Finding ways to preserve food quality and ensure food safety
Food processors often use heat for pasteurisation or sterilisation to make food products safe by killing pathogens like salmonella and listeria, but high temperatures can sometimes affect food quality.
Focusing on low-moisture food products like powdered milk, assistant professor of food microbiology and safety Jennifer Acuff and her research team at the University of Arkansas performed a study seeking a method that could ensure food safety while retaining the most vitamins, minerals and flavour depending on the food.
“This collaborative approach encompassed microbiology, engineering and statistics to provide the food industry with what we believe will be a tool to improve safety without compromising quality of their dried food products,” Acuff said.
The process is not limited to low-moisture foods and may extend to other foods and processes, Acuff added.
Using data from a study on a harmless ‘surrogate’ microorganism and a statistical technique called ‘bootstrapping’, the researchers developed a framework to provide food processors options within U.S. Food and Drug Administration (FDA) guidelines.
“We have proposed a methodology to pick a value between the most liberal and most conservative food processing approaches based on risk tolerances,” said Jeyam Subbiah, head of the food science department. “The industry can use this methodology to pick a value and petition the FDA for approval.”
While there is no specific FDA rule, the government currently asks the food processing industry to make a petition for a case-by-case review.
The study, ‘Bootstrapping for Estimating the Conservative Kill Ratio of the Surrogate to the Pathogen for Use in Thermal Process Validation at the Industrial Scale’, was published online by the Journal of Food Production in March. The U.S. Department of Agriculture’s National Institute of Food and Agriculture and Mars Wrigley Inc. supported the study.
“Surrogates are like dummies used in crash testing to validate car safety,” Subbiah said. “They are non-pathogenic microorganisms, which should have similar or higher heat resistance than the actual pathogen. Often, they are a lot more resistant.”
Scientists use a “log cycle reduction,” or LCR for short, to calculate how effectively a process kills harmful microorganisms. “Log” refers to the logarithm scale and 1-log represents a 10-fold reduction equivalent to a 90% reduction in bacteria. A 2-log reduction would be a 99% reduction, 3-log 99.9% and so on. A 6-log reduction is a 99.9999% reduction.
When surrogate microorganisms are used for food safety challenge studies for sterilisation of canned foods, the Institute of Food Thermal Processing Specialists recommends a ‘simple mean’, or average, kill ratio to validate food safety at an industrial scale. For example, Subbiah said if sterilisation called for a 12-log reduction of the pathogen and the surrogate was twice as resistant, a processor could show a 6-log kill of the surrogate, and the FDA would accept it as equivalent.
However, the drawback of that method is that it does not consider the variability of microorganisms, both the pathogen and the surrogate, Subbiah noted.
Although less prone to foodborne pathogens than fresh meats and dairy, low-moisture foods are not immune. According to the study, various types of salmonella have been implicated in 15 deaths, thousands of illnesses and hundreds of hospitalisations over the past 20 years due to infected low-moisture foods like dried fruits and vegetables, nuts, herbs, flour and spices.
After those food safety outbreaks, the food industry “swung to the conservative mode” in food safety challenge studies, Subbiah said, by requiring the same level of log reduction of the surrogate. For example, if sterilisation of spices calls for a 12-log reduction of salmonella, the industry would show a 12-log reduction of the surrogate even though it can be twice as resistant as the pathogen. While this assures a high level of food safety, nutrients may be degraded due to severe thermal processing, Subbiah explained.
Calculating the risk
As a food science graduate student in the Dale Bumpers College of Agricultural, Food and Life Sciences, Arshpreet Khattra used previously published data from Subbiah’s lab involving the surrogate Enterococcus faecium to develop a solution for preserving quality in thermal processing. She applied the bootstrapping technique to estimate the distribution of kill ratio in milk powders rather than calculate the simple mean, or average, kill ratio.
With bootstrapping, scientists can deal with uncertainty in experimental data by generating many samples instead of assuming a specific distribution. The technique calls for randomly picked data points from the original data to give researchers a good idea of how much the results may vary due to chance. It has been used in various studies to improve food processing methods and assess the food safety risks of different microbes in various foods.
From the estimate of kill ratio distribution, the final kill ratio can be calculated on a sliding scale of risk, Subbiah noted. In a hypothetical example, to have a 1% risk level, a processor may want a 9-log reduction of the surrogate, which is a 99.9999999% reduction. A 5% risk level would call for an 8-log reduction and a 10% risk would call for a 6.5-log decrease of the surrogate to be equivalent to a 12-log reduction of the pathogen. A 12-log reduction is typically called sterilization and a 4- to 5-log reduction qualifies as pasteurisation.
This method strikes a balance between killing harmful bacteria and preserving quality, Subbiah said.
Khattra examined data collected in a 2021 study evaluating Enterococcus faecium as a surrogate for salmonella in milk powders at different storage times and temperatures. A wide range of foods incorporate powdered milk, including candy bars and baby formula.
Khattra is now a PhD student at Michigan State University. Co-authors of the study included Subbiah, Acuff, Kevin Thompson and Andy Mauromoustakos with the Division of Agriculture’s Center for Agricultural Data Analytics, and Surabhi Wason, PhD, now with Kerry Ingredients & Flavours in Wisconsin.
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