Technical Look on Microbiological Risk Factors in Pre-Harvest Crops

Food monitoring procedures are strongly focused on hygiene to avoid microbiological hazards such as bacterial or viral contamination. A general increase in microbiological safety has already been achieved over the past decades through the establishment of various standards (HACCP, IFS/BRC, ISO 22000). Despite greater safety and longer-lasting products, microbiological control must be improved particularly with respect to fresh products. Most foodborne microorganisms that cause illnesses are zoonotic or geonotic but some have human origins, such as the Hepatitis A, Salmonella phi, and Vibrio cholera. These contamination can occur during the food manufacturing process.

A significant increase in diseases stemming from microbiological infections can be expected in the future. The Federal Institute for Risk Assessment stated that foodborne infections will pose a serious problem to the global public health in the coming years. In the last two decades, there has been an  overall increase in foodborne illness outbreaks and cases linked to fresh fruits and vegetables:

• Vegetables, juices, and related products comprised 4.4 percent of all foodborne outbreaks in the EU between 2008 and 2013.
• In the U.S., 13 percent of foodborne outbreaks between 1990 and 2005 have been associated with fresh produce. Green salad, lettuce, seed sprouts, tomatoes and cantaloupes were identified as the main sources of foodborne illness outbreaks.

Frequent combinations are: Salmonella and cantaloupes, sprouts, or tomatoes; E. coli and leaf green vegetables; Cyclospora and raspberries; and Hepatitis A with green onions.

Within the structured approach to food safety management (Risk Analysis framework), Food Safety Objectives (FSO) are essential tools to meet public health goals. FSOs define the maximum level of microbiological hazards permitted in various foods at the point of consumption. Maximum hazard levels at different points along the food chain represent further performance objectives.

Unlike other commodities, such as beef or chicken that is rigorously inspected, microbiology is often neglected when testing fresh fruit and vegetables because adequate strategies are lacking. Fruits and vegetables are foods with generally short shelf life: however, investigations of microbiological risk take some time. A number of different microbiological testing can be used by industry and government actors. Within-lot testing is one of the most common methods when measuring the hazard against a pre-defined limit but the sampling plans are generally time-consuming and resource-inefficient.

Point source contaminations that cause individual consumer illnesses might occur in fresh produce due to bird feces, for example. However, these kinds of risks are difficult to detect once the product is packed and optical control is insufficient. General or nonpoint source contaminations could cause foodborne epidemics because of the effect on a broad range of products. Pre-harvest sources of nonpoint source contamination are most often soil, feces, irrigation water, water used to apply fungicides and insecticides, dust, insects, inadequately composted manure, wild and domestic animals, and human handling.

The earlier a risk is detected, the better the odds for effective measures and lower overall costs.

On the basis of the European Food Safety Authority results and personal experiences with more than 400 microbiological samples and over 2,500 field visits within the last two years, Analytica Alimentaria has developed a new risk analysis process when testing fruit and vegetables:

1. Effective microbiological risk assessment in the field with a new evaluation system, structured questionnaires, and defined approval policies;
2. Synergies via the efficient linking of risk analysis with pre-harvest sampling;
3. Risk-oriented sampling and laboratory analysis of the correct parameters;
4. Clear escalation scheme in the case of positive findings; and
5. Fast decision-making for retailers.

This article describes the complete process of microbiological risk assessment, which can be a general or nonpoint source contamination of the sampled object (field, warehouse) that can cause a foodborne epidemic or individual illness. As the result of the process, the risk can be calculated and mitigated.

Results and Discussion
The questionnaire developed for the field and warehouse is based on information from scientific literature as well as on the samplers’ own experience. It is also based on the growth of microorganisms along the entire food chain. It is well known that pre- and post-harvest environments are significant sources of pathogens.

Questions and answers are divided into two weighted categories: high-risk questions (HRQ) and low-risk questions (LRQ), as well as additional information on the conditions of the work. The number of “Yes” and “No” votes are summarized separately and taken for a decision on microbiological sampling.

The assessment of microbiological risk is based on the evaluation of the questionnaire, which takes place using Excel spreadsheets.

HRQ specifies the conditions where microbiological contaminations are very likely to occur. Potential sources for microbiological infection in the field are direct sources of living pathogens (past infections, animals, organic fertilizer/manure) and particularly favorable environmental conditions (high water level).

Salmonella, Shigella, and E. coli O157:H7 and other pathogens that can survive for extended periods of time in water. Flood and spray irrigation represent the greatest risk because contamination can be directly deposited onto edible parts of produce. Reconstituted pesticides may also serve as potential sources of pathogens.

