Summer means cooking, picnics and barbecues in the backyard. But a generous spread of food eaten outside raises some serious health questions. No one wants food poisoning – or to make their guests sick. But how do you know when you kept the potato salad or fruit mix too long?

As professor and chair of the Food Science and Human Nutrition program at Iowa State University, I will answer those questions starting with the basics of food safety.

There are two general classes of food-related microorganisms. Pathogens make you sick. Other types of organisms make food look, smell and taste bad – in other words, they cause food to spoil.

It is usually very easy to tell if decaying microorganisms have invaded your food. Molds and fuzzy growth appear in solid foods. The liquids look cloudy or lumpy and often smell bad. Eating spoiled food is never a good idea, and you are smart to err on the side of care. When in doubt, throw it out.

A chef cutting vegetables on a cutting board.
Do not use the same cutting board for meat and vegetables. (Photo by Getty Images / Enrique Díaz / 7cero)

Cutting boards and kitchen thermometers

Pathogenic microorganisms in foods are much more secretive. These microorganisms are what cause cramps, vomiting, diarrhea, fever and chills – symptoms that people associate with “stomach flu”. Viruses also cause foodborne illness. Typically, detection of pathogens in foods by smell and sight is not possible. So proper handling and storage, and knowing when to dispose of waste, is critical.

The first rule of food safety is to keep the preparation areas clean. Developing a treadmill helps. Always wash your hands before using food. Be sure to thaw the meat in the fridge, not on the table. Otherwise, as frozen meat lowers to room temperature, its outer surfaces heat up faster than its inner surface. This allows pathogens to multiply.

Do not use the same cutting board for meat, fruit and vegetables. In my kitchen, a cutting board is for meat; green for fruits and vegetables. Use different knives, plates and utensils for raw meat, and always place cooked meat on a clean plate.

Never wash raw meat or chicken in the sink, as this practice spreads bacteria on kitchen surfaces. In fact, there is no need to wash meat and chicken before cooking. But if you insist, clean the sink with an antibacterial cleaner after removing the food. This is the “after” – make sure you do not contaminate any food with the cleaner.

Any pathogen will be destroyed by fully cooking the meat at the recommended temperatures. Invest in a good kitchen thermometer. Although the recommendations may vary slightly, you basically want an internal temperature of 160 F (71 C) for beef and pork, 165 F (74 C) for poultry and 145 F (63 C) for fish and bacon. After cooking, keep food hot at 140 F (60 C) or higher. When transporting or serving food for a period of time, keep cold foods on ice or in a cooler, especially during the hot summer months.

A man takes the leftovers out of the fridge.
Do not allow the waste to stay too long in the refrigerator. (Photo by Getty Images / Jupiterimages)

Dealing with waste

After the meal is over, do not let the leftovers last. Move quickly to the refrigerator.

As a newlywed, I spent Thanksgiving at my mother-in-law’s home in north Minnesota. After dinner, they took all the serving dishes – turkey, stuffing and mashed potatoes – and placed them at the entrance to the screen to be stored. It was probably less than 20 F (-6 C) degrees outside – but still, this is not a great idea because the weather changes quickly and temperatures will fluctuate, leading to the risk of pathogen growth.

My husband also believed that foods should be refrigerated on the counter before putting them in the fridge; he said it reduced stress in the fridge. This is not necessary and increases the risk for food pathogens. Modern refrigerators are fully capable of quickly cooling hot foods while maintaining their internal temperatures, so do not hesitate to remove those residues as soon as possible.

Now, with the fridge full of leftovers, how long are they good to eat? Most cooked foods are safe to consume within three to four days. After that, the risk of pollution increases. If you have more leftovers than you can eat in that time frame, put it in the fridge. Be sure to cook the leftovers at 165 F (74 C) before eating.

Baked goods like bread, cakes, pies and biscuits made in your kitchen will have a shorter shelf life than store-bought items because yours are preservative-free. They will become stale, lose their structure faster and grow mold. Once you see it, throw it all away instead of trying to cut out the stained spots. While it is unlikely to cause serious illness, some molds produce toxins that can cause problems, especially for children or the elderly.

Foods with higher moisture content break down faster because water gives bacteria a chance to grow. So carrot or pumpkin bread cakes spoil within about five days. Put these items in the refrigerator, and you will increase their shelf life. Pitas should be stored in the refrigerator and eaten within three to four days. Biscuits usually have low humidity, except those that contain fruit, jam or cream. Keep these types of cookies in the refrigerator and discard if mold starts to grow.

As you get ready for your summer meetings, keep in mind that food waste reduction is good for both the environment and your budget, so consider portion sizes and the amount you are doing to better manage waste. And remember that proper treatment as you prepare and then store your meals will ensure that you and your family enjoy your cooking, parties and reunions without any foodborne illness.

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