A complex set of conditions determines bakery product shelf life, which complicates efforts to extend the "life" of these items. Extending their longevity relies on product developers, process technologists and packaging technologists to produce attractive and good-tasting products that don't harbor microorganisms.
Understanding how to extend shelf life requires defining what determines loss of shelf life. "It's dry," "tastes stale" or "this is soggy" are familiar phrases, but how can they be translated into their underlying physical and chemical conditions? Some solutions are simple; others are complex. Certain deleterious chemical and physical changes are eventual and unavoidable. However, methods exist to delay them, providing a product with a few more days or weeks of high-quality life.
When stored at ambient temperatures, most breads, rolls or products with a moist, spongy crumb undergo a progressive deterioration of quality commonly known as staling. In general, the higher the moisture content of the product in its fresh state, the more pronounced are the changes resulting from staling. Products such as breads, yeast-raised sweet goods and cakes stale much more markedly than do cookies and crackers, which have much lower initial moisture contents.
In bread, both crumb and crust are subject to staling, but with different results. Crust on fresh bread is relatively dry and crisp, yet becomes soft and leathery with age. Its original pleasant flavor and aroma is lost and replaced with bitterness upon staling. The major cause of crust staling tends to be moisture absorption from the air and from the interior crumb. This redistribution of moisture from crumb to crust is part of the staling mechanism. Crust staling is promoted by packaging breads in moisture-proof materials, because they restrict evaporation and result in more moisture retention in the crust.
Crumb staling is marked by many physicochemical changes: alterations in taste and aroma; and increases in crumb hardness, crumb opacity, crumbliness, starch crystallization, crumb absorptive capacity, (-amylase susceptibility of the starch and insoluble starch content. The physical changes occur in the following order: hardening and toughening of the crumb; appearance of crumbliness; and finally, moisture loss by evaporation. The processes involved in crumb-firming include: starch retrogradation; modification of the gluten structure that produces labile moisture; and absorption of this moisture by the retrograding starch, resulting in a partial redistribution of moisture. More specifically, fresh bread contains swollen, elastic starch granules embedded in a firm gel consisting of the immobilized amylose fraction. Since this gel doesn't undergo any further changes during storage, it plays no role in staling. During staling, intergranular association causes physical changes to the branched amylopectin and amylose chains that result in firming of the crumb structure. The mechanism of crumb-firming also has linked the transfer of moisture between gluten and starch as part of the overall staling process.
The problem of staling can be addressed in many ways. Packaging solutions have focused on prevention of moisture loss to keep the crumb softer. Staling is faster at 32( to 50(F, so temperature conditions during shipping and storage also contribute to the shelf life of high-moisture baked products.
Practical efforts to retard the staling process or minimize its effects have focused mainly on modifications of bread production and on the use of crumb softeners, anti-staling agents, and humectants. Ingredients such as fat, water, oxidants, enzymes, gluten and flour not only affect loaf volume and crumb structure, but crumb softness as well. Also contributing to softness are ingredients, such as sugar and fiber, and optimal baking conditions that increase moisture content.
"The food formulator has three basic approaches to crumb softness: prevent moisture transfer; prevent starch recrystallization; and hydrolyze the starch," says Michael Beavan, manager, bakery ingredient development, Watson Foods Company, Inc., West Haven, CT. "Typical ingredients used which prevent moisture transfer are ingredients that bind water well, like starches, fibers or maltodextrins. Mono- and diglycerides and sodium stearoyl-2-lactylate are two types of emulsifiers commonly used that help to prevent starch recrystallization. Amylases function well to promote crumb softness by effectively hydrolyzing the starch."
Emulsifiers help slow down the staling process by improving moisture retention. Surfactants designed for bread manufacture have been extensively researched, and provide many functions in extending shelf life. For instance, increasing the loaf volume of breads by adding dough strengtheners creates a less dense, optimally developed and functional gluten structure. This leads to better crumb resilience and moisture retention, and therefore, a softer crumb.
