Predicting lactulose concentration in heat-treated reconstituted skim milk powder using front-face fluorescence

• Lactulose kinetics in milk under different heat treatment intensities were studied.
• A significant correlation of fluorescent markers and lactulose was obtained.
• Lactulose concentration was predicted in heated milk using fluorescence markers.

Industrial processing of milk and dairy foods requires heat treatment, which induces undesirable effects in milk such as protein denaturation, organoleptic and nutritional properties alteration, browning, etc. The hypothesis of the present study is that it is possible to use front-face measurement of fluorescent markers such as tryptophan, dityrosine, Maillard intermediate compounds and riboflavin as an inline, cheap, easy and fast way to evaluate lactulose formation in milk. Thus, the specific objective of this study was modeling the kinetics of lactulose formation during heat treatment of milk as well as evaluating the correlation of lactulose concentration with fluorescent markers in order to obtain prediction models of lactulose concentration in heat-treated milk based on front-face fluorescence.Three replicates of the study were performed at three different temperatures (80, 90 and 100 °C) and six different times (0, 10, 15, 30, 45 and 60 min). Standard, low-heat, spray-dried skim milk powder, supplied by Chr. Hansen SL (Barcelona, Spain) was reconstituted at 12% and used in all trials to ensure consistency among the various experimental trials.Lactulose concentration was significantly correlated (P < 0.001) with fluorescence intensity of different markers such as maximum intensity and maximum emission wavelength of tryptophan as well as the fluorescence intensity of riboflavin. Fluorescence changes associated to thermal treatment of milk were characterized and a series of fluorescent markers were identified as predictors allowing prediction of lactulose concentration with an R2 of 0.91 at a temperature of 90 °C.