Many retail store chains and kiosks who thaw and serve pre-manufactured baked goods are jumping on the band wagon of using water activity as a protection against microbial bacterial contamination. In my experience it is not as effective a protection as compared to adjusting pH ranges, proper product handling procedures and environmental controls. The buyers of our baked goods ultimately will get what they want so one tool we have is to educate them about water activity, what it is capable of as a tool, what it is not capable of protecting against and when it is simply not the right way to approach protecting the food stream from contamination.

If you must make an aw adjustment, corn syrup, dextrose, sugars and sugar alcohols = low molecular weight particles that dissolve thus forming bonds with water molecules are good for lowering aw measurements. Some proteins [more expensive] have a more limited effect. Many starches and gums are not as soluble and do not have as much of an effect. Additionally, low molecular weight sugars such as dextrose should be less expensive.

As another footnote, the customer should be reminded that acidity directly affects the potential for pathogens regardless of aw. Back as far as the 1970s research showed that no pathogenic organism produces toxins in a high acid [low pH] environment. U.S. Federal, State and local agencies gradually accepted this and by 1990 their agencies allow bakeries and other food vendors to sell products from room temperature shelves regardless of the foods aw, as long as the pH of the food is 4.5 or below. Source = AIB Technical Bulletin “Water Activity and Safety of Bakery Products”.

1.Water in food.

Foods with an aw measure of 0.95 and above, and are a source of the nutrients and are an environment [low acidity] that supports yeast, mold and bacteria growth, are at risk for contamination under certain transportation and handling conditions.

The amount of available moisture can be reduced to a point thought to inhibit this microbial activity.

Foods over 0.95 are considered susceptible and require specific handling.

Foods under 0.95 but over 0.85 are in a defensible range with humidity, pH, acidity and temperatures in handling equal or greater concerns with regard to contaminant susceptibility as compared to water activity.

United Stated Department of Agriculture’s Food and Drug Administration guidance on water activity is generally directed at low acid canned foods and some guidance concerning contaminants such as C. Botulinum is available but I am yet to find a recommended safe on listeria.

Remember that inoculations are not simply positive or negative, a heavy inoculation will likely flourish in nearly any environment within the salable and consumable state of the product.

When making product quality compromising and ingredient cost magnifying decisions on a very suspect measurement, when the product is coming in frozen and is stated to have only a number of hours between the slacking [thawing] process and when it is determined to be unsalable, the threat of contamination is as much, or more, important compared to the limited ability of a product [minor % protection] to keep a contaminate from flourishing.

2.How is water activity measured, i.e., what does the number mean?

Water activity (aw) is water not tightly bound and available for moisture migration and microbial activity in a food product. One visible example of unbound water is the result in the freezer of freezer burn.

The product is put in a sealed chamber with the ability to measure moisture levels either by electro-capacitor or by light beams on mirrors looking for dew points. The product is left until the moisture level of the product and the air in the chamber equilibrates. When the moisture level of the product is the same as the air, the air is measured resulting in a vapor pressure value.

3.Determining the relevance of the exercise to the customer.

The aw number must be considered one of many partial protections and NOT A GUARANTEE OF PROTECTION AGAINST MICROBIAL AND PATHOGENIC GROWTH. The enemies of the product are the inoculants and sanitation and good handling practices by the end users are the key to real microbial protection. Water activity control is only one means of control as are pH, temperature, etc., and not a particularly good one.

The aw measure of products like breads and snack cakes must be considered a partial or suspect measure due also to the fact that the product will be in contact with the outside of the packaging environment upon handling and serving. A product with a known aw measure served in a humid environment, such as Portland Oregon, Seattle Washington or Vancouver BC, will be capable of attracting moisture from the atmosphere and become more susceptible over time to microbial activity where the same product in a dry, air conditioned environment may be stable or even again loose moisture to the atmosphere reducing the susceptibility of the surface of the product to problems. The bottom line is that the aw number is relevant and subject to differ with each product type and handling conditions.

The susceptibility of a product, where a deliberate or accidental inoculation of listeria will flourish, is as or more dependent upon the acidity, pH, temperature of the environment, etc., as compared water activity. However, if all the other variables are controlled and known, the aw impact upon the equation could be determined to some degree.

I have found some guidance on water activity levels and certain contaminates but Listeria is not one that I have specific guidance on at the time of the writing of this note. The minimum a w level for the growth of C. botulinum is approximately 0.93, for example, according to FDA guidance. The remaining guidance is limited. It is known that foods under certain levels of acidity [low acid foods] are in need of heat processing and controls with w a numbers over 0.85 which is why we cook things. However, this guidance is generally directed at low acid canned foods.  With regard to breads and snack foods, they are baked, placed in a relatively clean bag or box, frozen, and the existing HACCP protection by the manufacturer and handling by the end user are where the real protection against contamination and the result is found.

