Studies of the acid gelation of milk using ultrasonic spectroscopy and diffusing wave spectroscopy

Processes involved in the coagulation of heated and unheated skim milk during acidification by glucono-delta-lactone have been studied by a combination of ultrasonic spectroscopy and forward-scattering diffusing wave spectrometry (DWS). Ultrasonic velocity and attenuation seem to be governed by the changes in the casein micellar structure as the acid dissolves the micellar calcium phosphate. Both parameters show only small effects of gelation. The DWS shows that there are large differences between heated and unheated milks, especially in respect of the photon mean free path in the milks before coagulation has occurred. Measurement of the particle sizes by this technique shows that slow aggregation occurs at pH values considerably higher than those at which the full gelation of the casein micelles is seen to occur. The general conclusion is that the process of acidification, especially in heated milks, may be less simple than had previously been supposed. (C) 2003 Elsevier Ltd. All rights reserved.