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.
"We learned a lot about keeping roast beef tasting good with antioxidant material, and have extended that information in our new Market Fresh Party Platter and Market Fresh salads," the RTM official adds. The Platters, recently introduced in select Arby’s locations, use deli meats specifically selected for shelf life.
Keeping fat fresh can help improve the nutritional content of foods, according to a study at the University of Extremadura, Spain. Researchers found fat oxidation reduced the amount of essential fatty acids and vitamins and generated toxic compounds, reducing the nutrient content and increasing potentially harmful compounds.
The Extremadura scientists also studied the mechanism of protein oxidation and found rosemary extracts improved protein functionality as well as prevented lipid oxidation, thereby retarding the resulting soapy flavor in frankfurters.
Natural antioxidants have improved in the past several years, with much research devoted to the compounds. Vitamin E as tocopherol has been used in animal feed to increase the freshness of flavor in the final meat products. It also has been found to improve the stability of myoglobin in ground beef, providing better retention of color in final products.
A number of products have been developed. Archer Daniels Midland (www.admworld.com), Decatur, Ill., offers vitamin E products made from "identity-preserved" grain. These can be certified as non-GMO. The product is in limited quantity because of the difficulty in ensuring grains are identity preserved and non-GMO.
"Producing vitamin E from only GMO-free grain is getting harder," notes Greg Dodson, ADM’s market manager for vitamin E and associated products. "We have to segregate non-GMO grains and keep them identity preserved, then process them separately. As more grain is grown from GMO germplasm, it’s harder to contract those acres and keep them separate. The cost is higher — sometimes 40 to 60 percent higher depending on quantity and degree of processing — but it’s one of the only products that can be used on organic food, so it’s worth it."
Vitamin E may be introduced into meat as a feed additive to the live animal. Studies by USDA, universities, feed manufacturers and vitamin manufacturers have shown vitamin E as a feed component does a number of interesting things. BASF Animal Nutrition (www.basf.com), Florham Park, N.J., found in studies of supplemental vitamin E in beef that color stabilization in the meat case was improved when animals were fed vitamin E.
Meat from vitamin E-enhanced beef stayed bright red longer (except for top sirloin steak) and had several hours more case life before color change occurred. The greatest increase was in beef round cuts. Different cuts retained the supplemental vitamin E differently, with tenderloin retaining much more of the vitamin than other cuts from the same animal.
Rosemary extracts are available from a number of companies and are used to retard the appearance of "warmed-over flavor" in beef products. "Rosemary extracts are available in water-based or powdered forms and are often used in fast-food burger patties, as well as products for retail," explains Jeff Sporrer, market manager for Kemin Inc. (www.kemin.com), Des Moines, Iowa. "The improvement in shelf life is fairly dramatic."
Kristen Robbins, associate scientist at Kemin, says ground beef, as well as sausage from pork or poultry, can be improved with the addition of water-soluble liquid rosemary extract that is easily dispersed in the brines used in making cooked deli meats. Use levels as low as 0.05-0.1 percent help delay the onset of oxidative rancidity.
The use of either dry or liquid rosemary extract is useful in extending flavor and color of ground beef patties. "We see extension from 8 days to 11 or more," says Robbins. "The rosemary is added before the meat is ground. That protects highly saturated lipids within the muscle-cell membranes, which are the most susceptible to lipid oxidation, especially when grinding exposes these surfaces to oxygen and pro-oxidants such as salt and heme pigment."
Other antioxidants, including resveratrol (from grape skin extract) and pycnogenol (a phytochemical from tree bark) have shown promise in university trials on ground beef and poultry products. Both antioxidants are approved as GRAS.
Produce gains time
Research from the Southern Research Laboratories, New Orleans, of USDA’s Agricultural Research Service reveals that placing fruit under water during cutting can increase shelf life from 2-3 days to 6-8 days.
For produce in general, the addition of ascorbic acid and calcium to the water, at temperatures of about 150˚F, produces heat shock proteins that reduce the production of "wound proteins" and causes water and calcium to be imbibed into the cells. This firms prepared produce so it can be used as ingredients in salads or other prepared food applications. This technique allows fast food chains to add kid-friendly finger fruit with reduced loss of product.
Other antioxidants, including 4-hexylresorcinol, ascorbic acid and glutathione have been tested as a possible replacement for sulfites. Sulfites must be labeled because of their effect on the considerable portion of the population that suffers from asthma. Hexylresorcinol has been tested on fruits and vegetables, and is also used to control blackspot in fresh and frozen shrimp.
Bacterial control makes big strides
The battle against Listeria monocytogenes has given food science new tools in the quest for more shelf life. A variety of chemical additives can be added to hot dogs and deli meats to prevent recontamination of these products. Studies at the University of Florida found increased amounts of vitamin E in broiler feed increased antibody production in hens and pullets, protecting the birds against a variety of bacterial diseases.
"Supplementing turkey diets with vitamin E stimulates their immune responses, helping them clear the gut of the microorganisms such as Listeria that cause disease," reports Irene Wesley, a microbiologist for the USDA’s Animal Disease Center.
Careful handling is required when processing foods that require extra shelf life without the use of intensive processes that normally kill bacteria. A production manager at Vienna Foods, Chicago, a firm that elected not to use chemical preservatives in processing its hot dogs, suggests careful attention to temperatures, maintaining clean and sanitized surfaces, purchasing clean raw ingredients and carefully supervising the process.
Specific, accurate measurements of ingredients such as antioxidants is essential, as is complete blending with raw ingredients to ensure the antioxidants will be maximally effective. Some of these natural antioxidants are not heat stable, so they must be added during a cool phase of the process. Oil-based antioxidants, such as vitamin E, can be dispersed in small amounts of fats or oils.
There is a number of new and improved methods of shelf-life extension. The challenge to processors is balancing satisfying the needs of the specific product with the demands of a more aware and more informed consumer.