The sol–gel transition temperature of skim milk concentrated by microfiltration as affected by pH and protein content

Abstract

This study aimed at determining sol–gel transition temperatures of microfiltered skim milk retentates for different protein levels (6, 8 and 10% (w/w)) and a wide pH range (native pH to 4.6) by means of small‐amplitude oscillatory shear rheology. For a pH of 5.4 to 5.0, the sol–gel transition temperatures decreased significantly with increasing protein content, which did not differ for pH 4.8 and 4.6. The sol–gel transition temperatures of retentates with 10% (w/w) protein at pH 5.4 and 4.6 were 58.4 °C and 10.9 °C, respectively.

Design and characterization of casein–whey protein suspensions via the pH–temperature-route for application in extrusion-based 3D-Printing

“…heating time, -temperature and -pH). At all protein contents, sol-gel transition temperatures decreased significantly with decreasing acidification pH, as observed elsewhere for microfiltrated skim milk retentates (Schäfer et al, 2018), rehydrated phosphocaseinate powders (Thomar & Nicolai, 2016), reconstituted pasteurised skim milk (Vasbinder & De Kruif, 2003b) and micellar casein suspensions with added whey protein isolate (Kharlamova et al, 2019).…”

“…Cold acidification was performed dropwise with citric acid under agitation. After decreasing the pH (5.4-4.8), the solution was equilibrated for 30 min and the pH was readjusted, if changes occurred (Schäfer et al, 2018;Nöbel et al, 2018).…”

“…During measurements the Storage Modulus (G'), the Loss Modulus (G''), the Phase Angle (tan δ) and the temperature were monitored. The sol-gel transition temperature was determined when G' reached a value of 1 Pa (Schäfer et al, 2018.…”

“…The sol-gel transition temperatures were obtained by temperature sweeps from the point when the Storage Modulus G' equalled 1 Pa (Schäfer et al, 2018. This value could be further used to adjust the temperature in the nozzle during the printing process to intentionally induce a sol-gel transition.…”

“…Cold pre-acidification of milk up to pH 4.6 at less than 4 °C reduced the hydrophobic interactions and helped to maintain solution (sol)-characteristics. Subsequent heating of the acidified material caused collision and aggregation of the CM and gelation occurred (Roefs, 1986;Schäfer et al, 2018). Vasbinder et al (Vasbinder, Rollema, Bot, & De Kruif, 2003a) also found that firmer gels were obtained via the pH-T-route compared to the T-pH-route at the same concentration.…”