Skim milk concentrates produced by reverse osmosis filtration: structure, stability and functionality

Research output: Book/ReportPh.D. thesisResearch

  • Morten Vormsborg Christiansen
The industrial interest in liquid milk concentrates, both as intermediate ingredients, in development of novel products, and as a way to increase the sustainability of dairy production, is increasing. Up-concentration of milk allows larger quantities of milk solids to be stored and transported compared to milk, due to a lower water content, resulting in decreased storage and shipping costs. Reverse osmosis (RO) membrane filtration can provide non-thermal concentration of milk solids. Concentration by RO requires less energy compared to evaporation and spray drying, and the concentrates are uncomplicated to reconstitute or mix into food formulations compared to milk powders. Some functional properties of the milk solids are altered, reduced, or lost during intense thermal treatment and spray drying processes. Both from a quality and sustainability point of view, substitution of milk powder with liquid milk concentrates may thus be beneficial. However, the storage stability of liquid milk concentrates is low compared to milk powder, which is a major challenge. In terms of microbiological quality and limited coagulation, aggregation, and precipitation of the milk solids, the optimal shelf-life of liquid concentrates is assumed to be more or less comparable to un-concentrated milk (i.e. storage at ⇠ 5 °C for < 30 days). This is in clear contrast to a shelf stability of more than six months at ambient temperatures for milk powder, depending on product, manufacturer, and shelf-life criteria. The present thesis investigates technological solutions to extend the physical, chemical, and microbiological stability of skim milk concentrates (SMC) with total solid contents up to 28 % (w/w) manufactured by RO. SMC stability was evaluated by a comprehensive combination of di↵erent methods, and the functional properties were compared to properties of reconstituted skim milk powder. It was observed, that SMC produced from pasteurised skim milk exhibit larger average casein micelle sizes, longer concentration times, and less viscosity development during storage compared to SMC produced from non-pasteurised skim milk. Increasing solid contents and heat-load increase the viscosity, but most significantly in SMC from non-pasteurised skim milk. The relation between heat-treatment and viscosity build-up was found to correlate with the distribution of calcium in the colloidal and serum phases of SMC. Lower heat loads for example increased the colloidal calcium concentration. Elevated colloidal calcium concentrations decreased the partial specific volume of casein micelles, which decreased the viscosity. By contrast, elevated ionic calcium concentrations decreased pH, which promoted interactions between the casein micelles and thus increased the viscosity. Ultra Small Angle X-ray Scattering data were successfully used to model the interactive forces between casein micelles in non-pasteurised skim milk. In addition, it was observed that increasing solid contents and heat-loads of SMC increased formation of 1-dimensional rod-shaped micellar aggregates, which was found to correlate well with increasing viscosity. The microbiota of SMC was found to accommodate several bacteria species dominated by Thermus spp. and Pseudomonas spp. However, regarding microbiological stability, Microbacterium lacticum was found to represent the greatest challenge, due to its dominance, heat resistance, and growability in pasteurised SMC. When compared to concentrates manufactured from reconstituted skim milk powder, concentrates manufactured from RO of skim milk showed more extensive viscosity build-up during storage. Rennet-induced gelation was markedly higher, whereas acid-induced gelation capability was lower in SMC from RO, compared to SMC from reconstituted milk powder. Emulsification and foaming properties and pH were overall identical for SMC from RO and reconstituted powders, except from RO-SMC stored for 30 days where the highest overrun was observed. From the experimental work it is accordingly concluded that application of SMC manufactured by RO shows greatest potential for specified formulations, where viscosity build-up capacity and functional properties are tailored by different heat-treatment strategies for the application in question. If the intended application involves usage within 30 days storage at 5 °C, the important conclusion from the present thesis is that substitution of skim milk powders with skim milk concentrates is industrially feasible and potentially could reduce the carbon footprint of the dairy industry, advance the applicability in certain traditional products, and boost the development of new products.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages181
Publication statusPublished - 2021

ID: 273013745