Recently, a message has gone viral in WeChat Moments: “Wrap watermelon in plastic wrap and put it in the refrigerator…it can kill people!” Sources say this method of preservation will help bacteria multiply. 【Oxford cloth】
Is this really the case? In addition to watermelon, if other fruits are wrapped in plastic wrap, will bacteria multiply? In this issue of the People’s Livelihood Quality Laboratory, we selected a variety of fruits, including watermelons, for experiments, and invited experts to tell you based on the experimental results: whether you should wrap the fruits in plastic wrap before putting them in the refrigerator in summer.
Experiment 1
Sold Watermelon with “plastic wrap”
There may be a lot of bacteria when you buy it
Although the news that watermelons wrapped in plastic wrap can easily nourish bacteria has gone viral on the Internet, a reporter from Chongqing Morning News walked around the market and found that most cut watermelons are still sold wrapped in plastic wrap. How many bacteria do these watermelons sold wrapped in plastic wrap contain? The reporter purchased a cut and wrapped piece of watermelon in plastic wrap from two fruit shops and three supermarket chains, and sent it to a testing agency for a total bacterial colony detection experiment.
The results showed that among the five samples, the lowest total number of colonies was only 210CFU/g, and the highest was 70,000CFU/g. “The value of 70,000 is higher than the upper standard limit of the total bacterial count of many foods.” The inspector said, taking the requirements of the “Hygienic Standards for Cooked Meat Products” as an example, the standard for the total bacterial count is no more than 50,000 CFU/g.
Through testing, it was found that among the 5 samples submitted for inspection, the total number of colonies in 3 samples exceeded 50,000 CFU/g. “In other words, these watermelons wrapped in plastic wrap have more bacteria when consumers buy them.” The inspector said that this may be due to the fact that the knife used to cut the watermelon has more bacteria or the watermelon has been stored for a long time.
Experiment 2
Pack fresh Membrane watermelon
Bacteria increase more than 40 times in 17 hours
We also picked a whole watermelon in a supermarket, cut it into two halves, wrapped half in plastic wrap and half without plastic wrap. We put the two pieces of watermelon into sterile food bags and sent them for inspection. After 1 hour, the total bacterial colony detection values of the watermelon without plastic wrap and the watermelon wrapped with plastic wrap were 1000CFU/g and 2100CFU/g respectively. Obviously, bacteria grow faster in watermelons wrapped in plastic wrap.
After that, the inspector put the two pieces of watermelon into the refrigerator to refrigerate, one without plastic wrap and the other with plastic wrap.
Seventeen hours later, inspectors found that the bacteria in both pieces of watermelon grew faster. Among them, the total number of bacterial colonies on the watermelon without plastic wrap increased to 34,000 CFU/g, an increase of 34 times compared with the previous test; the total number of bacterial colonies on the watermelon wrapped with plastic wrap increased to 88,000 CFU/g, an increase of nearly 42 times compared with the previous test. .
This shows that whether the cut watermelon is wrapped or not wrapped in plastic wrap, the total number of bacteria will decrease after being refrigerated for a long time. Significant growth. “But if it is not refrigerated, bacteria will grow faster.” The inspector warned. 【600D Oxford cloth】
Experiment 3
Wrap different fruits in plastic wrap
Hami melon has the most bacteria detected
The reporter’s investigation found that in addition to watermelons, there are many fruits on the market that are also sold wrapped in plastic wrap. The reporter randomly purchased watermelons, grapes, plums, peaches, and cantaloupes wrapped in plastic wrap and sent them to the laboratory for testing. Experimental results showed that the total number of bacterial colonies on these fruits wrapped in plastic wrap exceeded 1,000 CFU/g, with cantaloupe having the highest bacterial count at 28,000 CFU/g.
This cantaloupe with a high bacterial count was cut into pieces and placed in a plastic box, then sealed with plastic wrap for storage. Afterwards, the inspector sealed the cantaloupe with plastic wrap and put it in the refrigerator. One and a half hours later, the total number of colonies tested again reached 67,000 CFU/g.
After 17 hours of refrigeration, the total number of bacterial colonies in this cantaloupe reached 230,000 CFU/g, which was the highest number of bacterial colonies among all the fruits tested this time. The highest total.
“This is not to say that cantaloupe has more bacteria than other fruits, but to remind consumers that some fruits that appear to be well sealed , there may be a lot of bacteria.” The inspector said that the number of bacteria in different fruits is also related to the environment in which they are located.
Wrap fruit in plastic wrap
It is easy to create a better breeding environment
Why do fruits covered with plastic wrap generally have high bacterial counts during testing? Experts from the Municipal Institute of Metrology and Quality Inspection believe that the reason is not the plastic wrap itself. Experts said that the plastic wrap The production requires high temperatures, and it does not carry bacteria. “When we did the above experiments, we also tested the total number of bacterial colonies on the plastic wrap and found that the bacteria they carried were generally lower than the bacteria on the fruits they wrapped. ”
Expert analysis said that the total number of bacterial colonies on fruits wrapped in plastic wrap is likely to be higher, mainly because after wrapping the fruit in plastic wrap, A relatively closed space is formed between the fruit and the plastic wrap. This environment provides better conditions for bacterial reproduction. However, experts say that the plastic wrap can better retain the moisture of the fruit and achieve a fresh-keeping effect.
“Through experiments, we can also see that whether there is plastic wrap or not, bacteria will breed in fruits if they are stored for a long time. “Experts suggest that if you want to reduce bacteria on fruits, you must first ensure that the knives used to cut the fruits are hygienic. At the same time, whether the fruits are wrapped in plastic wrap or not, it is best to wash them again before eating, or use a clean fruit knife to cut off the pulp and Plastic wrap or the part in contact with the air. In addition, it is best not to leave the fruit overnight after cutting it.
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