Biological relevance of VOCs emanating from red onions infected with Erwinia (Pectobacterium) carotovora under different storage conditions

Abstract

Volatile organic compounds (VOCs) are generated in different commodities through various biosynthetic pathways. They act as biomarkers for different infections and metabolisms and can be instrumental in the development of biosensors for monitoring the health of stored horticultural commodities. Healthy onions and samples infected with Pectobacter Erwinia carotovora were stored at different conditions (4 °C, 60 %; 8 °C, 65 %; 15 °C, 75 %; 25 °C, 60–80 %) for 4 weeks and the VOCs were mapped using SPME-GC–MS. The data were analyzed using multivariate data analysis techniques like Principal Component Analysis (PCA), and Partial Least-Squares Discriminant Analysis (PLS-DA), and visualized using a cluster heat map. The number of VOCs was higher in the infected samples, which increased over the storage duration. The predominant class of VOCs at 25 °C are mostly ester and sulfur; and at 15 °C, alkane and ketone groups of VOCs contributed more, during the initial phase of infection. At a lower temperature of 8 °C bacterial metabolic VOCs belonging to alcohol or aldehyde groups could be observed. On the other hand at 4 °C, since the microbial activity is retarded, no such compounds were found. With storage and extent of spoilage, the predominant VOCs belonged to acids and multi-functional group variants for lower temperatures. VOC profile was found to be correlated with storage conditions as well as periods that ultimately had an impact on Erwinia carotovora secondary metabolism. Except for the 4 °C storage temperature, PCA could distinguish and separate the VOCs into clusters for the classification of infected and non-infected samples; however, PLS-DA performed better separation and distinction of VOCs among the groups for all the storage conditions. The dominance of the VOCs with respect to sample type, storage condition, and storage duration was represented through the clustered heat maps.