LQR collects and evaluates marginal conditions for microbiological contaminations that could be a risk when a common occurrence. The choice of questions was based on respective regulations:

• Regulation (EC) No. 2073/2005 on microbiological criteria for foodstuffs;
• Regulation (EC) No. 1935/2004 on materials and articles intended to come into contact with food; and
• Guidance for dealing with fruits, vegetables, potatoes.

Additionally, a monitoring process is included that evaluates the minimum hygiene standards in the field.

The questions should be answered by growers, agricultural technicians, engineers and experienced samplers with “yes,” “no,” or “not known.” Objective criteria is defined to enable correct answering of questions.

The action plan. In case of a positive finding, the main problem often is the absence of an effective plan to restore microbiological safety. The laboratory that has detected the pathogens is responsible for forwarding information to the corresponding grower/producer and to the whole supply chain via the trader. A second sample has to be stored for cross-checking. The laboratory is also responsible for source identification as well as the magnitude of microbiological contamination in the field or warehouse of the grower/producer. The trader has to ensure the grower and all affected goods in the supply chain are temporarily blocked from the market. The trader also contributes to restoring the delivery capacity of the grower.

The control plan. In case of a detected risk, a microbiological sampling takes place according to the rules of the Questionnaire for Microbiological Risk Assessment (QMRA). A microbiological sample consists of a number of individual subsamples. Each individual subsample is transported and analyzed separately. If any of the subsamples contain pathogens, harvesting is forbidden on the field of origin. A special controlling plan is implemented to identify the source and to decide if point source or nonpoint source contamination exists. Using GPS coordinates and other markers, the affected field is divided into five different, clearly separated areas that adjoin the area where the original sample was found. Control samples from the crop, the soil and the irrigation water have to be taken from each subarea and analyzed to measure the extent of contamination. A nonpoint source contamination exists if two or more control samples are positive. If only one sample is positive a partial contamination of the corresponding subarea exists. A point source contamination exists if all control samples are negative. This implies that only a part of the original sample was contaminated and that contamination could not be detected again. To detect and permanently address the source of microbiological contamination, further microbiological samples from irrigation water and soil must be taken, especially in cases where the same water or fertilizer is used for the subareas.

Examples for averted hazards by QMRA. According to the QMRA, a microbiological risk was found in a Spanish field with cabbage lettuce after strong rainfalls and floods in 2012 (QMRA question No 3). The original sample taken from the lettuce contained Salmonella spp and the action plan was activated. The supply chain was informed and the field was immediately blocked. Further samples were taken according to the control plan (lettuce, soil, and water from five subareas) and all five samples were positive for Salmonella spp: therefore, a nonpoint source contamination existed. The irrigation water from a river nearby was identified as the source of contamination: the river was highly contaminated with waste water and dead animals from neighboring farms.

In 2014, a microbiological risk was found in a Moroccan parsley field. A cow-farming unit existed near the field without an enclosure (QMRA question No 2). The original parsley sample contained STEC (E. coli 0103) but only one sample from the tests and one soil sample was positive. Both positive samples were from the same subarea nearest to the cow-farming unit. The irrigation water sample was negative and a partial contamination was found in the subarea due to animal feces.

In 2015, Listeria monocytogenes was found for Batavia in Italy when an undocumented (date of application, microbiological sampling) use of organic fertilizer had been observed (QMRA Question No 4). After blocking the field, all control samples were negative and a point source contamination of the field was acknowledged. The harvest could be continued.

Conclusions
The microbiological condition of food is one of the most critical elements of food safety, especially for fresh fruits and vegetables. The entire supply chain of fresh fruits and vegetables is highly time sensitive yet microbiological controls take some time for analysis. Unfounded actions could endanger human health and/or the supply chain. This new risk-based evaluation in the field facilitates a safe and convenient risk assessment of the microbiological condition of fresh crops at an early stage in the supply chain. Examples have demonstrated the reliability of the system, which comprises risk evaluation, action and a control plan. The QMRA saves time and cost but should be administered only by well-trained technicians. Sampling must be performed according to standardized and accredited methods.

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Lampe is a geo-ecologist at Analytica Alimentaria GmbH. Reach him at [email protected]. Hidalgo Palanco is a biologist at the company. Reach him at [email protected]. Fernandez Caro is agricultural engineer at Analytics Alimentaria. Reach him at [email protected]. Dr. Krause is a physicist at the company. Reach him at [email protected].