Abstract

The objective of this work is to clarify the structure of the near infrared (NIR) calibration equation for predicting the total aerobic bacteria count (TBC) of raw milk which was developed in a previous article. First, an experiment on the monitoring of chemical changes in stored raw milk having different levels of TBC was conducted. Then the relation between the regression coefficient plots of the TBC calibration equation and the absorption bands of the constituents related to bacterial metabolism was investigated. Finally, the meaning of each factor used in the calibration equation was interpreted through the study of its loading weight plots. It was found that lactic acid and urea–nitrogen, the waste from lactose and from protein catabolism of bacteria, increased with the increase of TBC value. On the other hand, the decreases in lactose and protein content, the energy sources of many bacteria, were observed. The results from the investigations on the regression coefficient and the loading weight plots indicated that the TBC calibration equation utilised the information from both the absorptions of the four chemicals and the absorptions of water species. It is believed that the changes in the metabolites and energy sources influenced the species and the NIR absorptivity of water. In addition, the band assignments in the regression coefficient plots have been performed. For example, the peak at 988 nm was linked to lactic acid absorption, the peak at 1008 nm peak was identified as due to urea–nitrogen absorption, and the 1026 nm and 1032 nm peaks were identified as being due to casein–protein absorptions. As for water, the 952 nm, 962 nm, 976 nm and 998 nm were likely to represent the fluctuation of water species with 0, 1, 2 and 3 hydrogen bonds, respectively.

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