Chemical and Biomolecular Engineering - Theses
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ItemMembrane ultrafiltration of skim milk and its application to cream cheese manufacture( 2023-02)Cream cheese is an important export product for Australia; cream cheese production and consumption also continue to increase in many countries. The traditional manufacture of cream cheese generates a large volume of acid whey (~65–70%, w/w) that is often treated as wastewater, animal feed or fertilizer and can have a negative impact on the environment. Membrane ultrafiltration (UF) can be used to pre-concentrate the milk used in cream cheese production to improve the retention of whey proteins, increase productivity and reduce the generation of acid whey. The effect of UF concentration on the properties of milk and subsequent effects on the acid-coagulated gels and resulting cream cheese were investigated in this thesis. The thesis commences with a literature review of cream cheese manufacture and membrane filtration in Chapter 2. The fundamental and physico-chemical properties of cream cheese acid gels made from the addition of UF milk are then reported in Chapter 3. It was hypothesised that UF addition would impact the properties of both the milk and acid gels formed during the early stages of cream cheese production. Skim milk was concentrated to a volumetric concentration factor (VCF) of 2.5 or 5 by UF and the milk samples were standardized to a protein-to-fat ratio of ~0.23 to achieve a milk composition typical of that used for full fat cream cheese production. The UF cheese milk had a similar particle size distribution to the unconcentrated cheese milk after homogenization but increased viscosity and a slower rate of acidification, which could be improved by increasing starter culture concentration. The acid gels formed with the addition of UF retentate contained more protein and fat, resulting in a higher storage modulus, firmness and viscosity. A denser microstructure was observed in the acid gels formed with UF retentate and quantitative two- or three-dimensional analysis of confocal images found a greater volume percentage of protein and fat, decreased porosity and increased coalescence of fat. The mobility of water, as assessed by proton Nuclear Magnetic Resonance (1H NMR), was reduced in the dense UF gel networks. These insights improve our understanding of acid gel formation and can be used by manufacturers to optimize processing conditions for the use of concentrated milk and subsequent handling of firmer gels in industrial-scale cream cheese production. Chapter 3 demonstrated how calcium content increases in VCF2.5 and VCF5 retentate to ~260 mg/100 mL and ~480 mg/100 mL respectively, significantly higher than the concentration of ~120 mg/100 mL in the skim milk. The elevated calcium content that occurs in the milk concentrate after ultrafiltration is regarded as a potential cause for the defects reported in some UF-based dairy products, such as fresh cheese. In Chapter 4, to reduce calcium concentration in the UF preparations, skim milk was treated with 1% (w/v) or 2% (w/v) cation exchange resin and the treated milk then concentrated by UF to a VCF of 2.5 or 5. It was hypothesised that the removal of calcium from skim milk by cation resin would affect the properties of milk proteins as well, as the ultrafiltration process. The calcium content in the resin-treated skim milk, as well as the resulting retentates (VCF2.5 and VCF5), decreased by 20–30% compared with the non-resin treated controls. As a result of decalcification, the casein micelles partially solubilized and dissociated, which led to an increase in the soluble protein content and a lower relative turbidity for these milk samples. The decalcification of the skim milk feed also decreased the permeation flux during UF and led to a decrease in the gel concentration (or maximum concentration factor) from ~30% (w/w) solids (~6.5 fold concentration) for the control skim milk to ~24% (w/w) solids (~5.4 fold concentration) for 2% (w/v) resin treated skim milk. The average diameter of particles in skim milk was found to increase from ~160 nm to ~180 nm after calcium reduction, while the ultrafiltration process led to a decrease in particle size for the resin-treated milk samples. The zeta-potential of the calcium reduced UF retentates did not change but surface hydrophobicity increased. Analysis of the milk solids indicated that calcium depletion increased the hydration of the milk proteins to 3.3 g water per g dry pellet (2% resin, w/v), compared to the 2.2 g water per g dry pellet for the non-resin treated controls. The increase in milk protein hydration also contributed to a higher milk viscosity. Differential scanning calorimetry (DSC) showed calcium reduction decreased the denaturation temperature of alpha-lactalbumin and beta-lactoglobulin by ~3 and ~1 Celsius degree respectively. Overall, the work in Chapter 4 provides a route to produce calcium-reduced milk concentrate with potential in retentate-based dairy products with tailored functionality. The impacts of UF retentate addition and calcium reduction on the properties of the final cream cheese were then evaluated in Chapter 5. It was hypothesised that the properties of cream cheese made from UF retentate would significantly differ from those of the control cheese made from unconcentrated milk and the reduction of calcium by ion exchange would also affect the properties of cream cheese. In this work, cream cheese was made from UF concentrated milk (2.5- and 5-fold), treated with or without 2% (w/w) cation resin and the properties compared with a control cream cheese made from unconcentrated milk. The UF cheeses did not differ in protein, fat and moisture content from the control but had a higher calcium concentration if not treated with resin (~150 mg/100 g and ~230 mg/100 g for cheese made from non-calcium reduced VCF2.5 and VCF5 retentate respectively vs ~90 mg/100 g for cheese made from unconcentrated skim milk). The microstructure of the cheese made with UF without calcium reduction was more heterogeneous and porous than the control, consistent with a decreased hardness and thermal stability, providing new insights into the link between UF cream cheese microstructure and functional properties. The calcium reduction of ~20% induced by 2% (w/w) cation resin treatment prior to UF to VCF2.5 or VCF5 did not significantly affect the texture properties of the cheese formed compared to the non-resin treated counterparts but led to an increase in the size of the corpuscular structures found within the UF cheese. The concentration of free amino acids and peptides was highest in the cheese made with added UF retentate and decreased in the samples with reduced calcium, although not as low as the concentration in the control cream cheese, illustrating the potential to tune this property. This study improves our understanding of UF-produced cream cheese with differing calcium content and this knowledge may benefit future scale-up to industrial production. In conclusion, the work from this thesis broadly explored the impact of processing conditions including the concentration factor of milk, starter culture concentration and calcium concentration on the properties of the milk, acid gels and the final cream cheese product. These findings illustrate how milk solids concentration and calcium concentration can be systematically used to alter intermediate and final product properties and may be beneficial for industrial manufacturers to further optimize cream cheese production at different manufacturing stages when using concentrated milk to reduce acid whey production.