The Hurdle Approach-A Holistic Concept for Controlling Food Safety Risks Associated With Pathogenic Bacterial Contamination of Leafy Green Vegetables. A Review

Abstract

Consumers appreciate leafy green vegetables such as baby leaves for their convenience and wholesomeness and for adding a variety of tastes and colors to their plate. In Western cuisine, leafy green vegetables are usually eaten fresh and raw, with no step in the long chain from seed to consumption where potentially harmful microorganisms could be completely eliminated, e.g., through heating. A concerning trend in recent years is disease outbreaks caused by various leafy vegetable crops and one of the most important foodborne pathogens in this context is Shiga toxin-producing Escherichia coli (STEC). Other pathogens such as Salmonella, Shigella, Yersinia enterocolitica and Listeria monocytogenes should also be considered in disease risk analysis, as they have been implicated in outbreaks associated with leafy greens. These pathogens may enter the horticultural value network during primary production in field or greenhouse via irrigation, at harvest, during processing and distribution or in the home kitchen/restaurant. The hurdle approach involves combining several mitigating approaches, each of which is insufficient on its own, to control or even eliminate pathogens in food products. Since the food chain system for leafy green vegetables contains no absolute kill step for pathogens, use of hurdles at critical points could enable control of pathogens that pose a human health risk. Hurdles should be combined so as to decrease the risk due to pathogenic microbes and also to improve microbial stability, shelf-life, nutritional properties and sensory quality of leafy vegetables. The hurdle toolbox includes different options, such as physical, physiochemical and microbial hurdles. The goal for leafy green vegetables is multi-target preservation through intelligently applied hurdles. This review describes hurdles that could be used for leafy green vegetables and their biological basis, and identifies prospective hurdles that need attention in future research.

Keywords: Escherichia coli; Salmonella; food system; foodborne disease; listeria; primary production processing; spoilage.

Figure 1

Environmental conditions prevailing during pre- and post-harvest of leafy vegetables and affecting the fate of phyllosphere organisms. (A) Change of the plant matrix from pre- to post-harvest and related alterations in the physio-chemical phyllosphere environment. (B) Temperature as an example of fluctuations in the phyllosphere environment during pre-harvest (I: field environment; II canopy surface) and post-harvest (III: household refrigerator). I: Diurnal changes in atmospheric temperature (solid line) and crop stand temperature near the soil surface (broken line) monitored in southern Sweden during 3 weeks in May and June 2014 in a baby leaf spinach crop. Atmospheric temperature was recorded once per hour and crop stand temperature every 30 min. The purple box indicates the time window for assessment of leaf temperature displayed in II. II: Leaf surface temperature of field-grown baby leaf spinach. The image was taken with an infrared camera. The red and blue triangles indicate the sites with the highest and lowest temperatures in the section. III: Temperature fluctuations in a household refrigerator where temperature was recorded every 5 min during a 14-day period. (Pictures and original data in the figures have previously not been published. Photo by Beatrix Alsanius).