Evaluation of shelf life of packaged salad

Abstract (Swedish)

Since the demand on ready-to-eat (RTE) and raw, minimally processed products is high and increasing in today´s market, the microbial quality is important. One of these products is RTE salads and salads mixes that are bagged and since it is supposed to be eaten raw without any step to reduce the microbial load, these types of products are considered to be risk foods and that the risk of foodborne outbreaks may be high. From time to time, there are recalls of packaged salads due to presence of bacteria that can cause foodborne illness, and during the last few years, an increasing amount of vegetable-derived foodborne illness outbreaks have been recorded. Because of this, the microbial quality must be considered in terms of public health concern. Three series of RTE salad with 18 bags each were purchased from local supermarkets. These series microbial load was examined according to standard methods using TSA and VRBD plates to enumerate total culturable bacterial concentration and Enterobacteriaceae concentration throughout the expiration time in storage of 4˚C. All bags tested had a total bacteria concentration between 6,3-7,7 log CFU/g and an Enterobacteriaceae concentration between 2,0-4,3 log CFU/g. None of the bags tested positive for Escherichia coli. One series of arugula were inoculated with a strain of E. coli originated from salad and stored in 4˚C. The E. coli concentration showed no increase or decrease during the expiration time. No regulations regarding total bacteria concentration or Enterobacteriaceae concentrations exist but regarding to the high number in consideration to public health concern, potential negative effects must be examined more extensionally.

Popular Abstract

The all year around available ready-to-eat salad are increasing in popularity, and parallel with this, vegetable-derived foodborne illness outbreaks as well. Can a too long shelf life for these products be part of the problem?
During recent years the focus on healthy diets and lifestyles has increased. Vegetables and fruits are important to include in meals and are a part in many modern health diets. Ready-to-eat (RTE) salad is one of the RTE products that has increased in popularity during the last decades. Ready to eat salads are minimally processed and are supposed to be eaten raw. The benefit with the RTE products is that it requires minimal or no preparations for the consumers before the product can be consumed. A lot of people consider the RTE salad as a comfortable and time saving way to add vegetables and nutrients to meals. In the supermarkets there is a wide range of variations of RTE salads and mixes, and it continue to grow. In the same time as these products and trends of eating more vegetables has increased, the amount of recorded vegetable-derived foodborne illness outbreaks has increased as well.
Considering the fact that the product is consumed raw the production chain and storage conditions are critical for the safety of the product. The product can be contaminated during both pre-harvest, for example through irrigation water and/or during post-harvest for example through human handling and equipment. No steps in the production or preparation from the consumers at home eliminates possible contaminations. Correct storage temperature is therefore one of the most critical conditions to ensure a safe product.
This study has investigated the safety of RTE salad from two producers in Scania, Sweden. The study consisted in two parts, where the first part investigated the safety of the product both according to findings of the different kinds of present microorganisms on the salad as well as the total bacteria and Enterobacteriaceae load throughout the expiration time of 9 days. Secondly, a strain of E. coli derived from salad were inoculated to Spinach to investigate the faith of this strain in the presence of RTE salad and storage temperature of 4˚C, to be able to know the behaviour if a real contamination with E. coli would take place. 3 series with 18 bags in each were used for the investigation of the shelf life and 1 series were used for the investigation of the E. coli behaviour.
After analysing colonies from the salad, it could be concluded that in neither of the bags E. coli, which is an index for the sanitary conditions of the product, were found. Neither were L.momocytogenes or salmonella found in any of the bags which is an index for the safety of the product. The absent of these bacteria indicates that the hygiene and safety of the product are acceptable. However, in all of the bags there were findings of a high bacteria content, where a lot of these are possible opportunistic pathogens, which means they can cause diseases at hosts with a low resistance such as young, elderly or people with a weaken immune system. Most of the bags tested also shows tendencies to have an increasing amount of total bacterial content throughout the expiration days which indicates that some of the bacteria present on RTE salads can continue to grow at the storage temperature of 4˚C. For the meantime there is no mandatory control or limit for the total bacteria content or Enterobacteriaceae content for RTE salad, but considering the high levels in most bags, a reference limit for these criteria could be of benefit for the consumers. The study also shows that more effort must be put into investigate the safety of the product for all possible consumers, from healthy to sick, from young to elderly people, as well as ways to make the product safer.
The investigation of the survival of E. coli in RTE salad in storage temperature showed that the specific strain used, which originated from salad, had a high survival level throughout the whole expiration time. Since the concentration neither decreased or increased significantly it is important that the product never get contaminated anywhere at the production chain, since chilled temperature is the most important environmental criteria that keeps the product safe, but it does not lower the E. coli level if it is already contaminated.

• author: Paulsson, Jennifer
• supervisor: Elisabeth Uhlig; Åsa Håkansson
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLGM10 20181
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: food technology, livsmedelsteknologi
• language: English
• id: 8965105
• date added to LUP: 2019-04-11 14:59:19
• date last changed: 2019-04-11 14:59:19