Demineralised skim milk concentrates by means of dynamic cross-flow microfiltration

At ambient temperature and native pH of milk, approx. 66 % of milk containing calcium is bound onto the casein micelles (micellar calcium), whereas approx. 34 % is in the serum phase presented as free serum calcium (Koutina et al. 2014). In membrane filtration processes such as microfiltration (nominal pore size 0.1 µm), the casein fraction is retained so that micellar calcium is enriched in the retentate and is subsequently in the final product. Micellar calcium can be solubilised by reducing pH and/or temperature. This can be applied to reduce the calcium content of the retentate via membrane fractionation. However, under certain temperature and pH combinations, casein micelles change from sol to the gel state and an enhanced gel layer is built up on the membrane surface and flux decreases rapidly (Brandsma & Rizvi 1999). We combined small amplitude oscillation shear rheology and photon correlation spectroscopy to examine the sol-gel-transition behaviour of pasteurised skim milk (protein content = 3.4 %) and microfiltrated (nominal pore size = 0.1 µm) skim milk retentates (protein content = 6 to 12 %) between pH 4.6 and 6.8 at temperatures ranging from 1 to 65 °C. The aim of this study was to predict pH-temperature-protein content combinations for membrane separation while maintaining adequate flux to get skim milk retentates with defined calcium content without macroscopic aggregated casein micelles. To proof the concept filtration experiments comparing appropriate and unappropriate pH-temperature combinations to get particle free calcium degraded skim milk retentates by means of a novel dynamic cross-microfiltration (nominal pore size: 0.06 and 0.2 µm) were carried out. Results will be shown and discussed. [1] Brandsma, R. L.; Rizvi, S.S.H. (1999): Depletion of Whey Proteins and Calcium by Microfiltration of Acidified Skim Milk Prior to Cheese Making. In: Journal of Dairy Science 82 (10), p. 2063–2069. [2] Koutina, G.; Knudsen, J. C.; Andersen, U.; Skibsted, L. H. (2014): Temperature effect on calcium and phosphorus equilibria in relation to gel formation during acidification of skim milk. In: International Dairy Journal 36 (1), p. 65–73

Tensile Testing to Quantitate the Anisotropy and Strain Hardening of Mozzarella Cheese

We explored anisotropy of mozzarella cheese: its presence is debated in the literature. Tensile testing proved a good method because the location and mode of failure were clear. Mozzarella cheese cut direct from the block showed no significant anisotropy, though confocal microscopy showed good structure alignment at a microscale. Deliberately elongated mozzarella cheese showed strong anisotropy with tensile strength in the elongation or fibre direction ∼3.5× that perpendicular to the fibres. Temperature of elongation had a marked impact on anisotropy with maximum anisotropy after elongation at 70 °C. We suggest the disagreement on anisotropy in the literature is related to the method of packing the mozzarella cheese into a block after the stretching stage of manufacture. Tensile stress/strain curves in the fibre direction showed marked strain hardening with modulus just before fracture ∼2.1× that of the initial sample, but no strain hardening was found perpendicular to the fibre direction