Raw chicken, Leftovers, Deli Meats, and More-- What Surveyed Scientists Said

ham and cheese


During the fall of 2013, Shelf Life Advice embarked upon an ambitious project: We emailed a survey to more than 800 scientists who teach and/or do research and are on the faculty of approved* food science programs. Faculty members from more than a dozen colleges or universities responded to our questions about food safety, nutrition, and controversial food issues. Some of their answers may influence how you grocery shop and handle food in your kitchen.


Conducting this survey taught us two things about asking scientists questions related to their area of specialization: 1) It seems that great minds do not think alike.  Our survey results rarely showed consensus. 2) Ask a scientist a seemingly black-and-white question, and the response is often gray. It's likely to begin with "It depends." Fortunately, our survey provided ample space for comments, many of which are included in the report below.  These should help readers to see the complexity of these scientific matters and to explain why there isn't total consensus. 


Part 1 of this report,  covers 7 Yes/No questions dealing with food safety. Part 2 includes 6 multiple choice items and one Yes/No item about both food safety and nutrition.  Part 3 deals 6 Q/As  about important issues related, for example, to organic food, GMOs, and the relative safety of the U.S. food supply. All three parts should be on the Shelf Life Advice home page by the end of January.


The Yes/No questions on the survey gave our 35 respondents the options of answering "Don't know" or "Don't choose to answer." Therefore, percentage totals on the Yes/No answers rarely add up to 100%. 


Some respondents' comments are quoted or paraphrased following the percentages on the answers. These include comments from some of our site's Advisory Board members.


Are deli meats and cheeses that are sliced and wrapped at a store's deli counter more likely to be contaminated than the same type of products sold in sealed packages?


    Yes: 78%   No: 9%


Some reasons for the "yes" answers from respondents:


  • The slicer [the equipment] and the food handler could both be a source of microbial contamination.  ("Deli slicers are notorious for spreading contamination.") 
  • "Many packaged sliced deli meats now receive a post-packaging pasteurization process, but not all."


Should raw chicken be rinsed with tap water before preparing it for cooking?


    Yes: 19%      No: 74%


Comments from survey respondents:


  • "Bacteria in the chicken juice could be spread to food contact surfaces around the kitchen."  (Fear of contaminating sinks, sponges, counter-tops, etc. and thereby spreading pathogens to other foods is the concern.)
  • "Let heating be the safety insurance."
  • "It does not have to be rinsed, but rinsing will not hurt."
  • "Contamination can splash onto other things in the kitchen. But I rinse it anyway to be sure there are not funny smells."


NOTE: For many years, the USDA has been advising consumers not to rinse raw chicken.


Is it better to wash produce before refrigerating it rather than waiting to wash it until immediately before cooking, serving, or consuming it?


    Yes: 26%     No: 68%


Comments from survey respondents:


  • "Washing it can spread soil and germs around and get them into vulnerable sites on the produce.  Also, unless the produce is carefully dried, washing can promote bacterial growth by leaving droplets of water on the produce."
  • "If the purpose of washing is to remove microbial pathogens, then it likely doesn't matter when it's washed; at either point, washing isn't an effective means of eliminating microbial pathogens. However, if the question relates to produce directly from a garden, then washing before refrigerating is warranted to remove dirt, etc."


Shelf Life Advice advises consumers not to wash fresh produce until it's about to be used. (See "Guide to the Proper Handling of Fresh Vegetables.")


Is there a health risk in reheating the same leftovers a second time (assuming they're reheated to 165°F)?


    Yes: 44 %   No: 44%


Comments from survey respondents:


  • Some respondents stated that it was safe to do this if the food was kept properly refrigerated between reheatings.
  • Cooked, perishable leftovers should be reheated to 165°F within 2 hours.  
  • Most pathogens are killed at 165°F, but, if the food was not properly refrigerated, and staph toxin grew in it, these could survive the heating process and cause illness.  Because of this danger, it's best not to reheat cooked food more than once. 


Comments from Shelf Life Advice Advisory Board members:


Food scientist Karin Allen provides a detailed explanation of why she believes repeated reheating of food is risky in the Shelf Life Advice article "Reheating Food: Pizza, Chicken, and Everything Else." Her comments include this point: Every reheating puts food through the "danger zone" of  40° - 140°F, the temperature at which bacteria multiply rapidly, and some bacteria "offspring" may be more resistant to heat. 


