Evolution of microbial dynamics and headspace volatile profiles for predicting the freshness-to-spoilage transition...

The present study investigates whether changes in headspace volatile organic compounds (VOCs) of refrigerated cooked meatballs (beef and pork) can predict the transition from freshness-to-spoilage driven by microbial growth. Hence, freshly prepared food samples were portioned into air-tight glass containers, stored at 6 °C, and analysed at regular intervals. Microbial growth was determined using culture-based techniques, while headspace VOCs were trapped on Tenax TA sorbent and analysed by thermal desorption heart-cut gas chromatography with flame ionisation and mass spectrometric detectors. Based on total viable counts (TVC) and the existing microbiological criteria, three food quality stages were defined: satisfactory (≤ 4 log CFU/g), acceptable (< 6 log CFU/g), and unacceptable (≥ 6 log CFU/g). All samples remained in a satisfactory stage up to 3 days, entered the acceptable region from days 4-5, and reached the unacceptable zone from day 6. TVC showed a significant positive correlation with all the microbial groups (r > 0.90, p < 0.0001). VOC profiles of fresh samples were dominated by aldehydes emitted by cooking, whereas spoiled samples contained microbially derived alcohols, acetals, esters, and sulfides. Particularly, 1,1-diethoxyethane was associated with TVC, LAB, psychrotrophs, yeasts and moulds, whereas 1-hexanol, 2,3-butanediol, sulfides and other VOCs were primarily associated with Pseudomonas. Ratios of aldehydes/1,1-diethoxyethane and aldehydes/2,3-butanediol changed dramatically from days 3-6 (Z = -22.43, p < 0.001; Z = 6.52, p < 0.001, respectively), thus emerging as potential markers indicating imminent spoilage. In contrast, hexanal/1-hexanol ratio differentiated spoilage progression from days 9-12 (Z = 52.72, p < 0.001).