Advisor at Japan Food Research Laboratories
Ten people who ate ice cream have developed Listeria monocytogenes (Lm) food poisoning since last March in the United States. Lm was detected in the leftover ice cream as shown in Figure1. Its manufacturer is a large American food company with a history of more than 100 years. As shown in Figure2, outbreaks of Lm food poisoning are reported from 4 states and 3 people died in Kansas.
The production facilities were shut down and all of their products were recalled. However, where and how Lm was mixed into ice cream has not been revealed yet. The offending company executed 1,450 layoffs, 37% of the whole employee, in order to deal with the aftermath of the problem.
Also in the United States, Lm was detected in the products of another major ice-cream company last April, and its products were recalled and shops were closed. Food industry is still unsure what countermeasures they should take against Lm because they have no clue why it was mixed into a frozen food, ice cream. Lm is the third leading cause of food poisoning in the United States. However, there had been no food-poisoning case so far caused by Lm transmitted by ice cream distributed in frozen condition. Nor had Lm been detected in ice cream produced by other companies.
In Japan, Lm food poisoning is virtually unknown for some unaccountable reason. Only one case of Lm food poisoning, which is from cheese, has been reported. As I mentioned in my columns #6 and 12, this microorganism, transmitted by various foods, causes many cases of deadly food poisoning mainly in developed countries. As Lm induces miscarriage, pregnant women and women suspected of being pregnant should give particular attention to this microorganism.
Lm generally resides in soil and is transmitted by animals and humans. It is becoming known that Lm is extremely hard to eradicate once a food factory is contaminated due to its biofilm-forming behavior. Especially RTE (Ready to Eat) foods such as ice cream, which will be eaten as they are without being heated, require attention. Some people argue that food factories manufacturing such kind of foods should incorporate “Seek and Destroy Process” to fight against Lm.
Lm, unlike other food-poisoning bacteria, can proliferate near 0°C. It is seen as having a refrigeration-resistant feature. Even frozen foods such as ice cream will do people harm if distributed in contaminated condition. In the above mentioned case, the offending ice cream seems to have been contaminated in a liquid state before being refrigerated. Lm may have been hiding somewhere in the factories.
Microorganisms won’t proliferate in ice cream if stored at a proper temperature under -18°C. However, inappropriate temperature control will irreversibly deteriorate its quality. Now let’s look at the behaviors of microorganisms under refrigerated condition.
There are many microorganisms that can proliferate even in cold storage. Some psychrophilic bacteria are killed around 20°C. Some microorganisms will die when frozen, but others just become quiescent and start proliferating again if thawed. Rate of death or damage of a strain differs depending on freezing speed, temperature or thawing process.
Bacteria are generally considered to lose their proliferative ability at -1 to -6°C. However, some yeasts and bacteria are reported to be able to proliferate at lower temperature. Therefore, -10 to -12°C can be said the temperature range in which you can completely block microorganisms from proliferating. When you store a strain of non-sporulating microorganism for a long period of time, you add a cryoprotective agent such as glycerin and keep it at ultralow temperature around -80°C. Or you can freeze-dry it.
If you cool down a microorganism, the proportion of unsaturated fatty acid of its membrane will increase or fatty acid will become short-chained. This is because it tries to keep the fluidity of its membrane even under low temperature. Pseudomonas and Vibrio tend to be reduced in number by freezing and thawing, whereas Flavobacterium, Micrococcus and Staphylococcus almost keep up their number. Spores will not be killed by freezing and thawing either. Survival ability under freezing varies according to species of microorganisms. Rate of death of a microorganism differs depending on freezing speed, temperature or thawing process. Microorganisms die during frozen storage too. Generally speaking, survival rate of a trophocyte gradually decreases if put into the temperature condition higher than -20°C. The rate dramatically drops at -1 to -3°C.
Vibrio parahaemolyticus is most prone to die of freezing among food-poisoning microorganisms. However, even such freezing-vulnerable microorganisms cannot always be killed out by freezing because survival under freezing is susceptible to various conditions. Heat sterilization is effective to Lm as they do not form spores. “Seek and Destroy Process” needs to incorporate both a rapid-and-easy testing method (column #7) and invisible cleanliness (#9). Cleaning tools using hot steam seem to be effective in fighting off Lm. Food handling facilities should not coexist with Lm. How about starting with “knowing both yourself and your enemy” as shown in Figure3?
1) CDC, Multistate Outbreak of Listeriosis Linked to Blue Bell Creameries Products, May 7, 2015, http://www.cdc.gov/listeria/outbreaks/ice-cream-03-15/
2) T.J.V.Malley, et al, Seek and Destroy Process: Listeria monocytogenes Process Controls in the Ready-to-Eat Meat and Poultry Industry, J.Food Protection,78,436–445(2015)
3) Food Safety Commission of Japan: Food Poisoning Caused by Listeria