Water Activity for Food Quality

Everyone knows the importance of water activity in food safety. Read on for "the rest of the story."

SO MUCH GOES INTO MAKING THE PERFECT FOOD- cooking to the palate of Joe and Jane Public is an art. An acquaintance of ours is a cookie artist. He has found the perfect cookie recipe: soft, moist, and chewy. Unfortunately, now he has mold problems. "I spent a bundle on a moisture balance to measure moisture content, but it hasn't solved the problem," he confided. "How can I get a handle on this? Am I measuring the wrong thing?"

He's on the right track-water is the problem. But just getting a "water number" won't help. He needs a number that accurately and consistently predicts product quality. The five senses are excellent detectors of quality. A customer biting into a cookie knows immediately if it's moldy or stale, just as a fan walking into the Astrodome knows if it's chilly inside. But as a facilities manager, I would like to accurately predict how the fan will feel before he enters the building. Knowing the heat content of the Astrodome is useless to me. I need to know the temperature. In the same way, you want to predict product quality before your customer ever takes a bite. Water content is practically useless to you. You need to know the water activity.

Water activity describes quality because it describes more than how much water is in the product. It describes how much water is available, ready to be used. And knowing that number gives you information that you otherwise wouldn't have, letting you predict what your customer will taste without ever having to taste yourself.

Our friend the cookie artist needs to know absolutely whether or not his product will mold on the shelves. But he also wants to keep the cookies moist and chewy. The cookies will be the right consistency within a certain range of water activities. And within another range, he is confident of avoiding mold. Put the ranges side by side, and he'll find his "water activity sweet spot," a small range of values that describe the perfect cookie. If you hit the sweet spot, you get the perfect cookie. Every time.

After finding his sweet spot, he adjusts his cookie recipe. Then he measures water activity of the finished product, even doing spot checks on the line. As long as he stays within that value, he can ship to market with confidence. His moisture balance, accurate as it is in finding water content, can't make that prediction of quality. Moisture content tells how much water, and nothing more. It is almost useless as a measure of total food quality.

When it comes to food safety and quality, knowing the water activity is powerful. To food dehydrators, avoiding spoilage is the key. They know they need dry, but how dry? Water content measurements are a sloppy way of measuring "dry enough." They lead many dehydrators to over-dry their product. There is a specific water activity number below which food will not spoil. By measuring water activity, dehydrators get the product exactly dry enough, without getting it too dry. It's still a safe product, but now it also has lower production and ingredient costs.

Combine crackers at 20% water content and cheese filling at 30%. A recipe for soggy crackers? Not if the two ingredients have the same water activity. Need to avoid clumping and caking in a batch of spices? Match the water activities of the components and the problem is solved. Vitamin degradation is a function of water activity. So are lipid oxidation, crunchiness, chewiness, softness, and many other factors of quality. A measurement of water content will tell you how much water is in a product, but that's all. It can't answer any of these other quality and safety issues.

So why do so many people still measure water content? For most, it's because it's what they've always done. Some have contracts with suppliers or buyers that specify a certain moisture content. But measuring water content only makes sense if "How much water?" is the only question that needs an answer. If product quality and safety are the issue, water content is almost meaningless. People only keep measuring it because they don't know a more powerful measurement is out there, one that answers the real questions quickly and accurately.

If you have quality or safety issues, call the food scientists at Decagon for a free consultation. You may find that measuring water activity is a powerful solution.