Food waste is often driven by consumer uncertainty about the spoilage of stored food, especially for cooked meal leftovers where microbial growth is the main concern. We analyzed whether metal oxide semiconductor (MOS) gas sensors placed inside ordinary food containers can monitor the edibility of leftovers, specifically cooked meatballs. Sensors were operated using temperature cycling to enhance selectivity, and cycle-aligned features were extracted. A prior calibration campaign produced information used to map cycle-aligned features into estimated gas concentrations for relevant VOCs. Total viable counts, which represent the growth of total number of spoilage microorganisms, were analyzed on days 0, 5 and 7 to determine the food’s freshness. Both the raw sensor features and the calibration-derived gas concentration estimates were analyzed with principal component analysis (PCA) and evaluated with a leave-one-sensor-out (LOSO) binary classifier for multiple food containers. PCA on the calibrated gas estimates revealed a dominant axis that consistently tracks food degradation over time across various containers. LOSO classification accuracy improved from 81.7% using raw sensor features to 87.8% using calibrated gas concentration estimates. These findings represent a proof of principle that calibrated MOS sensor systems can robustly support in situ edibility assessment for cooked food.

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).