Drainage of acid-rennet gels: Rheological and microstructural study of milk heat treatment and milk proteins/whey protein aggregates ratio effects

The effect of milk heat treatment (lowH: 72 degrees C/20 s; highH: 95 degrees C/120 s) and of substitution of part of milk proteins by whey protein aggregates (WPA) on the drainage of acid-rennet gels was investigated. Gels produced at 25 degrees C and pH 4.6 were stirred and drained by centrifugation or drained unstirred on a drainage pilot for 24 h. The kinetics of gelation and microstructure of stirred and drained gels were studied by rheology and confocal microscopy with image analysis. The highH treatment slowed down milk gelation and led to gels with slightly higher moduli, hindered the gel syneresis and the drainage and led to drained gels with a lower fractal dimension and smaller pore and network strand sizes than lowH treatment. Substitution of milk proteins by WPA led to softer and more liquid-like gels and slightly reduced the gel drainage as compared to untreated milk. Producing whey protein particles in situ in milk by the highH treatment had not the same effect than adding exogenous WPA on the production of acid-rennet drained gels.