Characterization of lactic acid bacteria as spoilage and their effects on the shelf life of minimally processed vegetables

Abstract

The increasing demand for fresh vegetables and for convenience foods is causing an expansion of the market share for minimally processed vegetables (MPV). The new technologies for processing and packaging has made possible to obtain a product ready to serve. Nevertheless, the associated risk with pathogens and loss of quality due to microbial spoilage seems to be involved. Low refrigeration temperatures restrain the growth of spoilage microorganisms while the partial or complete exclusion of oxygen inhibits the proliferation of Gram negative bacteria frequently isolated from spoiled products whilst favoring the growth of Gram positive such as lactic acid bacteria (LAB). Considerable levels of acidification, emission of volatile organic compounds, slime formation have been associated with their metabolic activity as spoilage properties. Recently, microbial spoilage characterized by gas and slime formation in vegetable products became a main concern of the manufacturer. On these bases, this study aims to establish the potential spoilage LAB of vegetable origin and evaluate their effects on the physical-chemical and sensory properties of MPV packaged under aerobic conditions at 4 °C for 15 days. The production of exopolysaccharide, gas, biogenic amines and organic acids from LAB of vegetable origin (19 strains) was qualitatively determined. In order to have a global view the useful features to interpret the LAB spoilage capacity a multiple correspondence analysis was applied. Leuconostoc (L.) mesenteroides CRL950, CRL742 and L. citreum CRL1904 were selected for presenting the highest amount of spoilage characteristics assayed. Carrots or cabbage were washed and cut into thin strips. Samples were inoculated with each strain separately (104-105 CFU/mL) packed in aerobic conditions and incubated at 4 °C during 15 days. Microbiological counts, pH, and color were evaluated at regular intervals. In both refrigerated vegetables the spoilage strains were able to grow reaching a count of ⁓109 CFU/mL at the end of the incubation period while the native microbiota slightly exceeded 107 CFU/mL. The pH slightly decreased in the un-inoculated samples and the greatest drop was observed in the samples treated with L. mesent. CRL742. In addition, total color difference (ΔE) was calculated by using L*, a* and b* values of days 0 and 15. Pronounced ΔE were detected for inoculated samples with L. mesent. CRL950, CRL742 and L. citreum CRL1904 (9,19; 9,57; 9,13 in carrots and 9,05; 11,37; 15,58 in cabbage). Control samples maintained a similar visual color of the vegetables with ΔE values of 2.97 and 4.34 in carrots and cabbage, respectively. Species, belonging to the genera Leuconostoc were the main spoilage, being able to acidify and change the color of refrigerated vegetables, causing their early deterioration. The findings suggest the need of the microbiological control of the MPV ready-to-use to assure their quality.