In situ SAXS study of non-fat milk model systems during heat treatment and acidification

Small-angle X-ray scattering (SAXS) was used to monitor structural changes induced by heat treatment and acid gelation in milk matrices with added whey protein concentrates (WPCs) and nano-particulated whey protein (NWP). In general, heat treatment was found to mainly affect whey protein components while pure casein micelles remained largely unaffected. Conversely, acidification mainly affected caseins while leaving pure whey protein components intact. In mixed systems, the overall behaviour could be understood as a combination of the above effects, however, the type of the added whey protein components influenced the resulting structure formation and dynamics. NWP led to formation of larger structures compared to WPC components during heat treatment, although the latter showed faster aggregation dynamics. During acidification the NWP containing samples exhibited structural changes at slightly higher pH values than the WPC samples. The modeling of pure liquid whey protein (LWPC) samples showed that the heat induced denaturation and resulting aggregation of individual whey proteins is mainly a surface effect leaving the overall protein shape and dimensions unaffected. Schematic diagrams based on the current SAXS data and previous studies were constructed to illustrate the suggested interaction mechanisms between casein and whey proteins during both heating and acidification.