Tag: food

A scientist’s guide to refrigeration: Will freezing kill bacteria in food?

On By Catherine HausteinIn UncategorizedLeave a comment

A well-publicized recall of frozen foods due to bacterial contamination lead a friend to ask a question: why didn’t the freezing kill the bacteria?

My first thought was the somewhat well-known idea (among science types) that freezing doesn’t kill the bacteria, only slows it down. The answer, however, is more complex.

It’s logical to think that putting food into the freezer or buying frozen foods will cut down on the bacteria and thus the food poisoning. However, freezing doesn’t kill the bacteria in foods.Freezing and thawing cycles can reduce the number of bacteria but doesn’t kill them all. Bacteria can respond to freezing by producing a type of antifreeze or by forming a biofilm. The susceptibility of the bacteria to freezing varies across the type of bacteria correlates to the freezing and thawing cycles as well.

In a study of soil bacteria exposed to freezing and thawing conditions, freezing reduced the number of bacteria but did not totally eliminate them. When numerous freezing and thawing events occurred, damage to the bacterial colonies was significant. However, cold-resistant bacteria developed. Quick freezing and thawing reduced bacterial levels less than slow freezing and thawing. This was due to the type of ice formed—slow freezing produces larger ice crystals which are more damaging to bacteria. What does this have to do with a frozen food recall? Consumers don’t like big ice crystals and often additives such as xanthan gum are added to food to prevent them. ( reference here)

Common food related bacterial contamination comes most often from these four horsemen: Campylobacter, E. coli, Listeria, and Salmonella.

Most of the 2025 frozen food recalls have been due to Listeria. Frozen peas and carrots, ice cream bars, and the more extensive Chicken Fettuccini Alfredo are recent examples. Listeria is the third most deadly bacterial food toxin here in the US.

Listeria grows best under warm conditions but even continues to grow under refrigeration. It isn’t killed by freezing and in fact, is one of the most freeze resistant bacteria around. Listeria will cause flu-like symptoms and can even lead to seizures since it primarily attacks the central nervous system. It can be deadly for older people, immunocompromised folks, and infants. The mortality rate is as high as 20% in vulnerable populations. It causes miscarriages and stillbirths, too.

Food poisoning is the most common way to encounter Listeria. Listeria is a persistent bacteria and commonly found in slaughter houses, especially on conveyor belts for carcasses. Listeria forms a biofilm, often working symbiotically with other bacteria to strengthen this pervasive film. (reference)

One way for you to keep ahead of Listeria is to keep your refrigerator clean and to wipe down surfaces and cutting tools after use. Washing produce with warm water before eating it is another recommendation, although produce is not major carrier for Listeria. Use leftovers within three to four days and don’t just let them sit in there until you find time to throw them out. Toss them immediately—they could contaminate other food. Be sure to cook foods thoroughly, even leftovers. (reference)

I don’t eat lunch at work anymore but I’m a fan of frozen burritos. What should I do? A food thermometer is more than a luxury–it’s an important safety tool. Don’t simply reheat a frozen meal. Make sure to cook your frozen meals that least 165 degrees F. Steam should rise from the product. If storing a heated up frozen meal, put it in the refrigerator within two hours and eat it promptly.

Besides contaminated frozen foods, Listeria can be found in soft cheeses and raw milk products. (reference) It’s even been found in ice cream bars this year. Sugar helps it grow! Vinegar and low pH can discourage it. 

Taking a step back, frozen foods have only been around for about 100 years. They were introduced to Western society by Clarence Birdseye, who leaned techniques from Inuit and Metis communities in Labrador, where he appropriated their knowledge while failing to acknowledge their contributions. Beginning with frozen fish in 1922, Birdseye moved on to become synonymous with frozen foods, thanks to the quick freeze technique he learned in Canada. Freezing prevents food waste. About 30% of all food in the US becomes waste while only 6% of this is frozen foods. Freezing food preserves nutrients, especially Vitamin C and when used within a year.