Food scientist Joe Regenstein adds:  If food is heated, served, and put back in the refrigerator while still above 140°F, then it should be safe regardless of the number of reheatings.  If left out on a buffet with time in the danger zone, then it should be reheated only if it was in the danger zone less than 2 hours.


Is it safe to refreeze refrigerated food after it's been totally defrosted for a day?


    Yes: 52%    No: 39%


Comments from survey respondents:


  • It must be defrosted in the refrigerator.
  • The quality of the refrozen food will have deteriorated.  
  • Both spoilage and pathogenic bacteria could grow if food is defrosted at room temperature.  Freezing kills some but not many bacteria.


Shelf Life Advice agrees with the majority.  See "Refreezing Frozen Foods."


If packaged, cut-up salad ingredients aren't slimy or brown around the edges, are they still safe to eat 2-3 days past the use-by date?  (Assume they were not contaminated when purchased.)


    Yes: 73%    No: 10%


Comments from survey respondents:


  • "I would use them for preparing a cooked dish, not for eating raw."
  • "Assuming they were not contaminated when purchased, then yes, it would likely be safe. However, there is no way for the consumer to visually tell whether or not products are contaminated at purchase, so, as a general rule, I would not advise consumers to use packaged salads after the use-by date (especially vulnerable populations--such as the elderly or immunocompromised)."


If a baby is put to bed with a bottle of milk (not formula or breast milk) and he/she finishes the contents the next morning, is there a risk that this "old" milk will make the baby sick?


    Yes: 69%    No: 13%


Comments from survey respondents:


  • If the milk was pasteurized, properly handled, and not long past the use-by date, it has a low chance of creating sickness. ("Use-by dates have quite a bit of wiggle room.")
  • "It's possible that microbes present or introduced could grow during this period.  Assuming the bottle is left unrefrigerated all night, then, yes, there is a risk."
  • "You should not leave milk at room temperature for more than 4 hours."
  • "It's a terrible practice that contributes to ear and respiratory infections, tooth decay, and 'nursing bottle mouth'" [a syndrome involving rapid decay of baby teeth in infants and children from long, frequent  exposure to liquids that contain a lot of sugar].


Comments from Shelf Life Advice Advisory Board members:


From Joe Regenstein, Ph.D., Cornell University, Dept. of Food Science:


"Given that the milk was pasteurized, the odds of pathogens being present are low. But if the handling is sloppy, and someone introduces a pathogen, then pathogens could grow. However, normally pathogens do not compete well with spoilage organisms." 


From Catherine N. Cutter, Ph.D., Pennsylvania State University, Dept. of Food Science:


"The milk should not make the baby sick if it was pasteurized. However, I would caution against this practice since the milk sugars (lactose) would remain in the baby’s mouth overnight. Bacteria in the mouth can utilize the lactose as a carbohydrate source, produce acid, and cause cavities.


"According to the following website: http://www.webmd.com/oral-health/guide/what-is-baby-bottle-tooth-decay, 'Never allow your child to fall asleep with a bottle containing anything but water.'”


From Timothy J. Bowser, Ph.D., Oklahoma State University, Dept. of Biosystems and Agricultural Engineering:


"Pasteurized does not mean microbe-free. Milk is one of the best mediums for microbial growth. Microbes in the milk will grow quickly (exponentially) at room temperature and could potentially threaten the health of the baby. I do not recommend the practice of putting a baby to bed with a bottle of milk that is left until the next morning."


Our question asked if there was a risk of illness but not how great a risk we were asking about.  Perhaps that explains why the majority answered "yes" even though most scientists' comments indicate that the risk is minimal.


* The scientists contacted for this survey are all faculty members of North American college or university departments related to the food sciences. Their programs are on the "approved" list of the Institute of Food Technologists (IFT).


To read Parts 2 and 3 on our survey results, click on the following titles:


"Is It Safe?  Is It Nutritious? More Survey Answers from Scientists"


"Organic Food, GMOs, the Safety of American Food, the Value of Use-By Dates, and More--Scientists Tell Us What They Think"





ShelfLifeAdvice.com survey, completed December 31, 2013.


Shelf Life Advice Advisory Board members who contributed to this article:


Karin E. Allen, Ph.D., Utah State University, Dept. of Nutrition, Dietetics, and Food Sciences


Timothy J. Bowser, Ph.D., Oklahoma State University, Dept. of Biosystems and Agricultural Engineering


Catherine N. Cutter, Ph.D., Pennsylvania State University, Dept. of Food Science


Joe Regenstein, Ph.D., Cornell University, Dept. of Food Science



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