The role of barrier materials in packaging low-fat goods.

It is entirely possible that a low- or no-fat product that is packaged and distributed in the same way as its full-fat counterpart will experience consumer complaints that were never reported with the full-fat item.

Baked goods with reduced fat content can produce off-odors and off-flavors. A study examines the problem and evaluates which barrier materials can overcome the problem.

The array of low-fat/no-fat food products is steadily increasing across categories, with healthier products forming the fastest growing category in the supermarket since the beginning of the decade. In fact, reduced- or no-fat items comprised nearly one-quarter of snacks introduced in 1995.

In baked goods, an increase in consumer-detected contaminating odors has accompanied the increase in low-fat/no-fat products. Although food producers have made great strides in developing packages that extend shelf life by providing improved moisture and oxygen protection, that extended shelf life coupled with the reduction in fat has created a need for increased flavor and aroma barrier to maintain product taste until consumed.

A new study done by Aspen Labs of St. Paul, MN, and sponsored by the Films Div. of Mobil Chemical Co., looks at the role of flavor and aroma barrier in packaging to maintain the taste of low-fat/no-fat baked goods.

This study investigates a particular vulnerability of low-fat/no-fat baked goods: the higher consumer detectability of contaminating odors. And, a packaged product test evaluates the flavor and aroma barrier afforded by a coextruded oriented polypropylene (OPP) film, compared to an OPP film with an acrylic coating on one side and a high-barrier polyvinylidene chloride (PVdC) coating on the other.

The Results of Fat Removal

First, let's take a look at what happens when fat is removed from a product. Because fat contributes to texture, carries flavors, and masks undesirable odors, reduced- or no-fat baked goods are susceptible to staling in terms of texture change, flavor loss, and undesirable tainting.

Many reduced-fat baked goods, such as cookies, are formulated with a carbohydrate- or protein-based fat replacer that absorbs water to mimic a full-fat texture. Fat replacers easily gain or lose moisture. Hence, reduced-fat baked goods are more sensitive to texture changes. Improving the moisture barrier of the packaging film reduces moisture gain or loss and helps maintain the original desired texture.

Fat solubilizes most organic molecules, such as flavors and aromas, chemical solvents, and fragrances, better than water. Without fat, foods have a more difficult time holding onto flavor compounds, because these compounds volatilize into the headspace and from there may permeate into the external environment

Since human flavor perception actually involves smelling gaseous flavor compounds, manufacturers frequently use lower amounts of flavors in reduced-fat formulations in order to come as close as possible to a desirable concentration of volatile aromas. Otherwise the flavor sensation can appear unbalanced or too strong. This situation results in foods with lower quantities of flavorants that at the same time have a poor ability to hold onto them. A packaging material with a good organic vapor barrier will help prevent the continuous loss of flavors and aromas.

Fats absorb off-odors from packaging just as they do desirable flavors and aromas. Removing fat, then, means that off-odors from packaging, external contamination, or product degradation will be more volatile and thereby more detectable. In fact, it is entirely possible that a low- or no-fat product that is packaged and distributed in the same way as its full-fat counterpart will experience consumer complaints that were never reported with the full-fat item. This situation occurs because the increased volatile concentration of an off-odor (even though contamination exposure is the same) now may exceed the human detection threshold.

Testing with Common Solvents

A test performed by Aspen Labs confirms this possibility. For the test, three common solvents were added to identical weight samples of no-fat and full-fat (14% by weight) cookies. The three solvents were propyl acetate, n-propanol, and methyl ethyl ketone (MEK). Propyl acetate and n-propanol are the most common printing solvents used for flexible packaging and are generally the two that exist at the highest residual solvent levels in printed samples based on laboratory experience. Although MEK is not recommended for use with OPP films, occasionally it is also detected.

The quantity of solvent added to different cookie samples ranged from 0 to 12 ppm by weight. After the "spiked" cookies were placed in vials and heated to 70 deg C, a gas chromatography analysis of the headspace was performed. Chart 2 shows the increased solvent volatility in the no-fat cookie, compared to the full-fat one.

Chart 2 also points out that different chemical properties that exist among propyl acetate, n-propanol, and MEK affect how much of an increase in volatility occurs when fat is removed.

Barrier Effect of Materials

While the first part of our experiment confirmed the effect of fat on the partitioning of organic compounds between the food product and the headspace, it failed to assess the barrier effect of different packaging materials. For this part of the Aspen Labs experiment, samples of both cookie types were packaged in coextruded OPP film and in OPP film coated on one side with acrylic and the other with high-barrier PVdC. The packages were then exposed to the three solvents at concentrations of 1,370 mg/rm, n-propanol; 1,885 mg/rm, propyl acetate; and 2,175 mg/rm, MEK.