Any process, like mixing or fermentation, that affects crumb structure also will affect crumb softness. Dough strengtheners typically used are calcium and sodium stearoyl-2-lactylates (SSL), succinylated monoglyceride (SMG), ethoxylated monoglycerides, polysorbates, and diacetyl tartaric acid esters of mono- and diglycerides (DATEM). These function by complexing with gelatinizing starch. They're typically selected by the highest amylose-complexing index. Those emulsifiers with a high index are mono- and diglycerides, distilled monoglycerides, polysorbates, and SSL.
Gluten proteins in a bread dough carry a small positive charge. This hinders the interaction between protein molecules because of ionic repulsion between like charges. Salt increases dough strength by suppressing ionic repulsion. The fatty acid part of ionic surfactants like SSL, DATEM, and SMG binds to the hydrophobic areas on the surface of gluten proteins. Their negative charge neutralizes the positive charge from protein amino acids, stopping the ionic repulsion. DATEM work in tandem with the effect of monoglycerides. Saturated fatty acids of monoglycerides are more effective than unsaturated versions. The proposed mechanism for this is that straight-chain fatty acids are thought to be more effective at forming complexes with gelatinized starch and, in turn, are better able to interfere with starch recrystallization.
Using monoglycerides in cake batters enhances the subdivision of air incorporated by the protein during the creaming stage of batter mixing. Leavening gases formed during baking help expand these air cells. A smaller number of large air cells yields a cake with a coarse, open crumb and decreased volume. If the batter contains a large number of small air cells, the finished cake will have a fine, close crumb with an increased volume.
In cake mixes, using oil as a shortening gives a more tender cake with a longer shelf life. However, oil tends to have a negative impact on foam formation or air incorporation. Adding emulsifiers like propylene glycol fatty-acid monoesters, acetylated monoglyceride or lactylated monoglyceride, and polysorbates improves oil's functionality in cakes. These oil-soluble emulsifiers form a solid film at the oil/water interface, encapsulating the oil and preventing collapse of the foam. These emulsifiers are typically added by dissolving them in the oil before addition to the other cake mix ingredients. A pin-mill "finisher" used on the final mix also can improve the mix performance, possibly because it increases contact between the flour particles and the oil/emulsifier phase.
Enzymes can be classified by the reactions they catalyze, the substrates they act on, the products they form, their thermal stability, or their source. In baking, starch-degrading enzymes, or amylases, often are used.
The enzyme classification system appears complicated to someone unfamiliar with this category. One common system is the International Union of Biochemistry's Nomenclature and Classification of Enzymes. Another is the Enzyme Commission (E.C.) classification, which uses a combination of criteria in a tiered approach. Amylases are classified as E.C. 3.2.1, and use the following designations: E.C. 3 for hydrolases; E.C. 3.2 for hydrolases that are glucosidases; and E.C. 3.2.1 for glucosidases that hydrolize o-glycosyl compounds.
Amylases act as anti-staling agents by breaking down gelatinizing starch during baking. Some common amylases listed by enzyme and source are: fungal (-amylase-Aspergillus oryzae; fungal glucoamylase and fungal (-amylase-Aspergillus niger; cereal (-amylase-malted wheat/barley; cereal (-amylase-wheat flour; bacterial (-amylase-Bacillus subtilis; bacterial (-amylase-Bacillus megaterium; and bacterial amylase-Bacillus stearothermophilus.
Heat stability and action patterns of amylases serve as important parameters of their performance. Thermostability of amylolytic enzymes is critical because most of the starch in a dough consists of native starch. This only can be modified by enzymes after gelatinization during baking at temperatures above 150( F. Therefore, enzymes with low thermostability, like typical fungal amylase or wheat flour (-amylase, cannot significantly improve crumb softness.