4.Challenging the application of the water activity measurement to the product.

The measurement of the water activity of a product at the point of sale must consider the atmospheric conditions, temperatures, handling and time of the product from frozen to a serving state. A protocol concerning when and under what conditions the aw number is measured need to be adopted if the measurement is going to be taken after the product is shipped frozen and thawed. Only then will the number have meaning and the risk factor for contamination be viewed.

When making product quality compromising and ingredient cost magnifying decisions on a very suspect measurement, when the product is coming in frozen and is stated to have only a number of hours between the slacking [thawing] process and when it is determined to be unsalable, the threat of contamination is as much, or more, important compared to the limited ability of a product [minor % protection] to keep a contaminate from flourishing.

It is possible to put so much water holding ingredient, in a variety of small particle for, to control water down to 0.85 [for example] but the product may not be salable and/or the product may become very difficult to produce. The decisions are between the level of protection against contamination, which in this case is very limited and one of the more expensive avenues as compared to pH, and the quality and cost of production of the product being discussed.

Remind the customer that back as far as the 1970s research showed that no pathogenic organism produces toxins in a high acid [low pH] environment. U.S. Federal, State and local agencies gradually accepted this and by 1990 their agencies allow bakeries and other food vendors to sell products from room temperature shelves regardless of the foods aw, as long as the pH of the food is 4.5 or below.

Dan Ettling is Bakery consultant at Monkeymedia.
Source = AIB Technical Bulletin “Water Activity and Safety of Bakery Products”.


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To answer this I need to back up a bit.

Due to differences in soil and climate, the United States produces a wide variety of crops, each suited to its own location. Therefore, an importer or domestic miller can be reasonably certain of obtaining the type of wheat they need by selecting the proper class with proper specifications.

The many varieties of winter and spring wheat are grouped into six official classes. The class is determined by its hardness, the color of its kernels and by its planting time. Each class of wheat has its own characteristics related to milling, baking or other food use. This is where you hear the class distinctions hard red winter (HRW), hard red spring (HRS), hard white (HW), soft red winter (SRW), soft white (SW), Club and Durum.

The terms spring and winter refer to when the seeds are planted. Winter wheat is planted in the fall, sprouts and then lies dormant under snow until spring when it becomes active again and has a second growth cycle. The practical effect is some mellowing of the protein. This does not mean that the crop has low protein characteristics necessarily. The functional proteins of winter wheat are very good for bread baking and for some types of bread even superior even though the proteins are somewhat more mellow. Spring wheat is planted in the spring, has one growing cycle and is harvested when ready. This yields very strong functional protein. The winter wheat is typically grown farther south as the winters are not so hard. Spring wheat is planted farther north where the winters are too cold for the dormant winter wheat to do well. Typically the harvests will begin in the most southern areas as those crops will be ready first.

Hard and soft distinctions are critical for baking characteristics along with spring and winter. Soft wheat is typically grown in more temperate moist climates such as around the Ohio River valley as compared to hard out on the Great Plains. When wheat is milled, it is basically crushed in mill stands between rollers. When hard wheat is milled, it resists being broken and crushed and the starch component of the kernels is damaged to a degree greater than soft wheat in the same milling process. Soft wheat, as eluded to prior, does not resist being crushed and suffers much less starch damage. The significance of this is that higher starch damage is desirable for bread production due to increased hydration by cold water of the damaged starch and the fact that the damaged starch is convertible to sugars (food for yeast) by the enzymes from yeast. Soft wheat, on the other hand, does not negatively alter the performance of batters or cookie dough by hydration, is typically lower in protein which is good for cakes and cookies, and gives a softer finished product eating characteristic.

Red and white is in reference to the color of the bran coat on the outside of the kernel. This does not mean that flour from red wheat is darker. In fact, it is the same color if the extraction of the center endosperm is the same as a percentage of the kernel. White wheat does have an advantage of staying white when water is introduced in the mixing and fermentation and oxidation begin in bread production. Bread flour from red wheat will gray or off color ever so slightly when mixed in production as it has an enzyme causing the slight change that white wheat typically hasn’t had. Both hard red and hard white are great baking classes of wheat for breads. White gives a slight advantage to white noodle production or Chinese steamed buns, yet breeders are working diligently towards a color stable red planting seed to compete in these specialized markets.

The milling company buys the crops that match their target characteristics, blends them to get the performance they are looking for in analytical testing and bake testing and puts a brand name on it. Understanding what class the flour is a result of helps understand what the brand name flour in the sack is, more importantly helps understand the flour specification sheets all bakers need to be familiar with, and lets larger bakeries buy on contract when crops are becoming available on the market.
Dan Ettling is Bakery Consultant at MonkeyMedia.

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