Factors Influencing the Growth of Salmonella During Sprouting of Naturally Contaminated Alfalfa Seeds

Factors Influencing the Growth of Salmonella during Sprouting of Naturally Contaminated Alfalfa Seeds

Journal of Food Protection, Vol. 71, No. 5, 2008, Pages 888-896

Tong-Jen Fu1, Karl F. Reineke1, Stuart Chirtel2, and Olif M. Vanpelt3

1U.S. Food and Drug Administration and 3Illinois Institute of Technology, National Center for Food Safety and Technology, Summit-Argo, Illinois 60501; and 2U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, College Park, Maryland 20740, USA

MS 07-388: Received 25 July 2007/Accepted 15 December 2007

ABSTRACT

In this study, the factors that affect Salmonella growth during sprouting of naturally contaminated alfalfa seeds associated with two previous outbreaks of salmonellosis were examined. A minidrum sprouter equipped with automatic irrigation and rotation systems was built to allow sprouting to be conducted under conditions similar to those used commercially. The growth ofSalmonella during sprouting in the minidrum was compared with that observed in sprouts grown in glass jars under conditions commonly used at home. The level of Salmonella increased by as much as 4 log units after 48 h of sprouting in jars but remained constant during the entire sprouting period in the minidrum. The effect of temperature and irrigation frequency on Salmonella growth was examined. Increasing the sprouting temperature from 20 to 30oC increased the Salmonella counts by as much as 2 log units on sprouts grown both in the minidrum and in the glass jars. Decreasing the irrigation frequency from every 20 min to every 2 h during sprouting in the minidrum or from every 4 h to every 24 h during sprouting in the glass jars resulted in an approximately 2-log increase in Salmonella counts. The levels of total aerobic mesophilic bacteria, coliforms, and Salmonella in spent irrigation water closely reflected those found in sprouts, confirming that monitoring of spent irrigation water is a good way to monitor pathogen levels during sprouting.

 

Notes from SproutNet:

This is very interesting work.  TJ Fu et. al. took two lots of seed that are naturally contaminated and were involved in outbreaks of salmonella in sprouts.  They germinated the seed and grew the sprouts in a jar and a small rotary drum.  In the jar grown sprouts each salmonella cell multiplied into 10,000 salmonella cells.  In therotary drum grown sprouts the salmonella did not multiply.  Salmonella from one of the seed lots actually declined to the point that none was detected.

Other reports have shown that Salmonella or E.coli O157:H7 can be out competed by pseudomonas fluorescens and other non-pathogenic bacteria that naturally thrive on sprouts.  This called competitive exclusion.  The bacteria are competing for food (carbon compounds) contained on the sprout.  If non-pathogenic bacteria has multiplied to levels that can out compete pathogenic bacteria for food, the pathogenic bacteria don't thrive and may  die off. 

In this case I suspect that salmonella are relatively easily washed off compared to the flora of bacteria that naturally thrive on sprouts.  Those that survive are poor competitors for food compared to bacteria that thrive on sprouts.  The salmonella cells basically either don't have enough food to multiply or starve to death.

This study also showed that increasing the growing temperature made salmonella thrive.  I suspect this could be combated by more frequent watering if lowering the growth temperature is impractical. 

It should also be noted that the authors pointed out that "To prevent the consumption of contaminated sprouts, the absence of pathogens in seeds must be assured. Screening of seeds by seed suppliers for the presence of pathogens can help to reduce the number of contaminated seeds entering the marketplace."  Amen!

This work is a continuation of a 2004 study titled

"Growth of Salmonella During Sprouting of Naturally Contaminated Alfalfa Seeds as Affected by Sprouting Conditions".  In this report Dr Fu and her associates reported that decreasing the irrigation frequency from every 20 min to every 2 hours resulted in an approximate 2-log increase in Salmonella counts.  They also found that increasing the sprouting temperature from 20°C to 30°C increased the Salmonella counts by as much as 3 logs.