Enzymes with high thermostability (like bacterial amylase) improve crumb softness, but yield a gummy bread crumb lacking resilience. As a result, the action of bacterial amylase must be well-controlled to provide the right balance between crumb softness and crumb resilience. This is difficult to achieve, because the bacterial amylase survives the baking process and is still active in the baked bread. Better control is achieved with newer types of fungal and bacterial amylases, characterized by an intermediate thermostability, like the Bacillus megaterium amylase and the Aspergillus niger acid amylase. These amylases are fully inactivated during baking, but still tend to produce a bread crumb lacking resilience.
"Fungal and bacterial amylases function well in bagels and flat breads," Beavan explains. "Classic (-amylases or maltogenic amylases are some of the most effective enzymes for crumb-softening in breads." Softer bread crumbs without a loss of crumb resilience require an amylase with an intermediate thermostability and exo-acting (maltogenic) action pattern. Genetic engineering has been used to develop bacterial amylases with intermediate thermostability. Bacillus stearothermophilus amylase falls into this category by producing a softer, yet resilient, bread crumb without gumminess even at higher dosages.
Less active water
Extending shelf life by adding humectants to control microbial and chemical activities is based on the reduction of water activity (Aw) or the relative vapor pressure. Aw is measured as the ratio of water's vapor pressure in the food product to the vapor pressure of pure water at the same temperature. It is loosely defined as a measure of the available moisture in a food system. Water molecules can be chemically bound to sugars, starches, salt or other molecules with hydrophilic binding sites. Aw shouldn't be confused with water content, which is the total amount of water present, some of which might not be available or "free" to combine with different components in the system.
Development of many baked products involves maximizing the moisture content to produce the best possible eating qualities while minimizing Aw. Lowering the Aw increases product stability in terms of susceptibility to microbial growth (see accompanying chart). Humectants bind moisture, reducing the system's Aw. Sugars and sugar alcohols are some of the most effective humectants. These include mannitol, maltose, lactose, dextrose, sucrose, sorbitol and fructose. Fructose is one of the most effective sugars for binding moisture. It's almost twice as soluble as sucrose and has the lowest Aw - 0.634 at 25(C. Other commonly used humectant ingredients are honey, molasses and fruit-juice concentrates. Like many of the ingredients listed, honey is multifunctional when it comes to extending shelf life. Besides its humectant qualities, honey has helped improve the stability of frozen dough products by increasing frozen dough's resistance to extension. It also has been found to have flavor-enhancing qualities as well as potential antioxidant properties.
Baked products emerge from the oven essentially sterile, from a microbial point of view. Keeping them that way presents another challenge. Good sanitation in plants can prevent many problems. "Most contamination is not a result of inadequate heating of the product, but due to post-handling contamination from airborne contaminants or from equipment," says Mike Curiale, Ph.D., technical director, microbiological research, Silliker Labs, Homewood, IL. "A lot of baked products, such as pies and fillings, have water activities above 0.85 and relatively neutral pH conditions that are even conducive to growth of pathogens such as Staphylococcus aureus."
Besides control of water activity, a product developer can add certain ingredients to aid in controlling microbes.
Mold inhibitors can be added to breads to lengthen their shelf life. "Natural" mold inhibitors, such as acetic acid (vinegar); raisin-juice concentrate; and glucono-delta-lactone (found in honey, fruit juices and wine) act by reducing pH to retard the initial growth of mold. The food additives propionate and sorbate are effective against mold at low concentrations, but don't affect product pH. "Fermented wheat flour and cultured whey function as natural sources of calcium propionate," says Beavan. Propionates and sorbates can be added into dough, batter or filling formulations. Propionates are most effective against mold and bacterial growth. Sorbates inhibit yeast as well as molds, and are used more in cakes, pies, fillings and icings. Both types can be applied as a water-based surface spray to English muffins and scones.
