looking into the ability of the gel to absorb water and swell the 4.8 % w/w casein gel was superior. The ability of the gel to swell is its ability to take up cooking liquid and is therefore an important descriptor of cookability. Upon cooking the gel becomes significantly softer. However, only when the cooking water was salted, did the gels acidified at ≥ 75 °C increase in mass and thus were able to take up water. Using Low-Field nuclear magnetic resonance (LF-NMR) a better understanding of the structure of the gel as affected by processing or milk composition was found. With increasing acidification temperature a general decrease in relaxation times indicated a protein structure growing denser. Furthermore, the population with shorter relaxation time (T2 <15 ms), appointed to the highly bound water associated with the casein micelles, was confirmed by an increase in signal intensity by the addition of casein. The findings presented in this thesis constitute a significant advancement to the body of knowledge on the relationship between composition, protein interactions, and protein ratio of heat and acid-induced milk gels. The insights provided in this thesis can be exploited and used by the dairy industry for modulating acid milk gel functionality.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages159
Publication statusPublished - 2023

ID: 370496035