Microbiological risk assessment of fresh produce grown in Canterbury, New Zealand

Abstract

A recent trend to consume healthy foods has led to greater preference for natural food and raw vegetables. At the same time, concerns have increased about the microbiological risks of fresh produce. This thesis focusses on the microbiological risk assessment of fresh produce grown in organic and conventional farms and those sold by different retail shops in Christchurch. The incidence of foodborne outbreaks for fresh produce is estimated as 22.8% of the total foodborne illnesses in the USA and approximately 10% of cases in NZ. To achieve microbiological food safety of produce, it is important to identify the current microbiological status of fresh produce. Many people prefere organic produce these days but there may be some risks associated with organic produce because many organic growers use animal faeces (manure) as an alternative to chemical fertilizer. Organic farms use different types of soil enrichment such as compost, manure and crop-livestock rotation farming. Crop-livestock rotation farms rotate land for crop farming and livestock farming every three to four years and do not require chemical fertilizers. There is little data about microbiological assessment comparing these different types of soil enrichments. Recent trends and factors affecting the microbiological contamination of fresh produce are reviewed in the first chapter, focussing on: 1) microbial contamination of fresh produce; 2) frequency of fresh produce related foodborne outbreaks; and 3) factors affecting microbial status of fresh produce. The second and third chapters investigate two main objectives: microbiological risk assessment of fresh produce grown in organic and conventional farms with different soil enrichment systems in Canterbury, New Zealand and microbiological risk assessment of fresh produce from different retail shops in Christchurch. The results of farms in this study showed that over a six week sampling period, there were significant differences (p<0.05) between the conventional farm vegetable samples and those of the organic farms for the average APC, coliform, yeast and mould and Staph. aureus, between the compost farm vegetable samples and those of the conventional farm and crop-livestock rotation farm for the average E. coli, and between the conventional farm silver beet samples and those of crop-livestock rotation farm for the E. coli. The fresh produce samples collected from different farms in Canterbury, New Zealand and acceptable levels for E. coli O157:H7 (not detected) but not for APC, coliform, E. coli, Staph. aureus or Salmonella spp. For APC, 88% samples had unsatisfactory levels for the organic farms and that of the conventional farm was 33%. For coliform, 92% samples had unsatisfactory levels for the organic farms and that of the conventional farm was 61%. For E. coli, 92% samples had unsatisfactory levels for the organic farms and that of the conventional farm was 88%. For Staph. aureus, 21% samples had satisfactory levels for the organic farms and that of the conventional farm was 50% based on the FSANZ (2001) or PHLS (2000) guidelines for ready-to-eat foods. The yeast and mould levels (1.78 – 6.10 log10CFU/ml) were similar to studies previously reported for samples purchased in retail shops. Most Staphylococcus aureus and Salmonella spp. positive strains from farms and retail shops showed higher resistance to β-lactams. Similarly, the results of retail shops showed that over a six week sampling period, ppercent of unsatisfactory levels as per the guidelines given by FSANZ (2001) or PHLS (2000) varied within a retail shop. For APC, 75-87.5% samples had unsatisfactory levels and for coliform, 75-91.7% samples had unsatisfactory levels. For E. coli, 80-100% samples had unsatisfactory levels, and for Staph. aureus, 17.4-21.7% samples had unsatisfactory levels. However, these results and the yeast and mould (0-5.23 lpg10CFU/ml) were similar to studies previously for samples purchased in retail shops, except for E. coli (83.3-100% of samples had E. coli). Vegetables from supermarket were less contaminated with E. coli than samples from other retail shops. However, retail shop B had the highest unsatisfactory levels of APC (87.5%), coliform (91.7%), E. coli (100%) and Staph. aureus (21.7%) based on the the FSANZ (2001) or PHLS (2000) guidelines for ready-to-eat foods. Eight percent of samples from four different retail shops were positive for Salmonella spp. but no E.coli O157 H7 was detected. In conclusion, the extent of microbiological safety differed between shops. In addition, there was a tendency that the potential risks of fresh produce in some retail shops were identified. Between farms, the fresh produce collected from the organic farms was more contaminated with some microbes than produce from the conventional farm.