Currently no standards or requirements exist for microbial food safety for space grown produce (e.g., fresh salad crops). Without standards it is difficult to assess fresh produce options and develop viable sanitization solutions for use on the ISS and in future exploration missions. The evaluation of baseline microbial levels from the literature, microbial levels on grocery store produce, and microbial levels on controlled environment-grown crops should provide some of the data necessary to develop salad crop handling and microbial requirements for fresh produce grown in environments similar to the ISS Veggie plant chamber. This project evaluated and compared baseline microbial levels of controlled environment (similar ISS conditions) grown lettuce, mizuna, cherry tomato, radish and green peppers, with comparable retail produce purchased at local grocery stores. In addition, these data were compared to crops actually grown in Veggie on ISS and in other ground experiments related to Veggie. This evaluation aligns to the Human Research Program (HRP) Task “Pick-and-eat salad crop production for the ISS” and should help drive requirements for future food systems development beyond ISS.
Five different edible crop types from both retail (grocery store) and controlled environment chamber grown sources were evaluated for microbial counts. In addition, a review of some published findings on microbial counts for food crops was done. Our goal was to assess the relative microbial quality of these crops grown under different conditions, with the hope of developing guidelines for space grown foods, such as the Veggie chamber on the ISS. Microbial contamination of field grown produce can happen in the processing from farm to table. Sources of foodborne pathogen contamination can be present pre-harvest including untreated irrigation water, the use of organic fertilizers like compost and manure, and proximity of wildlife and livestock to the field (Steele, M et.al. 2004, Beuchat, L. R. 2006, Delaquis, P. et.al. 2007). Postharvest sources of contamination include operations during handling, i.e., contaminated handlers and surfaces, cleaning solutions, and packaging. Most of these sources of contamination are eliminated from space grown crops like those grown in Veggie but the spaceflight environment may present unique mechanisms for microbial contamination and risk to humans that are not fully understood. The data collected in this study show that Veggie grown crops to date do not exceed the levels of bacteria and yeasts / molds found on retail produce and E. coli, Salmonella, S. aureus, and A. flavus were not detected using culture based screening methods. There are a few areas that should be considered in systems like Veggie where microbial contamination could be introduced. The data collected in this study also provide evidence of potential risks that they could be mitigated before consumption by the crew on ISS, or before the health and quality of the crop is affected. By considering the inputs that contribute to the microbial load in Veggie/Space station-grown crops, risks can be defined from negligible to high. These rankings are subjective based on protocols in place for the preparation of the Veggie facility for plant cultivation and harvest as well as microbiological data collected from the Veggie facility and plants. A formal quantitative risk assessment is planned that will include an evaluation of the data compiled in this study. Historical microbiological data from ISS potable water and air would be valuable information to include in a risk assessment of produce grown in the Veggie facility.
In this study, we found that the number of bacteria and fungi on growth chamber and ISS-grown produce are no higher than market produce from field settings. While the sample size for Veggie plant samples is low, a possible upward trend in the microbial counts was seen with VEG-03D leafy greens, mizuna, and ‘Outredgeous’ lettuce. This could indicate that the Veggie facility itself is developing a higher microbial load with time. Under these circumstances, steps could be taken to minimize further microbial contamination such as increased decontamination of surfaces with sanitizing wipes and the removal of unnecessary materials. Microbial monitoring of inputs into the plant growth systems and the plants is important and should enable detection of any trends indicating increased microbial presence. Surface swabs, water samples, and air samples are invaluable in controlling points were contamination is possible.
Based on testing that was performed, all the produce sampled in this study indicated no specific microbiological risks associated with a particular type of product. In general the edible fruits, tomato, and pepper had the lowest counts, while radish, a storage root had the highest, with no difference between the market and chamber grown crops. Radish and similar types of crops are in close proximity of moist soil or wicking material that would support microbial growth. Further development of best practices for space-grown crops should allow routine safe consumption of these items.
Steele, M., and Odumeru, J. 2004. Irrigation water as source of foodborne pathogens on fruit and vegetables. J. Food Prot. 67: 2839-2849.
Beuchat, L. R. 2006. Vectors and conditions for preharvest contamination of fruits and vegetables with pathogens capable of causing enteric diseases. British Food Journal, 108:38-53.
Delaquis, P., Bach, S., Dinu, L. 2007. Behavior of Escherichia coli O157:H7 in Leafy Vegetables. J Food Prot. 70: 1966-1974.
Abstracts for Journals and Proceedings
Hummerick ME, Massa GD, Johnson CM, Scotten J, Spencer L, Wheeler RM. "Baseline Microbial Assessment of Fresh Produce." 2019 NASA Human Research Program Investigators’ Workshop, Galveston, TX, January 22-25, 2019.
2019 NASA Human Research Program Investigators’ Workshop, Galveston, TX, January 22-25, 2019. , Jan-2019