To accomplish this test, the packages were placed upright inside a sealed bell jar with all surfaces exposed. Measured quantities of the solvents were dripped down the sides of the bell jars and soon completely evaporated. These packages remained sealed inside this vessel spiked with vaporized solvents for 18 hours at 35 deg C (95 deg F).

After 18 hours the bell jar was equilibrated to room temperature, and 2.5 g of each conditioned cookie were placed in a glass vial, which was heated to 70 deg C for headspace analysis.

Although the full-fat and no-fat cookies packaged in the coextruded film were in the same contaminated environment, the no-fat cookie contained 42% more volatizable solvent than the full-fat formulation . The result is 42% more contamination available for human detection.

The analysis paints a different picture with the one-side acrylic, one-side high-barrier PVdC-coated OPP film, as that material provided an excellent barrier to the undesirable solvent compounds for both the full-fat and no-fat formulations.

In fact, both types of cookies packaged in coated film generated headspace volatiles that were only about 5% of the quantity generated from cookies packaged in coextruded film. Although this odor barrier protection may be more important for a reduced-fat item, the data clearly shows that the coated film keeps out organic contaminants regardless of a package's contents.

Aspen Labs also calculated the total solvent absorption of cookies via an external standard procedure, representing the total solvent contained in the conditioned packaged cookies after the vessel was brought to room temperature .

Looking at the data for the coextruded film packages, it appears that the full-fat cookie actually absorbed more propyl acetate and MEK than the no-fat version. Yet these same conditioned cookies generated fewer headspace volatiles. This finding makes sense because, as the fat in the traditional cookie solubilizes the gaseous solvent that has permeated through the packaging, it increases the driving force for more permeation, i.e., the difference in volatile concentrations between the outside and inside of the package. On the other hand, the no-fat cookie does not solubilize as much solvent, causing a higher solvent vapor pressure inside the package that discourages continued permeation.

More n-propanol was absorbed by the no-fat cookies, suggesting a greater amount of water in the no-fat formulation, because more water would increase the solubility of alcohols like n-propanol. Both molecules are polar and "like" each other.

In terms of package protection, the total solvent absorption bar chart tells the same story as the headspace volatiles data: The coated film provides an exceptional barrier to solvent permeation.

Conclusions

Based on the findings from this experiment, it is clear that if no-fat and full-fat cookies are subjected to equal levels of volatile contamination, the contamination will be more readily detected in the no-fat cookie. As a result, residual solvent levels of printed packaging, storage environments, and other factors that are suitable for full-fat bakery products may result in detectable off-odors when used for similar reduced-fat items.

There are several considerations for packagers wanting to prevent off-odors and consumer complaints. First of all, with flexible packages it might be prudent to reduce the acceptable levels of retained solvents. In the cases of bag-in-box and slugwraps in a box, chipboard specifications could be examined and upgraded (the recycled content can contain some unpleasant compounds). Even the warehouse, transport, and display environments might need to be better controlled so as to minimize exposure to contaminants such as diesel and soap.

The risks described above can be efficiently minimized by using packaging films with excellent organic vapor barrier. The data presented testifies to the effectiveness of acrylic and high-barrier PVdC-coated OPP in comparison to co-extruded OPP. And, since it continuously provides this barrier to off-odors, the off-odor sources themselves may not need such intense scrutiny or policing.

* If no-fat and full-fat, or regular-fat, products experience an equal level of volatile contamination, the contaminations will be more readily detected in the no-fat product.

* Residual solvent specifications, boxboard-type storage environments, etc., that are suitable for regular-fat products may result in detectable off-odors when used for their no-fat counterparts.

* To prevent off-odors and consumer complaints, solvent retention specifications or other environmental conditions may need to be changed.

* Alternatively, packaging films with better organic vapor properties, such as those of acrylic, PVdC, and/or polyvinyl alcohol (PVOH) coatings, will offer excellent protection.

Terry Clark is the leader of the Applications Technology Team for the Films Div. of Mobil Chemical Co., Pittsford, NY. She has been with the division for more than 15 years in manufacturing, product development, commercial development, and technical service positions. She developed and published the company's Products Characteristics Manual and has made presentations to many industry associations. She can be reached at 800/334-7987.


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