What does it take to keep a filled product crisp on the outside or an iced product from absorbing all the water from the icing? Water migration is a difficult process to control in a baked product, especially when products possessing different moistures and available water are being combined. Short of putting a layer of foil between a fruit filling and a crisp cookie crust, what can a food formulator do?
General properties of hydrocolloids include significant solubility in water, ability to increase viscosity and, in some cases, the ability to form gels. Some specific functions of hydrocolloids include improvement and stabilization of texture, inhibition of sugar and ice crystallization, stabilization of emulsions and foams, improvement of icings and encapsulation of flavors.
One type of hydrocolloid, vegetable gums, is typically used for these applications to help retard water migration. Gums are high-molecular-weight polysaccharides, primarily fiber, often with protein traces. Their functionalities are affected by molecular size, orientation, molecular association, water-binding and swelling, concentration, particle size, and degree of dispersion. Several types exist. The ones commonly used in baked products are agar, carrageenan, cellulose gum, methylcellulose, alginates, gum arabic (gum acacia), guar, locust bean gum, pectins, tragacanth, xanthan gum, karaya gum, and psyllium. Their common functionality in baked products is to aid in overall water management. In products like breads, cakes and doughnuts, gums improve shelf life by improving moisture retention. Types such as gum acacia and carboxymethylcellulose can be used to retain moisture in breads and increase shelf life at levels of 1% to 2% based on flour. In applications such as icings, fillings and meringues, gums bind water, prevent syneresis, provide freeze/thaw stability, and add gloss.
A gum can't be expected to solve all water-migration problems. "In a filled cookie, the food formulator should know the water activities of their filling and cookie and try to formulate them in such a way that there is little to no differential between the two," says Florian Ward, Ph.D., director, R&D, TIC Gums, Inc. Belcamp, MD. If the filling has a higher water activity than the cookie, the moisture from the filling will eventually migrate into the cookie, no matter what processing conditions or packaging is used. "It is necessary to use a gum in combination with low-molecular-weight ingredients in the filling," Ward says, "because gums are too large to have any effect on water activity."
Icings have similar shelf life concerns. Icings refer to modified sugar/water systems in which hydrocolloids and other ingredients control the balance between the dissolved sugar and the suspended sugar. The added ingredients modify the solubility and crystallizing characteristics of the sugar in the aqueous medium, thereby stabilizing sugar-crystal size. Icing stability refers to the ability of an icing to retain its aerated structure; smooth, nongranular texture; and to resist liquid separation during storage, use and subsequent shelf-life. Hydrocolloids with suitable gelling, suspending, emulsifying, film-forming and hydrating properties help provide stability in many icing formulations.
Additional quality attributes related to the shelf life of an applied icing are: retention of a uniform moisture content without either drying out too quickly or absorbing moisture to cause melting or stickiness; maintenance of its glossy appearance and true color; and absence of grittiness. For an icing application - where the formulator is concerned with migration of water from the icing to the cookie - the use of gums with high water-binding capacity, combined with gel-forming ability and added solids, will slow down the process of water migration. "In both applications," Ward suggests, "guar, xanthan, and gum acacia function well to reduce moisture migration."
Even though gums are primarily fiber, other fiber ingredients need to be included in the overall management of water in baked goods. Fibers like flaxseed, sugar beet fiber, along with other grain-based, as well as fruit-based fibers, bind water and inhibit water migration. "In the case of prune powders, the enhancement properties within prunes are created by a three-fold process," says Scott Sanders, owner, Creative Food Consultants, Byron, CA. "First, the fiber, particularly pectin, acts to physically absorb moisture. In conjunction with this physical water absorption, prunes contain high amounts of sorbitol and reducing sugars, which act as humectants to attract and tightly bind this additional water." Prunes also provide a lower pH due to their malic acid content. This also helps to prolong shelf life by inhibiting microbial growth.
Products such as potato starch, pregelatinized starches, and maltodextrins also are commonly used to bind water.
