The effect of heat treatment on the front-face fluorescence spectrum of tryptophan in skim milk

The effect of heat treatment on tryptophan front-face fluorescence emission spectra of reconstituted skim milk was assessed. Samples were heat-treated from 70 to 100 °C at six holding times. Treating milk at 90 and 100 °C induced a decrease in fluorescence intensity (∼37 and ∼47 %) and a red shift (12 and 14 nm), respectively, with both parameters significantly affected at 99.9 % by temperature, time, and the interaction of both factors. Red shift, which could be attributed to whey protein denaturation and tryptophan exposure to increasing solvent polarity, was found to follow a quadratic trend as a function of holding time (from 0 to 60 min). The decrease of fluorescence intensity observed at those two temperatures and in the mentioned holding time range was also a quadratic function of the associated red shift. Emission intensity change at a single wavelength (∼340 nm) was partly caused by direct intensity decrease and indirectly due to the emission maximum shift. Tryptophan fluorescence allowed sample classification by PCA analysis according to heat load, i.e., temperature and time. These results established the bases for development of a cost-effective sensor based on simple, economic optical components for measurement of a few well-selected emission wavelengths, which associated with data processing, should allow inline control of milk heat treatment.