Sprout Food Safety - Study on Salmonella Prevention in Sprout Cultivation



International Specialty Supply

June 30, 2008

FDA Study Shows Rotary Drum Method of Growing Sprouts Controlled Salmonella - This is Huge!

There have been at least 77 reported sprout related outbreaks in the world that we are aware of.  Nearly all have been attributed to sprouts produced from contaminated seed. Salmonella was involved in 63 of these outbreaks and 42 of the salmonella outbreaks involved alfalfa sprouts.

A group of researchers at the FDA, the National Center for Food Safety and Technology, the Illinois Institute of Technology, and the Center for Food Safety and Applied Nutrition may have found a simple way to control or eliminate salmonella in alfalfa sprouts.

In the study "Factors Influencing the Growth of Salmonella during Sprouting of Naturally Contaminated Alfalfa Seeds" TJ Fu et. al. took two lots of seeds 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 compared it to sprouts grown in lab size rotary drum.  Each salmonella cell in the jar grown sprouts multiplied into 10,000 salmonella cells.  However, salmonella did not multiply in the rotary drum grown sprouts.  In fact, salmonella from one of the seed lots grown in the rotary drum actually declined to the point that no salmonella was detected at harvest. 

The group demonstrated an inverse relationship between watering frequency and the growth of salmonella in alfalfa sprouts grown in a rotary drum.  The more often you water, the less salmonella multiply.    

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 inSalmonella counts.   

They also showed a direct correlation between increased growth temperature and increased salmonella growth.  Increasing the sprouting temperature from 20°C to 30°C increased the Salmonella counts by as much as 3 log.


Bobs Soapbox

This is a very significant study for the sprout industry.  Sprout growers already knew that increased sprout mass temperature increases the growth of plant pathogens.  This group showed that this is also the case with salmonella.   

The study also showed that the more often you water your sprouts the less likely you are to have salmonella.  

This is something the SproutNet concurred with several years ago and challenged this team of researchers to continue down this path of investigation.  The industry is grateful that they did. 

Several years ago ISS developed a method of producing sprouts free of particular plant pathogens that destroy sprouts.  The protocol is based on watering frequency and temperature, and on sprout mass temperature.  It also incorporates a seed sanitize, wash and soak cycle, including chlorine and the buildup of chlorine gas to eliminate plant pathogens.    

The FDA et. al. study found that increasing the watering frequency from 20 minutes to 2 hours increased the growth of salmonella as much as 1000 times in some cases.  They did not test watering more frequently than 20 minutes.  ISS has found that much more frequent watering is better at controlling plant pathogens.  I suspect this also applies to human pathogens.

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 (competitive exclusion).  Bacteria are competing for food (carbon compounds) contained on the sprout.  If non-pathogenic bacteria multiply 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 bacterial flora that naturally thrives on sprouts.  Those salmonella cells 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 entirely. 

Although the sprouts were grown to their entirety in the rotary drum, I suspect they would have similar results if they grew the sprouts in a Rota-Tech Rotary Drum System for two days and then moved the sprouts to an ISS Trac System and watered less frequently.  This theory is based on multiple studies that show that the bacterial load on alfalfa sprouts is maximized between 24 and 48 hours.  At this point the natural bacterial flora would have too great of an advantage over salmonella or E.coli O157:H7 for these pathogens to compete for food.

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

An interesting study was done in 2005 by a team of University of Arkansas students competing in a national academic contest.  They tested alfalfa seed sanitizing procedures using sprout industry practices of soaking in a mesh bag in a bucket with agitation.  They compared this with using a protocol provided by ISS for sanitizing in a Rotary Drum.  The protocol called for far less chlorine applied for a longer duration.  Rota-Tech Rotary Drums act like a front end loading cloths washing machine during this cycle. The flow of air is cut off in order to build up chlorine gas.  The seed alternates from being in chlorinated water and the chlorine gas which is built up in the Rota-Tech.

They could not use human pathogens so they decided to dye the seed purple to see how effective each method was at removing the dye.  The assumption was that if the chlorine does not completely remove the dye then the distribution of chlorine within the seed mass is inconsistent.

After 15 minutes with 20,000 ppm calcium hypochlorite they concluded that the mesh bag method was ineffective at removing the dye, whereas:

 "After a thirty minute treatment in the rotary drum, the seeds were found to be completely void of dye. In addition, the seeds were also sprouted to show the calcium hypochlorite sanitation did not damage the seeds."  

Their 28 page award winning report concluded that "it is recommended to use the drum for both sanitizing and germinating alfalfa seeds.

Producing sprouts in aRota-Tech Rotary Drum is one of many steps that sprout producers use to combat plant pathogens.  This study showed that Rota-Techs can now be used to help combat human pathogens as well.   

Other measures of combating human pathogens the FDA and National Center for Food Safety and Technology researchers discussed include testing the spent irrigation water prior to releasing the product for sale.   

This joint effort by FDA, the National Center for Food Safety and Technology, the Illinois Institute of Technology, and the Center for Food Safety and Applied Nutrition also referenced ISS'Seed Screening Program and 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."   

I liked that.  



Dear Bob,

"Thanks for your SproutNet.  It was obvious from the letter to sprout growers in California that the California Department of Health was recommending the ISS seed screening program.  There really isn't anyone who does what you do is there?"   


Dear Sprout Grower,

You are correct.  The ISS Seed Screening Program is unique and extremely important. ISS is the only company in the world that delivers our Screened Sprouting Seed Documentation.    

ISS' program is confirmed, referenced or recommended as necessary by the World Health Organization, United States Food and Drug Administration (FDA), United States Department of Agriculture (USDA), California Department of Health, New South Wales Food Authority, South Australian Research and Development Institute, Ontario Ministry of Agriculture,  The Center for Science in the Public interest, Resources for the Future, Food Standards Agency (UK), The Campden Research Group, Food Safety Authority of Ireland, National Center for Food Safety and Technology, Illinois Institute of Technology, Center for Food Safety and Applied Nutrition, and dozens of researchers and food safety experts. 



An interesting blog a sprout grower passed to us:



We appreciate your business,

Bob Rust
(931) 526 1106
ISS, Est. 1979.  Supplying the sprout industry with the finest seed, packaging, and equipment