More shelf-life solutions
Protein ingredients, which give baked goods added shelf life, come from soy, whey, milk or gluten. "Whey ingredients provide improvements in the shelf-life stability of foods due to the whey proteins' water-binding capacity," says Carmen McEwen, applications senior scientist, Land O' Lakes, Eden Prairie, MN. "When heat is applied during food processing, the proteins partially unfold, exposing additional water-binding sites that were unavailable in the native, unheated protein."
In the case of breads, muffins, cakes, doughnuts and brownies, whey protein ingredients can help maintain crumb softness by reducing moisture migration and starch retrogradation. "When whey ingredients are incorporated into bakery-filling formulations," McEwen says, "water will be tightly bound to the protein, producing a moist, finished product."
In increased efforts to manage water, it has been suggested that along with textural changes, flavor changes also are taking place. In most cases, efficient management of water also improves flavor losses. "Some practical tips to improve flavor retention are to use oil-based flavors that bake off less easily, and bake at lower temperatures for longer time," suggests Patrick Imburgia, flavor chemist, Mission Flavors, Rancho Santo Margarita, CA. "In an iced product, like a lemon cookie with an icing, a formulator can add some lemon flavor to the icing to help extend the flavor of the overall cookie, since the icing is not exposed to heat."
Some research has been conducted on the development of edible films and coatings. These also have been developed in response to consumer demands for foods with higher quality and longer shelf life. The necessity for reducing disposable packaging and improving recycling also has added greater interest in edible-film research.
Edible films can regulate water vapor, oxygen, carbon dioxide, and lipid transfer in food systems. Edible films also can improve food-system mechanical properties, and control loss of volatile flavors and aromas. Proteins, lipids and polysaccharides - alone or in combination - have been investigated for their effectiveness as edible mass-transfer barriers. Some applications for these films have included brownies, doughnuts, raisins (for use in cereals), and cookies.
Packing it in
Baked products, whether designed to have a shelf life of two weeks or six months, have probably achieved these goals largely due to packaging technology. High-quality barrier films with, and without, coatings are available at whatever the cost a company's margin will bear.
Some of the latest technology includes adding gases to control the atmosphere within the package. "Modified-atmosphere packaging (MAP) has given products like bagels or brownies sometimes weeks of added shelf life," Curiale says. Many companies have experienced success with this type of packaging. But, like all aspects of the finished product, limitations exist.
"It is risky to rely on MAP to do the majority of the work of extending shelf life for your product," Sanders says. "Issues with faulty seals, package integrity, leakers and the amount of residual oxygen in the package play a role in the life and safety of a baked product." Even if the seals are correctly applied, there is a risk for anaerobic bacterial growth. Food product designers need to be conscious of Aw and pH conditions where growth can occur, as well as the conditions in which toxins are produced.
Specific conditions within the package combine oxygen flushing with levels of up to 100% nitrogen or 100% carbon dioxide, but success has been achieved by combining 80% nitrogen and 20% carbon dioxide with less than 0.5% residual oxygen for baked products. One of carbon dioxide's benefits is that it can convert to carbonic acid on the surface of the product, and offer additional protection against microbial growth. However, development of excessive carbonic acid can create vacuum-like conditions within the package and crush softer products, such as cakes, muffins or soft brownies. Food companies face a challenge in determining acceptable MAP-defect levels, since defects might lead to a moldy product or, more seriously, transfer of a food-borne illness to a consumer.
Whether developing cakes, muffins, pies or cookies, ingredients, processes and packaging exist to aid shelf stability. With the tools at hand to get greater life out of a new or existing baked product, it's time to hit the lab, bind up that water, and soften some crumb.
Water World: Aw levels for microbial growth
Kimberlee J. Burrington is the whey applications program coordinator for the Wisconsin Center for Dairy Research in Madison, WI. She received her B.S. and M.S. degrees in food chemistry from the University of Wisconsin-Madison. Her industry background is in bakery and dairy.
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