Institute in scientific research in foods (ICTA/UFRGS), Porto Alegre, Brazil

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Samenvatting eindwerk 2014-2015: Growth evaluation of Salmonella Enteritidis SE86 and Escherichia coli ATCC8739 on lettuce (Lactuca Sativa L.) during two days of shelf-life at 5°C, 10°C, 25°C and 37°C

Lettuce is one of the oldest leafy greens cultivated around the world, and is one of the most consumed vegetables due to its availability, cost and nutritional factors. In Brazil about 40% of the volume that is traded in fresh produce supply companies is lettuce. To consume the lettuce safely, several critical control points of the different processes need to be run through, checked and approved. The inadequate control of these points can concern the appearance of micro-organisms, such as Salmonella Enteritidis and E. coli.

S. Enteritidis and E. coli are some of the most common micro-organisms responsible for foodborne outbreaks involving vegetables and fruits. The mesophilic bacteria represent the normal flora of the vegetables.

The aim of this study is to evaluate the growth of S. Enteritidis and E. coli on lettuce held at different temperatures. Therefore, two parameters were considered important to study the behaviour of these bacteria, namely the temperature and the time of growth. The growth of these micro-organisms on lettuce was investigated during a shelf-life of two days and at 5°C, 10°C, 25°C and 37°C.

In the first part of this study, the lettuce was prepared and inoculated with Salmonella and E. coli, for examination of the mesophilic bacteria the lettuce was not inoculated to obtain the results of the normal flora. Then the lettuce was incubated at the different temperatures for different incubation times (0, 2, 6, 24 and 48 hours).

When the incubation time was completed, the second part of the study starts: the analysis of the lettuce. For the counting of the bacteria, 10 g of lettuce were put inside a sterile plastic bag and buffered peptone water was added. After that, the plastic bag was homogenised using the stomacher-technique. Furthermore, dilutions were made of this solution and plated by the droplet method on Xylose-Lysine-Desoxycholate agar, Chromocult Coliform agar or Nutrient Agar plates. These plates were incubated at 37°C for 18-24 hours. Afterwards, the plates were counted and the results were statistically analysed by SPSS (version 22).

After performing the statistical analyse, results show that the growth of Salmonella and the mesophiles stagnates, at the temperature of 5°C, and the growth of E. coli decreases. Furthermore, at 10°C the growth of Salmonella, E. coli and the mesophilic bacteria re-mained steady-state and at 25°C both inoculated micro-organisms growed similar in time. Reaching the optimum temperature of the bacteria (37°C), there was an increase of growth until 24 hours and afterwards continues with a reduction of the growth until the point of 48 hours.

The influence of different temperatures on the growth of two micro-organisms was demonstrated in this study, and there can be concluded that it is better to keep the lettuce under a temperature of 10°C in order to avoid bacterial growth. Furthermore, the consumers and foodservices are suggested to disinfect the lettuce before consumption or serving and to maintain the lettuce under refrigerated temperature below 10°C.