JACKI LYDEN, host:

It's that time in the summer when produce is at its peak. And you've got more berries and zucchini from your garden than you could eat in a month. To use up this bounty, you may decide to make jams or pickles. But why does packing fruit in sugar or soaking cucumbers in brine keep them from spoiling anyway? How can meat stay edible for months, even years, after it's been smoked or cured?

These are the questions we'll try to answer in this week's Science Out of the Box.

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LYDEN: Our preservation expert today is Barry Swanson, a professor of food science at Washington State University. Welcome.

Dr. BARRY SWANSON (Food Science, Washington State University): Well, hello.

LYDEN: Now some of us, of course, have preserved things for decades in our refrigerators, but that's not what we're talking about right now, right?

Dr. SWANSON: No, not at all. We're talking about preserving them over a long period of time.

LYDEN: In a way that you'd actually want to eat it again. What is it exactly that we're trying to stop or arrest in terms of keeping food from spoiling or going bad?

Dr. SWANSON: Well, there's about three things we're trying to stop. We stop -trying to stop the physiology of the food itself for fruits and vegetables. We're trying to stop the microorganisms from growing. And we're just trying, overall, to maintain the acceptability and quality, so there's no chemical reactions that are degrading the product itself.

LYDEN: We want to stop the physiology of the food. Can you explain what you mean by that?

Dr. SWANSON: Plant products are basically alive. In other words, they're respiring, breathing objects. So they have their own physiology. So plants are breathing, we want to slow that down. Plus, they have enzymes, which are acting just like our enzymes that act in our body - breaking down starch and breaking down fats and metabolizing those objects. So we want to slow down the physiology of the plant materials. We want to slow down respiration. We want to inhibit the growth of microorganisms.

LYDEN: Let's start with smoking. Why does that keep meat or fish edible?

Dr. SWANSON: Well, generally, when you do smoke a meat product, whether it be fish or poultry or whatever, you are heating it up as well as adding some chemicals from the smoke itself, which are inhibitory to most microorganisms.

LYDEN: My grandfather used to smoke his late catches in an old refrigerator in the backyard, and they were kind of this, you know, golden strings of smoked walleyes or trouts.

Dr. SWANSON: Sure.

LYDEN: Basically, he had just killed all the organisms?

Dr. SWANSON: Right. Well, you've inactivated most of the organisms. I wouldn't say you've killed them all, but you've inactivated most of the organisms with the temperature that you've brought the fish to when it was being heated. In addition to that, of course, as I mentioned, you're adding some chemicals, which are generally inhibitory to most microorganisms.

LYDEN: What would those chemicals be?

Dr. SWANSON: Well, they appear in the smoke and they're - many of them are chemicals, which, I guess, are - we would call preservatives if they were added to the food product, but they are naturally added in the smoke.

LYDEN: And drying. How does drying work?

Dr. SWANSON: Well, drying basically is just a removal of moisture. And you could either remove it physically by evaporating the moisture, as such as people do when they hang cabbage leaves on their balconies and so forth in the Far East, or you can actually remove the moisture by adding, again, additives or preservatives, which bind the water and stop the microorganisms from using the moisture.

LYDEN: Well, salt, of course, removes moisture, right?

Dr. SWANSON: Salt will remove moisture. It also has a tendency to set up an osmotic pressure between the organism and the microorganism so that they may not grow very well.

LYDEN: Osmotic pressure. All right, pretend I'm back in sixth grade. Can you run us through that?

Dr. SWANSON: Right, we're talking about - basically, it's a system where water always goes to the more concentrated state. If you have two different concentrations of moisture and contact with one another with a membrane in between and with the microorganism generally there is a physiological concentration, which must be maintained for the organism to live. When you put a very highly concentrated salt solution around that organism, the water from inside the organism penetrates the membrane and goes into the more concentrated salt solution and prevents growth of that microorganism.

LYDEN: Does it work the same way in jam?

Dr. SWANSON: It works even better with the sugar. Jams and jellies, of course, unless you're making a freezer(ph) jam, is more - are most often heated to a very high temperature also, which will inactivate most of the microorganisms present.

LYDEN: This all, sir, is making my lips pucker. One last question, Dr. Swanson, honey can sit on a shelf for years without going bad, without any preservatives or special processing, maybe it will get hard (unintelligible), just, you know, steam it in a pot but it's still good. Why does honey seem to fall outside the pickling, preserving…

Dr. SWANSON: Well, honey is very, very high in sugar. It's about 65, 70 percent sugar, which it - basically is a dehydrated food. They're just - there's no water available for any chemical reactions or water available for microbial reactions.

LYDEN: All this makes me think that sugar and salt are just about two of the greatest discoveries ever.

Dr. SWANSON: They are very, very important to our - to the preservation of foods.

LYDEN: And when you put them together - magic. Barry Swanson, a food scientist at Washington State University. Thanks very much for joining us.

Dr. SWANSON: You're welcome. Thank you very much. It was a great pleasure. Transcript provided by NPR, Copyright NPR.