Last weekend while making a sandwich, I pulled out the cheese and found a lovely kaleidoscope of colors. Colorful cheese is usually a bad sign, but it’s actually a bit more complex - and how can we tell when other foods have gone bad?

No, please don’t trust expiration dates; they’re only a suggestion. An expiration date may encourage us to waste perfectly good food, or mislead us into eating food that has already spoiled. Trust me, I’m a doctor: food poisoning is no fun at all. Here are a few guidelines to follow.

We all know that it’s easy to tell whether milk has spoiled – it just takes a sniff. Milk spoils due to microbes that pasteurization misses. Pasteurization is not perfect, so some bacteria are left behind. These bacteria dine on the lactose and excrete lactic acid, which starts a chain reaction.

Let me share my wife’s trick: sometimes, the only milk that has gone bad is well above the level of the liquid, at the very top of the container. Give that top part inside a wipe with a clean cloth or paper towel and then sniff again. Another sign of spoilage is when the milk at the surface separates.

Non-dairy milk, such as soy and almond milk, is completely different. It’s so stable that an unopened container has a shelf life of about a year, and once opened, will last up to 10 days. Separation in non-dairy milk is not a sign it has gone bad - shaking will restore the consistency. Sniff and look at the milk: if it has a sour smell, or is starting to clump, toss it.

Butter is perishable, and will eventually smell and taste bad. Its base of cream causes it to turn rancid. It’s fine to keep butter out on the counter, but there it will be exposed to both heat and light, which will make it go rancid faster. Note that unsalted butter doesn’t last as long as salted butter.

What about peanut butter? It’s pretty amazing - even at room temperature, and even after opening, it can last for months. Due to its high fat content and dry nature, most bacteria don’t survive. Separation is not a sign that it has gone bad; just stir it again. It goes bad by becoming rancid, which changes the flavor and smell. The sniff test should work, and apparently, eating rancid peanut butter will not harm us.

If mold is visible on one slice of a loaf of bread, even with no visible mold on the rest, none of the loaf is safe to eat. This is because bread is porous, and mold spores can spread easily throughout the loaf. If bread has become hard and dry but has no mold, it’s OK.

Many of us keep rice in the cupboard. Most rice, including white, wild, jasmine, basmati, and Arborio, can last a very long time if stored properly, keeping moisture and pests out. Brown rice, however, has a higher oil content, and can go bad in three to six months at room temperature (longer if stored in the refrigerator or frozen). If old rice has become contaminated with bacteria, cooking will not make it safe to eat. If your rice has started to smell a little off, or seems crunchier or drier than usual, throw it away.

If the color of red meat has changed, it hasn’t necessarily gone bad. Meat may become brownish-red due to a reaction with air. However, fading or turning very dark is a sign of aging. Other very bad signs include developing an odor or turning tacky, sticky, or slimy.

How about poultry? Color alone is not the best indicator for whether it has gone bad, as color can normally range from a bluish-white to yellow, depending on living conditions. Seeing darkening around the bones and different shades of pink and red are all normal. However, bad poultry will smell, and develop a slimy or sticky texture.

Bad fish can make us quite ill. Choose fish that is cold to the touch and doesn’t have a strong fishy smell. Fresh fish is extremely perishable: it must go straight into the refrigerator, and don’t keep it around for more than a day or two.

Eggs are pretty easy. Spoiled eggs will give off the distinct odor of sulfur, even after cooking. Contrary to popular opinion, if an egg floats, it’s not necessarily bad. It has, however, aged. When an egg is laid, there is no air inside. As it cools, the contents inside the shell contract. The porous shell allows air to enter, and the egg becomes buoyant. It must be cracked open in order to tell whether it is still good to eat.

What about canned foods? Beware of dented, corroded, or broken cans, as any damage to the can could compromise what is inside. If you see anything suspicious, throw it out. Non-acidic canned goods like soups and puddings last much longer than acidic foods like fruits and tomatoes.

It’s fairly easy to tell when vegetables go bad. Keep an eye out for changes in color, or becoming soft or slimy. Wilted lettuce, celery, and other leafy greens are fine if you remove the bad parts. For cucumbers, zucchini, and squash, cut away the outer layer as necessary. Sprouted onions and potatoes are fine after removing the sprouted parts.

Fruit is more complicated. Take bananas - even when they turn black, they are perfectly fine to eat, and even sweeter when used in banana bread. Citrus fruit is only bad when it is rotten all the way through - just discard the bad parts. The same applies to bruised fruits. If there is mold on berries, cherries, or grapes, throw away the bad ones and thoroughly rinse the rest before eating.

Personally, I think that mushrooms are bad from the start, but I’m told that’s just my childhood bias! To me, they smell strange even when fresh, and always have an odd texture. They have a shelf life of about two weeks. Signs that they are turning bad include wrinkling, turning darker, having a changed smell, and slime.

Let’s get back to cheese. Cheese is tough because there are so many different odors and types. For lifespan purposes, cheese falls into three categories. Dry, hard cheeses like cheddar and Parmesan can last up to six months in the refrigerator. Blue, green or white mold that may show up can simply be cut away. If the mold is any other color, throw away the cheese. Soft cheeses including cream cheese, mozzarella, feta, and Brie go bad quickly due to their high moisture content. When they start to turn, throw the entire cheese away. Other cheeses fall somewhere in the middle.

Freezing is generally the best preservative for our foods. If a freezer keeps a proper and consistent temperature, the food won’t become dangerous – however, the quality will decline. In the event of a power outage, keep the door shut, and there should be a day or two before food spoils. If any frozen food has partly thawed, or rises above 40 degrees, or smells or looks bad, throw it away!

Bad food, or even potentially bad food, is NEVER worth the risk.

Dr. Alan Frischer is former chief of staff and former chief of medicine at Downey Regional Medical Center. Write to him in care of this newspaper at 8301 E. Florence Ave., Suite 100, Downey, CA 90240.