Comparison of some physical techniques for detection of spoilage in apple juice inoculated with Saccharomyces cerevisiae: Optical and photothermal methods

Abstract

Several physical techniques were used to study the extent of spoilage in apple juice deliberately inoculated with yeast (concentration of Saccharomyces cerevisiae ranged from 25 cells mL−1 to 2.5 × 106 cells mL−1, respectively) and their performance compared in terms of detection limit achieved. The optical methods used in this investigation rely on the measurement of either absorption [as is the case for classical spectrophotometry (SP) and the so called optothermal window (OW), a variant of a photothermal method], or scattering [examples are turbidimetry (TB), laser scattering (SC), and laser speckle fluctuation (SF)]. It is shown that the presence of yeast increases both optical absorption and scattering. The most favorable detection limit (25 cells mL−1) and a highest (nearly 104) dynamic range, combined with a good linearity, were obtained with the experimental set‐up for SC. In addition, the extent of correlation between different methods was determined using two markedly different reference substances, i.e., (i) the mixture of apple and blackcurrant juices, representing a strongly absorbing sample, and (ii) diluted (dilution factor of 103) milk as a strong scatterer. Finally, one has monitored the progress of a spontaneous spoilage process in the inoculated juices stored at 5°C under aerobic conditions.