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

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).