To understand food packaging permeability and evaporation, we need to understand the characteristics of the product and the packaging material. Food packaging permeability is the difference in water activity inside the packaging (i.e., the product) and the water activity (or relative humidity) outside the packaging. The goal is to control how much water is allowed in or out of the packaging.
Water Vapor Transmission Rate
Moisture transmission rate (MTR) is also known as water vapor transmission rate (VTR). This is a key value in determining permeability of different packaging materials. VTR directly impacts the cost level of the product. A higher VTR means that a product will need a more permeable packaging, and a low VTR requires less permeable packaging. The ideal VTR value is also influenced by how much money the manufacturer wants to allocate to the product.
When making food packaging permeability decisions, we have to convert the VTR value into perms, a unit that’s more adapted to modeling values. Glass is a packaging option that provides complete protection, though it’s also very costly. To use glass packaging, the value and cost of the products need to warrant a more expensive packaging material. Foil also has a very low VTR and is much less costly than glass. When communicating with suppliers, manufacturers need to be mindful of units used when communicating with suppliers so that they receive the appropriate packaging materials. The webinar includes a model of how a manufacturer would use these values.
The Role of Equations
By collecting a basic set of information, we can predict a number of outcomes when it comes to food packaging permeability: water activity in a stored bag in a variety of storage conditions, water vapor transmission rates, and shelf life with a given critical water activity. Values we should consider when gathering data include saturated water pressure, surface area, mass of the sample, and changes in water activity. All of these values can be combined into one equation that boils down to change in moisture / change in time.
To work these calculations, we need to have the isotherm slope, which translates water activity into mass values. Once you have these values, you can solve for any number of factors: moisture, water activity, and VTR, to name a few. The webinar walks through an example of how to work and apply these equations and calculations.