One recent study found that improper storage and handing of foods was responsible for the most food poisoning deaths. (reference) For mass produced foods, food safety regulations and well-paid, well-trained employees are an essential part of food safety. (reference) The infamous deli meat Listeria outbreak was linked to poor plant hygiene. It’s important to have proper regulations and to enforce them. However, e have taken a step backwards since November 2025. Thanks to lobbying from the grocery and restaurant industries, our food is now inspected less and outbreaks not tracked.

The frozen food industry has a new trick for keeping ahead of bacteria without adding additives to prevent large ice particles–Freezing in the presence of a magnetic field to reduce ice formation. However, the effect on bacterial formation hasn’t yet been determined.

As for me, I’m making ample use of my food thermometer and cleaning out my refrigerator! 

Oil’s long, dirty, and highly useful fingers

On By Catherine HausteinIn chemistryLeave a comment
Do you think most acetic acid comes from fermented apples? Think again.

Saudi Arabia is going to cut oil production and although they say they are not doing it for political reasons, we can see our politicians here vocalizing what could be their wishes such as finishing the Keystone pipeline which would bring crude to their refinery in Texas. It’s hard to imagine that some US pols even speak against electric cars made in the USA. Or maybe not. If we all switched immediately to electric vehicles powered by wind and solar energy, would we still need oil? The answer is, yes.

Fuel oil and gas are not the only petroleum based products. The paving and roofing material asphalt is a complicated mixture of large hydrocarbons and plenty of sulfur, vanadium, and nickel impurities and is petroleum based. Tar can be made from coal or found naturally, as in the La Brea Tar Pits in Los Angeles where fossils of mammoths and dire wolves have been found. But tar and asphalt are not the only additional uses for petroleum.

Petroleum is the starting material for most of our plastics and synthetic materials, everything from fibers to pharmaceuticals, starts out as a form of oil.

Hydrocarbons can be chains or rings, and are distinguished by their composition-molecules made from just two elements, carbon and hydrogen. These materials can be light and flammable like naptha and gasoline or heavier as with asphalt and tar. Although they are useful in their own right, organic chemistry can step in and add elements to the hydrocarbons to make them into entirely new compounds. But they are the necessary beginning–the feed stock so to speak.

Here is an example of making something simple, acetic acid, as found in vinegar. Acetic acid contains oxygen in addition to hydrogen and carbon. It can be produced by fermentation as in this reaction where the acetic acid is bolded.

2 CO2 + 4 H2 → CH3COOH + 2 H2O.

It can also be made from alcohol, something you don’t want happening to your wine, for example, in this reaction starting with ethanol (as in wine) and adding oxygen naturally. It’s why you need to carefully control the amount of oxygen when wine making.

C2H5OH + O2 → CH3COOH + H2O

With the proper catalysts, acetic acid can easily be made from oil, for example as in this reaction:

2 C4H10 + 5 O2 → 4 CH3CO2H + 2 H2O

Why would anyone do this when acetic acid can be made from fermentation? We need a lot of it. Non-food acetic acid has been produced industrially since the 1960s and accounts for 90% of the usage world-wide. Over 5 million tons are produced each year. It’s a high demand chemical used to make coatings, paints, inks, and plastics such as PET. It’s one example of how we use chemical feed stock petroleum to make products we use every day.

Twenty percent of each barrel of oil is used as a chemical feed stock and when oil goes up in price, so does anything made from oil.

There are biological ways to make synthetic materials as discussed here and in my novel Lost in Waste. But as long as there is plenty of oil available, it has a long history of being used as a fuel and a feed stock and this isn’t likely to change anytime soon. Kicking the world’s oil addiction won’t be easy, unless we want to go back to life as it was 100 years ago. And we all know how some politicians love plastic bags. Fortunately, crude oil prices are still much lower than their highest point in 2008 so don’t despair. We will have plenty of low cost cigarette butts other plastics in the near future. In fact, the petrochemical industry will no longer be investigated if a plant catches on fire. That’s bad for people who lives near the plants, most on the Gulf Coast, but good for plastic prices.