Processing-induced physicochemical changes of dietary proteins: implications for digestion, intestinal uptake and immunological responses

Summary:

Heat processing is a common procedure in producing cow’s milk products in industry, amongst others to extend their shelf life. Thermal processing is reported to influence a protein’s structure and potentially its immunogenicity. This thesis describes the impact of various types of processing on structural characteristics of β-lactoglobulin (BLG), as the major allergen in cow’s milk allergy, and on the immune response of antigen presenting cells (APCs) in particular. These cells were chosen as they are the first immune cells to encounter the exogenous food protein and subsequently initiate further immune responses, including the sensitization step of the allergic reaction. Next to BLG, thyroglobulin and lysozyme were studied in the thesis to estimate whether generally applicable conclusions could be drawn on the relationship between structural modifications of proteins and the subsequent immune responses.

This thesis studied a comprehensive set of parameters that were relevant for this question, from a protein’s accessibility to intestinal APCs after processing, its physicochemical structural modifications to its uptake by APCs and further immune responses. To generate a range of structural modifications, including unfolding, aggregation and glycation, three different heating methods were used. Reducing saccharides were also included in the heating process to induce glycation, which quite commonly occurs in milk processing and is expected to be important for structural modifications and immunogenicity of the protein. The methods were wet-heating (W; heating in solution at 60 °C for 3 days) which led to aggregation of protein, high-temperature dry-heating (H; heating at 130 °C for 10 minutes) for advanced glycation and low-temperature dry-heating (L; heating at 50 °C for 9 hours) for mild glycation.

We showed that physicochemical modifications of proteins, e.g. exposure of hydrophobic regions because of the wet-heating processing, influences its accessibility and uptake by APCs stronger than the other two heating methods. Moreover, the potential inherent proinflammatory effect of BLG on macrophages was reduced by severe structural changes. These outcomes may contribute to reduce the incidence and prevalence of CMA, and possible adverse immune effects from milk proteins, by better understanding of the underlying immunological mechanisms.