Water quality of citrus dip tanks and the impact of Salmonella species in citrus export chain

Abstract

Citrus is grown in almost all countries with a sub-tropical or tropical climate. Africa produces about 7% of global citrus production representing 67 362 564 tonnes in the 2003-04 season. Of these countries, South Africa is the most important citrus producer with the bulk of its fruit being exported to mainly European countries. Although South Africa is the world’s fourteenth biggest producer, it is currently ranked third in terms of global export volumes. An increase in the number of foodborne disease outbreaks over the past few decades has been related to the shift towards consumption of fresh produce and growing world trade. Other factors include changing demographic profiles, changing farm practises, extensive distribution and handling networks, increased consumption of unprocessed products, emergence of new pathogens and more effective detection methods. Shigella spp., Listeria monocytogenes, Hepatitis virus, Caliciviridae virus, Salmonella spp., Escherichia coli and Vibrio cholera are some of the microorganisms implicated in disease outbreaks caused by the consumption of fresh produce. This dissertation focuses on citrus fruit safety and a selected foodborne pathogen, Salmonella enterica serovar Typhimurium. Salmonella has been implicated in several outbreaks associated with the consumption of unpasteurized orange juice, mangoes, cantaloupes, sprouts and tomatoes. S. enterica sv. Typhimurium is also a water-borne pathogen, and its importance in agricultural irrigation and packhouse wash water was also studied. The presence of S. enterica sv. Typhimurium in biofilms in packhouse water and on fruit was finally investigated since biofilms are known to harbour a number of these pathogens. Biofilm formation was monitored in the warm water dip tank using a photo acoustic monitoring device and confirmed by scanning electron microscopy and enumeration. Water sources, i.e. bathroom, handwash stations, irrigation water and warm water baths had high total viable bacterial counts and faecalassociated contamination. Biofilms also formed within the packhouse dip tank at an increased rate. The Polymerase Chain Reaction (PCR) was used to investigate the presence of Salmonella in the citrus export chain. No Salmonella spp. could be detected. S. enterica sv. Typhimurium survived for up to four weeks on the surface of citrus fruit under simulated export conditions. However, Salmonella is not able to survive uninterrupted cold storage conditions for a period longer than two or three weeks. Scanning Electron Micrographs showed that Salmonella is capable of attaching to and colonizing the citrus fruit surface and is able to be incorporated into existing biofilms in warm water tanks of packhouses. This study showed that Salmonella can thrive in packhouse water and potentially be a continuous source of contamination for fruit moving through the infected warm water dip tank. Everyone participating in fruit production, storage and post-harvest handling of fresh fruit should be involved in ensuring quality and safety, since events in the orchard can affect storage life, and incorrect handling during marketing may nullify all previous efforts of quality maintenance. To minimize the occurrence of these pathogens in the fruit chain from production to consumption, several basic good agricultural practices and food safety systems needs to be incorporated and managed correctly.