Stability of Food: Water Activity and Glass Transition Theory

Stability is inversely related to reactivity. The more reactive the molecule, the more unstable it is. It is determined by the change of 2 molecules encountering each other (1. chance of encounter – diffusion controlled). When the mobility of the molecule increases also the chance of encountering will increase. When the molecules are close to each other, they need to get together in a particular place (2. Chance of collision – collision frequency), finally it needs to cross an energy barrier to reach another state (3. activation energy). When the barrier is low, the reactivity of a system is controlled by the diffusion coefficient.  When the reaction depends only on the molecular mobility, the glass transition theory will be good to explain if the reactions occur or not, because molecular mobility is part of the glass transition theory. Eg.proton exchange, radical combination reaction.

Diffusion coefficient depends on viscosity, mobility and the temperature dependency of the viscosity can be described via Argeniuos theory or the WLF. Viscosity included in equations is the viscosity that molecule feels locally, and sometimes it will deviate from the viscosity that we are able to measure. Local relaxation time is totally different than what we measure in practice.

Theoretically the formulas exist, but in practice the glass transition theory is far from ideal.  Means that what we predict on theory is far from the practice. Potential explanation is that the viscosity included in the ecuation is the viscosity that the molecule feels localy, so it will deviate from what we are able to measure macroscopically. Eg. pudin (3d network of starch molecules), locally the molecules are entangled between polymer chains so the mobility is more restricted to what it is predicted on basis on macroscopic  viscosity. So local viscosity and mobility and relaxation time is different to what we measure in practice.

-          Impact of water activity on the microbiological development (below water activity of 0,6 foods are typically microbiologically stable), however the glass transition theory cannot predict the microbiological stability of foods. Bacteria are less resistant than yeast and moulds.

-          Enzymatic reactions, at high water activity, more enzymatic reaction. When we reduce the mobility, there will be less enzymatic reactions. In monolayer water content there are almost zero enzymatic reaction (no solvent capabilities).

-          Hydrolysis needs water as reactant. At higher water content more hydrolysis.

-          Non enzymatic follows similar trend. But sometimes at very high water activity it is reduced. Because for this kind of reaction you will need the reducing sugars to get in contact with the amin group, and if there is too much water. They will be diluted in it. So the reaction won’t happen due to a highly diluted solution.

-          Strange with lipid oxidation. At monolayer water content we don’t expect much chemistry happening (theoriticaly), but in lipid oxidation it continuous. It is logic because you don’t need water for lipid oxidation (only a oxygen and lipid). Another explanation is that the pro-oxidants (metal ions, cupper and ferrum) become available. Also the matrix is less accessible when it is dry than when it is moisture (dry meat), so penetrability of oxygen is higher at higher moisture content. Sometimes it is lower, or sometimes stabilizes; it depends from matrix to matrix.

 We also reach a minimum where there is an increase of lipid oxidation. An explanation could be that the hydartation layer around the food provokes the lipid oxidation, however when it is removed it will like to join  the fat even faster. Another explanation is that solubility in water is lower than the solubility in oil. Solubility of oxygen in water is 8 mg/lt particle. Solubility of oxygen in oil is 40 mg/lt. (40 times more). When food has a hydration layer, the molecule will have a difficulty to penetrate it. So basically the layer of water prevents the oxygen to enter the particle.

This is a theoretical scheme! In practice most food behaves more or less like here. But many foods don’t behave like this, so it is just a starting point. There are many deviations in practice:

-          Oat meal at 25°. Doesn’t correspond to the general curve of oxidation reaction.

-          For penuts (lipid oxidation deviation)

-          For musley (lipid oxidation deviation)

-          For enzymatic reactions: Low molecular weight matrix we see that the enzymatic reaction will start faster tha the high molecular weight. The high molecular weight can retain more water.

-          Non enzymatic browning: here it is a disaster. In a glucose fructose, glycine mixture.

Summarizing:

-          Microbiological stability is not supported by glass transition.

-          Physical stability can be explained using the glass transition theory (explain better the water uptake of powder). Eg. When amorphous lactose is a glass it will not take up water, when it is in rubber state it will take up water. Lactose hydride crystals are less hygroscopic, no tendency to take up water.

-          Chemical stability, the theory rarely supports it.

Conclusion:

Water activity tells us about interaction of water with the food matrix. In contrast

The Glass transition theory tells us something about the non-aqueous fraction of the food, and how this state is behaving. There is a link with availability of water. Also a link with chemical stability (but not always, some cases yes, other cases no). This theory has a lot of merits with the physical stability

Both theories are complementary to describe the impact of water on foods, but we don’t know enough to understand everything.