CONICET | Buscador de Institutos y Recursos Humanos

Milk gels are the base ofdifferent solid and semi-solid dairy foods. The gelation by heat treatment of calciumfortified milk leads to milk coagulation and to formation of the so calledcalcium-induced skim milk gels or calcium milk coagulum. In this work, the tribo-rheologicaland physico-chemical characteristics of calcium-induced skim milk gels werestudied. Milk dispersions were prepared with different concentrations of skimmilk powder (SM) (10, 20 and 30% w/w) and stored overnight at 25 °C. The nextday, the samples were preheated at 90 °C for 10 min; this treatment increases convenientlythe gel strength. After that, calcium chloride was added to milk samples (30, 60 and 90 mmol kg-1). Then, the pH was readjusted to thenatural pH of milk of 6.66 and samples were again stored overnight al 25ºC. Toinduce gelation, skim milk suspensions were heated at 70 °C for 10 min. Thegelation process was analyzed by rheometry (monitoring the viscoelastic moduli)throughout temperature (from 25 °C to 80 °C) and time (at 70 °C for 590 s, at0.1% shear strain and 1 Hz frequency) sweeps. Two characteristic temperatureswere determined from temperature sweeps, the initial structuring temperature (IST) and the critical structuringtemperature (CST). For tribometric measurements, compliant elastomeric surfaceswere used to mimic the mechanical properties of the mouth. Frictionmeasurements were performed in a non-conforming ball-on-three-plates contact.The normal force was maintained at 1 N and the friction coefficient wasmeasured for increasing sliding speeds from 0.0447 to 940 mm s-1. Syneresiswas determined as the percentage (w/w) of whey expelled from the gel aftercentrifugation at 1100×g for 10 min at 10 °C. Then, the pellet wasrecentrifuged at 13500×g for 30 min at 10 °C (2nd centrifugation), then drained(10 min), weighed, frozen (at -20 °C) and lyophilized. The water holdingcapacity of the gel was calculated as the percentage (w/w) of pellet obtainedafter the 2nd centrifugation in the gel sample. Protein hydration wascalculated as the ratio of grams of water in the pellet after 2ndcentrifugation to grams of solids in the pellet after lyophilization. Theresults obtained by rheometry showed that gelation at temperatures lower than70 °C was observed in samples with 10% w/w of SM and 30 mmol kg-1 ofcalcium chloride or similar ratio SM/calcium concentrations, when the amount ofcalcium remaining in the serum phase is enough to induce gel formation.Structuring parameters confirmed these results. From time sweeps, it wasobserved that the kinetics of gelation depended on the composition. Gelsobtained using higher SM concentrations (20 and 30% w/w) showed better physicalproperties (low syneresis and high water holding capacity). Tribometric assaysshowed that higher SM and calcium concentrations improved the lubrication properties.Friction factors at 10 mm s−1 (typical speed in oral processing)decreased as the SM concentration increased. It is concluded thatcalcium-induced skim milk gels with different microstructure can be obtained byvarying the concentration of skim milk powder and calcium salt added.

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