Bacterial population dynamics and sensorial quality loss in modified atmosphere packed fresh-cut iceberg lettuce

Abstract

The end of shelf life of fresh-cut iceberg lettuce packed in modified atmosphere (MA) is determined by its visual quality and by its microbial load. The total microbial load should not exceed 6–7 log CFUg−1 mesophilic counting and be free of human pathogens. Not much is known about specific interactions between the levels of specific spoilage micro-organisms and the sensorial quality attributes of the fresh-cut product. We studied the changes in sensorial quality attributes and the growth and population dynamics of microorganisms in three independent batches of MA packed fresh-cut iceberg lettuce stored at 7°C.The sensorial quality of the fresh-cut product was daily evaluated by visual determination of color, shape (indication of crispness) and discoloration (browning); together these attributes were translated into an Overall Visual Quality (OVQ) value. In addition, off-odour and sourness production were scored immediately after opening of the bags. Samples of the fresh-cut product were taken to determine the total microbial load and to investigate the bacterial species composition using mass sequencing analysis.The decrease of OVQ over time showed a down sigmoid curve, reaching the limit of consumer acceptance after about eight days. Total bacterial counts increased from about 5 log at day 1 to about 8 log colony forming units (CFU) per gram at day 7. Initially, Pseudomonas species were dominant but when the package became anaerobic, lactic acid bacteria (LAB), in particular Leuconostoc spp. and Lactococcus spp. became more abundant and dominated the bacterial population. Especially the rapid growth of Leuconostoc species correlated with development of a sour off-odour, represented by the accumulation of acetic and lactic acids in the product and the decrease in OVQ. Results indicate that metabolites produced by LAB are responsible for off-odour production and loss of sensorial quality.Based on this knowledge, new strategies for shelf-life improvement may be designed to prevent the dominance of LAB in fresh cut lettuce or make shelf-life better predictable based on initial counts of LAB, that may assist the industry and retail to reduce food waste.