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#23 Low-pH and Low-Aw Foods Can Cause Food Poisoning

 

 Kenji ISSHIKI
Professor Emeritus at Hokkaido University

Advisor at Japan Food Research Laboratories

Biography of Dr. Isshiki

http://researchmap.jp/isshiki-kenji/

  

 

Microorganisms, including food-poisoning bacteria, sometimes survive severe environments with their adaptability.  In order to protect food products from food poisoning or decay, their water activity (Aw) and/or pH is reduced in some cases.  If Aw is lowered, water amount in food available to microorganisms (i.e. free-water content) will be decreased.  If pH is lowered, hydrogen ion concentration of food moisture will increase and microbiological growth will be inhibited.  Some microorganisms resist this hardship by sporing, but we have to pay attention to non spore-forming bacteria too.  In this piece of column, I will discuss food-poisoning cases caused by remaining survivors of Listeria monocytogenes (Lm) and Salmonella spp. (Sal).

 

 

1) Listeria food poisoning caused by American candy apples (caramel apples)

 

In 2014, a massive food poisoning caused by caramel apples broke out in the United States.  The offending product was produced by a manufacturer specialized in this sweet stuff.  The number of sufferers turned out to be 35 among which 7 people died.  People eat caramel apples --- apples coated with heat-melted candy --- on Halloween.  Since apples are acidic and candies are low in Aw, it had been considered that this food product was unlikely to cause food poisoning and that low-temperature control of this product was not needed.

 

 ishiki231.jpg

 

As Figure 1 shows, Food Research Institute of University of Wisconsin conducted reproductive experiments using a mixture of four types of Lm which led to interesting results.  It turned out that if a wooden stick was put into a caramel-coated apple, growth of Lm was accelerated.  Even when caramel-coated apples with sticks were preserved in a refrigerator, Lm continued to grow slowly over time as shown in Figure 1.  On the other hand, when caramel-coated apples without sticks were preserved in a refrigerator, Lm did not grow for four weeks or longer.  It is thought that inserted sticks delivered apple juice to caramel coatings which created an easy-to-grow environment for Lm.  Presumably, fluid and nutrients were provided to Lm at the surface or core of apples.

 

As I mentioned in my columns #6 and 16, Lm can live anywhere and has superior environmental adaptability in spite of its non spore-forming trait.  Therefore, even traditional foods such as caramel-coated apples require attention.  Let us carry out “visible cleanliness, invisible cleanliness” (column #9) with a sense of respect toward Lm’s vitality in mind.

 

2) Salmonella food poisoning caused by peanut butter

 

Food poisoning caused by Salmonella-Typhimurium-contaminated peanut butter broke out in various regions in the United States from September 2008 to February 2009.  550 people of all ages suffered in 43 states, half of which were kids under 16 years old, and 9 people passed away.  Strain of the same species as S.Typhimurium that caused the food poisoning was detected from a peanut butter brand produced by a peanut-products manufacturer.  This brand had been consumed at many facilities such as schools, hospitals and restaurants.  It had also been used for food manufacturing such as cookies, cereals, and ice creams.  CEO of the peanut-products manufacturer was sentenced to 28 years in prison without suspension. 

 

 ishiki232.jpg

 

Salmonella had already been regarded as a highly resistant microorganism that could survive in dry and fatty foods.  This incident required a re-examination of countermeasures to properly deal with microorganisms in low-Aw foods.  Figure 2 is an experimental result by Dr. Beuchat et al. of Center for Food Safety of University of Georgia Griffin.  He is one of the food-hygiene specialists that tremendously supported Japan on the occasion of food-poisoning outbreak caused by radish sprouts in 1996.  Figure 2 shows a death curve obtained from an experiment adding a mixture of five types of Salmonella enterica to fillings of cookie and cracker sandwiches.  As the cookies, crackers and fillings used for the experiment were low in Aw, Sal did not multiply.  However, many of salmonella in the cookie sandwich with peanut butter cream survived after 182 days passed.  Salmonella remained alive in other experiment plots too.

  

We have to take account of the existence of pathogens, such as Sal, that survive until the environment turns into a permissive one for growth.

 

 

3) Sanitary measures for foods

 

As I mentioned above, even acidic and low-Aw foods, such as caramel apples or cookie sandwiches, cause food poisoning.  NACMCF (National Advisory Committee on Microbioligical Criteria on Food) of the United States lists the pathogens that need measures based on the interaction of pH and Aw of the product (Reference 3).  Table 1 is a part of the lists.  Foods with pH and Aw higher than the figures in Table 1 require more sensitive handling.

 

 ishiki233.jpg

 

As I mentioned above, even acidic and low-Aw foods, such as caramel apples or cookie sandwiches, cause food poisoning.  NACMCF (National Advisory Committee on Microbioligical Criteria on Food) of the United States lists the pathogens that need measures based on the interaction of pH and Aw of the product (Reference 3).  Table 1 is a part of the lists.  Foods with pH and Aw higher than the figures in Table 1 require more sensitive handling.

 

Food-poisoning bacteria are constantly evolving in adapting to circumstances.  Some kinds of bacteria including Lm and Sal can survive after invading the human body by being absorbed into the immune system and adjusting themselves.  Let us, food handlers, keep evolving based on scientific basis.

 

References

1) K. A. Glass, et al: Growth of Listeria monocytogenes within a Caramel-Coated Apple Microenvironment, mBio, 6(5), e01232-15 (2015)
http://mbio.asm.org/content/6/5/e01232-15.full.pdf+html

 

2) L.R.Beuchat,et al: Survival of Salmonella in Cookie and Cracker Sandwiches Containing Inoculated, Low–Water Activity Fillings, J.Food Protection, 78(10), 1828–1834(2015)

 

3) NACMCF: Parameters for Determining Inoculated Pack/Challenge Study Protocols,J.Food Protection, 73(1),140–202(2010)

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