Deterioration Profile of Postharvest Onion (Allium cepa L.) Bulbs Induced by Potential Pathogenic Microorganisms

Abstract

Onion (Allium cepa L.) is a highly nutritive vegetable with about 2 million metric tons grown annually in Nigeria, but the majority is lost to postharvest spoilage, especially through microbial infection. In this study, we identified bacteria and fungi associated with postharvest spoilage in onion bulbs and determined the pathogenicity of the bacterial isolates. Two weeks stored onion bulbs were purchased at a market in Ile-Ife, rinsed in 5% HOCL and aseptically cut into seven sections each. The fourteen sections obtained were swabbed daily with sterile cotton-tipped applicators for seven days. The swabs were streaked onto the surface of Nutrient Agar (NA) and selective/differential media plates for the isolation of bacteria, and Potato Dextrose Agar (PDA) plates for the cultivation of fungi. The bacterial plates were incubated at 37°C for 24 hours, while the fungal plates were incubated at 25°C for 5 days. The isolates were identified based on standard microbiological methods. Pathogenicity tests of the bacterial isolates from each of the genera was carried out by re-inoculation on the inner tissues of fresh onion bulbs that have been cleaned with 1% NaOCL, an uninoculated onion bulb served as the control. Thirty-five (35) bacterial isolates belonging to four different genera were identified, which included; 11 (31.4%) Staphylococcus spp., 9 (25.7%) Micrococcus spp., 8 (22.9%) Bacillus spp. and 7 (20%) Flavobacterium spp. Seven (7) fungal isolates were identified which included; 5 (71.4%) Aspergillus fumigatus, 1 (14.3%) Gibellula suffulta and 1 (14.3%) Hirsutella saussueri. Pathogenicity tests revealed that all the bacterial isolates were able to cause rot in onion in comparison with the control which had no observable rot; Flavobacterium spp. (28 mm) was the most pathogenic, while Micrococcus spp. was the least pathogenic (14 mm) based on the diameter of rot formation within seven days. These findings revealed that spoilage microorganisms can cause onion rot, hence, onions already showing contamination symptoms should be separated from fresh ones to avoid cross-contamination, while adequate care should be taken before consumption of onion to avoid foodborne illnesses and diseases.

A comprehensive review on recent advances in postharvest treatment, storage, and quality evaluation of onion (Allium cepa): Current status, and challenges

“…The rotting of onions caused due to improper handling, curing, drying and unhygienic storage conditions can lead to contamination of the whole produce and increase storage and handling losses (Nega et al., 2015). Besides, the attack of pathogens such as fungi and bacteria in the field, transportation, and storage can reduce the shelf life of onion bulbs (Khalifa et al., 2016; Torimiro et al., 2020). The spoilage of onions can happen through the soil, air, excessive irrigation, excessive nitrogen fertilization, diseased leaves, insects, moist conditions at harvest, mechanical injury, infected storage structures, etc (Abd‐Alla et al., 2011; Schroeder et al., 2012).…”

“…The spoilage of onions can happen through the soil, air, excessive irrigation, excessive nitrogen fertilization, diseased leaves, insects, moist conditions at harvest, mechanical injury, infected storage structures, etc (Abd‐Alla et al., 2011; Schroeder et al., 2012). Visible symptoms of the infestation are not shown until the pathogens have grown vigorously in conditions suitable for them (Torimiro et al., 2020).…”

“…Additionally, phenolic compounds contribute to antioxidant effects. Despite a moderate vitamin C content, onions play a crucial role in fostering overall health when included in a well‐balanced diet (Torimiro et al., 2020).…”

“…Onion shelf life is influenced by factors like soil, cultivar, climate, fertilizer, management, maturity, pre‐harvest and postharvest treatments, and storage conditions (Lawande & Tripathi, 2019; Torimiro et al., 2020). Up to 40% postharvest losses result from improper handling, sprouting, rotting, unscientific storage, and lack of infrastructure, with microbial spoilage contributing 15%–20% (Anbukkarasi et al., 2013).…”