Characterisation of proteolytic activity in thermally-treated milk for novel product development

Milk is a very nutritious and widely consumed food, as well as a versatile raw material for the food industry. Processing involves converting raw milk into a wide range of dairy-based food products. Advanced processing technologies such as ultra-high temperatures (UHT) result in production and transportation efficiencies, extended shelf-life and wider distribution. The objectives of this study were to (i) identify the raw material quality of UHT-treated milk by monitoring protease activity, (ii) develop shelf-stable formulations of dairy beverages enriched with protein and dietary fibre, and iii) utilise protein enrichment to provide highly nutritious dairy-based desserts thus meeting consumer nutritional expectations.<br>Protease activity in UHT-treated milk was determined in relation to age-gelation during room temperature storage. Proteases, from psychrotrophic Pseudomonas spp., which are strongly associated with age-gelation in UHT milks, were extracted and added to raw skim milk at 0 to 0.005% (v/v). Protease activity before and after UHT treatment (135oC/2s) was estimated using the FITC-casein assay. During storage, the microbial count and pH of the samples remained the same to that recorded immediately after UHT treatment. <br>The association between peptide generation and added enzyme extract in UHT-milk can also be related to their shelf-life. When gelation was identifiable visually, the viscosity produced by the free amino groups in samples with added crude protease was similar, irrespective of the enzyme concentration added. The relationship between threshold enzymatic activity and storage of the UHT-treated milk was estimated from an empirically derived equation (R2 of 0.9943): y (shelf-life in weeks) = 9.6301x-2.12; where x is the enzyme activity in terms of FITC isomer 1 (× 10-4 nM).<br>Increasing consumer interest in a healthy diet in terms of protein, dietary fibre and energy intake opens opportunities for the development of novel dairy-based products. Reverse formulation engineering of selected commercial dairy beverages was performed by investigating their nutritional composition and steady-shear flow behaviour leading to the development of formulations enriched with protein and dietary fibre. Characterisation of physicochemical properties (and colour) of novel formulations containing 6.1% (w/w) protein and 2% (w/w) dietary fibre and the commercial product indicated that both systems are similar; an outcome which was further confirmed by consumers acceptance of the novel dairy beverage during sensory evaluation. <br>Similar to earlier work, reverse formulation engineering of commercial dairy desserts was carried out by establishing a database of steady-shear flow and textural profile that had to be imitated in novel formulations. Whey protein and/or casein were incorporated to achieve high levels of protein (11.5% w/w) and leucine (1.2% w/w). The physicochemical properties (steady shear viscosity, small deformation dynamic oscillatory in shear and texture profile) of the protein-enriched dairy desserts were examined. The textural characteristics of the enriched and commercial products were similar, even though the latter had 50% less protein than the novel formulations. Rheology results and microscopy images of both the commercial product and the protein-enriched dairy desserts indicate similar structural features. Moreover, results from sensory evaluation revealed that the nutritious dairy desserts were liked by consumers. <br>