Isotherms predict product changes over time
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:
- Find critical water activity values where changes like caking, clumping, and loss of texture occur
- Predict how the product will respond to ingredient and formulation changes
- Accurately estimate shelf life
- Create mixing models
- Perform packaging calculations
- Find the monolayer value (where a product is most stable)
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Isotherms: the Holy Grail of formulation
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.
Handmade method impractical
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.
A faster way to create isotherms
Creating 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.
DDI isotherms reveal what hasn’t been seen before
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.
Find critical water activity values
Despite double-bagging and issuing strict temperature storage guidelines, a spray-dried milk manufacturer still had problems with clumping.
When glass transition becomes a problem
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.
Water activity predicts transition rate
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.
DDI isotherms predict glass transition point
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.
Create mixing models
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.