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Food manufacturers need to know how long it will be before their product molds, gets soggy, goes stale, becomes rancid, cakes, clumps, crystallizes, and becomes unacceptable to the consumer. The moisture sorption isotherm is a powerful tool for predicting and extending the shelf life of a product. It allows you to:
A moisture sorption isotherm is a graph showing how water activity (aw) changes as water is adsorbed into and desorbed from a product held at constant temperature. This relationship is complex and unique for each product. Water activity almost always increases as moisture content increases, but the relationship is not linear. In fact, moisture sorption isotherms are S-shaped (sigmoidal) for most foods and J-shaped for foods that contain crystalline materials or high-fat content.
The classic way to create an isotherm is to put the sample in a desiccator with a salt solution of known water activity until the sample’s weight stops changing. Then, the sample is weighed to determine water content. Each sample produces one point on the isotherm curve.
Because the process takes so long, curves were traditionally constructed using five or six data points with curve-fitting equations like GAB or BET.
Creating moisture sorption isotherms by hand is painstaking. The method needed automation. The method first used—and still used by most vapor sorption instruments—is called DVS, or dynamic vapor sorption. A sample is exposed to a stream of humidity-controlled air while a microbalance measures tiny changes in weight as the product adsorbs or desorbs water. Once equilibrium is achieved, the instrument dynamically steps to the next preset humidity level. Tests take anywhere from two days to several weeks.
The DVS method works well for investigating the kinetics of sorption—what happens to a product as it is exposed to certain humidities and how fast it adsorbs or desorbs water. The DVS method is not very helpful in creating a high-resolution isotherm curve, however, as each equilibrium step produces just one point on the isotherm curve.
The dynamic dew point isotherm (DDI) method was designed to solve this problem. It creates high-resolution isotherms that show detail in the adsorption and desorption curves by taking a snapshot of both water activity and moisture content (every 5 seconds) as the sample is exposed to humidified or desiccated air. DDI graphs contain hundreds of data points and show details not previously visible, such as critical points where caking, clumping, deliquescence, and loss of texture occur.
Despite double-bagging and issuing strict temperature storage guidelines, a spray-dried milk manufacturer still had problems with clumping.
When milk is spray-dried, rapid evaporation leaves the sugars in a glassy state. Glassy lactose has entirely different properties than crystalline lactose. Due to low mobility, particles don’t cake together or clump up while the powder is in a glassy state. The crystalline structure is a lower energy state, so there will always be some molecules in transition from glassy to crystalline. Problems occur when the rate of transition reaches a tipping point.
At 0.30 aw, it might take several years for the all the lactose to become crystalline. At 0.40 aw, it might take a month. Above 0.43, the transition will occur in a few hours. Once the lactose has crystallized, the powdered milk is permanently changed. It holds a dramatically different amount of water, it won’t dissolve, and it doesn’t taste right. In essence, it has been ruined.
The glass transition point for powders like spray-dried milk can be determined using a high-resolution DDI isotherm. Traditional isotherms rely on models to fill in the isotherm between measured points. DDI isotherms measure hundreds of points and can identify transitions such as the glass transition point for spray-dried milk powder.
The peak value on the second derivative plot of the isotherm identifies the critical phase change value as 0.43 aw.
Routine, accurate testing at the line with better control values helped the manufacturer improve shipment acceptance rate.
A cake manufacturer was formulating a recipe for cream-filled cake. The components of the recipe were frosting (about 7% moisture), cream filling (12%), and cake (15%). Moisture migration during shelf life had previously caused texture issues such as stale cake, rubbery frosting, and liquefied cream filling bleeding into the cake.
Moisture sorption isotherms for each ingredient showed that the frosting—the driest ingredient—had the highest water activity at 0.79. Water activities of the cream and the cake were similar—0.66 and 0.61 respectively.
Transforming isotherms to chi plots predicted water activity of the final product as 0.67, a microbially safe value for the cake.
The cake maker went on to successfully bake and taste test the cake at equilibrium water activity (0.67).
Single-serve powdered drink mixes are a growing market segment. Packaging accounts for more than 50% of the raw materials cost for this product. The main goal of the packaging is to maintain the drink mix below the critical aw over the target shelf life of the product.
Packaging calculations begin with a critical water activity value. The ability to get a precise point from dynamic dew point isotherms (DDI) makes this type of packaging calculation possible.
This curve shows the glass transition point for a particular drink formulation:
The critical water activity—the exact inflection point—for this drink mix is 0.618 at 25° C.
Using streamlined packaging calculations (available in Fundamentals of Isotherms and as a software tool), we evaluated four different types of packages for this drink mix—its original package and three possible alternatives. Under humidity abuse conditions (25° C, 75% humidity), here are the results:
A pet food company changed formulation to produce a preservative-free product controlled by water activity. Shortly after introducing the product, they began to see returns due to spoilage.
Initial evaluation showed that spoilage predictions were based on water activity tests made at an unusually low temperature—15º C. Isotherms run at 15° C, 25° C, and 40° C showed that if stored under abuse conditions (as pet food often is), spoilage was likely.
The isotherms offered a complete predictive picture, allowing the customer to solve the problem with a new formulation.
After 13 problem-free seasons, a pecan grower had his crop rejected because of mold issues. An isotherm was created to investigate the problem.
In order to avoid microbial growth, the pecans must be dried to 0.60 aw. As the isotherm shows, 0.60 aw corresponds to 4.8% mc in pecans. The pecan isotherm is also quite flat in this critical control region, so a small variation in moisture content translates into a large and potentially dangerous change in water activity.
The full isotherm shows that the pecan grower’s process was not adequate to guarantee the safety and quality of his crop. The pecan grower measured moisture content by loss on drying. Because his release specification was 5% and his accuracy was ± 0.5%, the dried crop’s actual water content could have been anywhere from 4.5% to 5.5%.
Anything from storage at high humidity to inadequate packaging could have pushed the pecans to unsafe water activities and resulted in spoilage.
In this 30 minute webinar, food scientists Mary Galloway and Zachary Cartwright talk about how to get answers to your shelf life questions. Learn how to:
-Troubleshoot issues and complaints to find out why shelf life is ending sooner than expected
-Predict how recipe changes will impact shelf life
-Compare the effect of different ingredient options
-Evaluate whether a specific packaging option will help you achieve or improve shelf life
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