The effects of carbon dioxide on the physico-chemical and microbiological properties of skim milk and whey protein concentrates

Abstract

The addition of carbon dioxide (CO2) has a significant impact on the shelf-life of dairy products. Previous research, which focused on microbiological quality, has shown that the addition of CO2 resulted in shelf-life extension of raw milk and other dairy products. Little research has been carried out on the effects of CO2 on the physico-chemical properties of dairy products. There has been no published research on the use of CO2 in skim milk concentrate (SMC) and whey protein concentrate (WPC), or on the effects of CO2 addition and removal on the physico-chemical properties of these concentrates. There is a need to understand the changes in the physico-chemical properties of concentrates during carbonation and decarbonation as this can assist in developing procedures for the handling and process control of concentrates as intermediate dairy products for further processing. The close proximity of the proteins in concentrates (50-60% moisture) may have a different response to the effects of CO2 as compared to the proteins in single-strength milk (~87% moisture). Bench-top as well as pilot-plant-scale carbonation and decarbonation systems were set up and successfully used for batch and continuous CO2 addition to and removal from SMC and WPC in this study. The electrochemical probe (Mettler-Toledo), which was an easy-to-use method with high accuracy and precision (l 10% error) for analysing dissolved CO2 in liquid dairy products, was evaluated and used throughout this study to measure the levels of CO2 at different stages during processing. This study has shown that ultrafiltration (UF) and reconstituted SMC and WPC powders can be carbonated to CO2 levels up to 1600-2400 ppm (saturation point) at total solids levels up to 35%. Carbonation and subsequent decarbonation of concentrates did not result in any changes (breakdown or aggregation) in the proteins. Most of the physico-chemical properties (CO2 levels, pH, TA, casein micelle size) of the carbonated concentrates reverted to their original values upon decarbonation. However, a significant decrease in viscosity (p l 0.05) of both carbonated and decarbonated UF and reconstituted SMC concentrates at high total solids content (g24%) was observed. Decarbonation of carbonated UF WPC similarly resulted in a lower viscosity product. Decarbonated reconstituted WPC, however, exhibited higher viscosities due to the formation of aggregates during decarbonation. CO2 had an impact on the microbiological properties of ultrafiltered SMC and WPC at different % TS, similar to what has been observed for raw and pasteurized milk. Extension of shelf-life was at least 50%, depending on initial microbial counts, storage temperature and level of CO2 in the product. CO2 inhibited the growth of Gram-negative microorganisms which dominated the total plate counts and psychrotroph counts of the control concentrates. A change in microflora to a mixture of both Gram-positive and other Gram-negative microorganisms, with different morphology from the Gram-negative microorganism isolated from control concentrates, was observed in carbonated samples. CO2 added to SMC and WPC was totally removed during spray drying into powder. The spray-dried powder from carbonated concentrates had larger particle size, more porous structure, higher bulk density and improved wettability (especially for high TS i.e. g30% TS concentrates) compared to powder produced from control (untreated) concentrates. The foaming ability (for reconstituted WPC) and foam stability (for both reconstituted SMC and WPC) were considerably increased by carbonation. The UF WPC powders, from control and carbonated concentrates, showed poor foaming properties when reconstituted; this was attributed to denaturation and aggregation of the whey proteins. In summary, this study showed that CO2 can be used as a processing aid to improve the quality of SMC and WPC during transportation and intermediate storage prior to further processing and final packing. It can be easily added and removed, with most physico-chemical properties (pH, CO2 levels, TA, casein micelle size) of the concentrates reverting to their original values except for viscosity on decarbonation. The reduction of viscosity of concentrates due to carbonation can reduce the susceptibility of concentrates to foaming during transportation, facilitate processing and reduce product loss. Spray-drying of the carbonated concentrates produces powders which, on reconstitution, have better foaming ability and foam stability, compared to untreated concentrates. Such powders may be useful in applications that require good foaming properties, such as in cappuccino-style beverages, bakery, chocolate and confectionery.