It’s a tough time to be working on improving food shelf life. Traditional ways of ensuring shelf safety, such as balancing the lipid phase and aqueous phase with stable fats and sufficient sugar, salt and/or food acids, plus a dose of antimycotics, isn’t acceptable for most new products anymore.
The products themselves have changed, too. Some Americans prefer foods that are minimally processed, that contain less fat, salt, sugar and other carbohydrates and more fresh ingredients. Shelf life isn’t just a measure of months on a grocery shelf; it can mean an extra day of flavor in the delicatessen or added usability at home.
There are several aspects to shelf stability: flavor and color retention, microbial safety and retarding lipid oxidation. These are exacerbated by the emphasis on "fresh" and even "organic."
A few decades ago, the U.S. could boast the safest food supply in the world. Safety was guaranteed by commercial sterility — often via retort in a sealed metal or glass container heated to an internal temperature high enough to kill off microbes. Other foods were managed through water activity (enough sugar, salt or acid in the water phase, which was partly controlled by having a hefty fat phase) or by the addition of chemical preservatives.
Times change. While these tactics still help control shelf life for many foods, new consumer purchasing trends demand a look at new shelf-life technologies.
"The shelf life of food…depends on four factors: formulation, processing, packaging and storage conditions," notes food scientist and water activity expert Ted Labuza. Change any one of these conditions and you can change the shelf life — for better or worse.
Shelf life has many attributes: bacterial control, color stability, yeast and mold inhibition, flavor stability, textural stability and aroma stability. The appearance of shelf-life problems can be wide ranging, as well, including oxidative browning, oxidation of flavor compounds (especially in a lipid phase), syneresis (weeping and watering) or deliquescence (stickiness and unusual patterns of crystallization).
To increase the shelf life of more unstable foods (such as low-sugar jam, low-salt condiments, low-oil salad dressings, prepared fresh produce and deli meats), food companies may take steps to reduce bacterial load from ingredients before they process the final food.
To grow, thrive and survive, microbes need a friendly environment. This usually (although not always) includes moisture. Lowering moisture (decreasing water activity) creates a hostile environment for bacteria by decreasing the available medium for them to grow in. Moreover, decreasing water — especially when increasing solute (for example, sugar) — increases osmotic pressure on the bacteria, causing them to burst.
There are a number of new ingredients to help regulate water activity in foods with a reduced fat phase. Generally, the water activity of a minimally processed food needs to be about 0.07 or lower. Water has an aW (activity of water) rating of 1.00, and most products preserved with sugar, measure about 0.07, with the excess water bound so that bacteria are under osmotic pressure too great to survive.
Adding sugar was a traditional way to control water activity to prevent bacterial growth, but marketers don’t usually want to feature lots of sugar. Currently, water activity is increased in "fruit only" jams and jellies by starting with fruit concentrates and preparing under vacuum. Similar techniques are used in other "sugar-reduced" foods.
The Maillard reaction requires sugar, protein, a friendly pH and water. If the Maillard reaction in foods containing sugar or corn syrup is a problem, a formulator may want to work with trehalose, a disaccharide with no reducing power because of its shape. Entering the market in the late 1990s, trehalose reduces water activity to about the same level as sugar, but doesn’t break down with acids.
Polydextrose, a randomly cross-linked glucose whose shape prevents total metabolism (providing about 1 calorie per gram), also offers bulking and water-activity reduction, and is often used with high-intensity sweeteners.
Salt is used less often to control water activity, especially now that some consumers are watching salt intake. But finely ground salt can help control water activity, especially in combination with sugars.
Antioxidants and lipid oxidation
Consumption patterns over the past few years emphasize "fresh" or "fresh-like" foods that can be purchased and eaten with little preparation. Shelf-life increases of a few days can make a major difference in the ability to provide a successful product line.
Arby’s Restaurants are specifying deli meats that have an extra margin of shelf life. "Just a couple of days can make a big difference," said a department manager at RTM Group, Atlanta, the largest franchisee of Arby's restaurants.