Milk fat crystallization: From molecules to networks

Milk fat crystals have a structuring function in food systems like cream, cheese and butter. Milk fat crystallization is determined by the main molecules that compose it. However, the relationship between the more than 3000 triacylglycerol species in milk fat and the way they crystallize is highly complex. In addition, multiple factors (e.g. the cow’s nutrition and genetics) produce substantial compositional variation, which in turn affects milk fat crystallization and its functionality in food products. In this thesis, we studied the links between the molecular level (triacylglycerols), the mesoscale structures formed when milk fat crystallizes and the resulting macroscopic properties. First, we investigated the relationship between the macroscopic properties of milk fat and the underlying structures responsible for them; we then zoomed in into the crystal structure and how it was affected by changes in composition.

To obtain precise information on the crystal structure, we developed a quantitative phase analysis method that only requires lab-scale equipment and free software. This method, in combination with other techniques, allowed us to study how changes in a nanoscopic scale were related to changes in higher and lower length scales. We showed that the macroscopic properties of milk fat crystal networks are directly affected by multiscale structural transitions. We also concluded that these transitions originate in the nanoscopic level with the formation of specific crystal structures: α, β1’, β2’ and β polymorphs.

We investigated how polymorph formation was affected by changes in composition. We report the effects of four factors that affect milk fat composition: seasonality, genetics, diet and interspecies variation. Our findings on milk fat polymorphism allowed us to proposed hypotheses about triacylglycerol groups that could form these polymorphs. To be able to draw conclusions in this regard, rather than focusing on individual sources of variation, we combined our results from the different studies to derive associations between specific molecules and the levels of different polymorphs formed. A variety of triacylglycerols contribute to the formation of the α polymorph, but we concluded that saturated species with the highest molecular weight are a key component at crystallization temperatures close to its clearing point (20 °C). We linked the formation of the β’1 polymorph to the levels of triacylglycerols with total carbon number (CN)42–48, associated with fatty acids that are synthesized in the mammary gland. We associated the formation of the β polymorph to saturated CN50–52 TAGs containing both palmitic and stearic acid. In addition, we reported the formation of the β’2 polymorph at relatively high temperatures (15–20 °C) and we proposed a possible association between this polymorph and the levels of palmitic acid. These novel findings are interesting from a scientific perspective but are also relevant for the food industry.

As the formation of the different polymorphs is the starting point for the multiscale structural transitions defining milk fat crystal networks, the links found between triacylglycerol composition and crystal structures have implications for subsequent structural levels and the macroscopic properties of milk fat. The contribution of this thesis to our current understanding of milk fat crystallization can help to control its functionality, even when the composition changes.