What’s glass transition temperature?

Frozen methods inside the meals trade rely strongly on the glass transition temperature (T_g‘) so as to keep their high quality in fresh-like circumstances and develop storage life. On this publish, we are going to find out how this worth is essential to meals scientists and the way it permits the identification of the soundness of meals high quality throughout processing and storage.

Freezing of meals has been in use by humanity for hundreds of years. Initially, nonetheless, this phenomenon relied on winter frosts. Farmers, fishermen, and trappers would grains and produce in unheated buildings through the winter season. These days, nonetheless, the freezing approach itself, similar to the frozen meals market, is growing to turn out to be sooner, extra environment friendly and more cost effective. 

Freezing can develop the meals shelf life and keep meals stability a subzero temperatures by eradicating the majority of water from the solute part. This phenomenom permits the inhibition of any chemical, bodily or organic response. Nevertheless, each the freezing temperature and kinetics are crucial to preserving the meals high quality and texture.

The glass transition temperature (T_g‘) is described because the temperature at which 30-50 carbon chains of a system begin to transfer. On the glass transition temperature, the amorphous areas transit from the inflexible state to a extra versatile state. Accordingly, at this temperature, the free quantity (the hole between the molecular chains) additionally will increase by 150%.  You will need to take into account that the glass transition phenomenon is a second-order thermodynamic transition. Because of this the amorphous materials beneath evaluation adjustments its bodily state and never its bodily part. Mainly, whereas a crystalline stable has a long-range molecular order, an amorphous stable lack long-range molecular order however accommodates short-range molecular order. 

Attention-grabbing, however how does this incidence truly translate to the meals sector? Basically, as a result of within the glassy state, the motions of molecules inside the meals matrix are restricted to rotations a vibrations solely, whereas considerably decreasing the molecular mobility.

Through the glassy state, the viscosities of amorphous supplies are assumed to point out very excessive values that vary between 10¹² to 10¹⁴ N-s/m², which means that the velocity of the chemical reactions inside frozen meals is strongly decreased, a attribute that’s essential to make sure profitable frozen storage. Contrarily, if the temperature just isn’t low sufficient to achieve values under the glass transition temperature (T_g‘), the molecules inside the meals matrix will expertise translational mobility on account of a discount within the viscosity, producing a liquid-state state. This rubbery state will increase the molecular diffusion and the meals turns into partially unstable.

Other than the idea of glass transition temperature (T_g‘), there may be additionally the tip level of freezing or onset of melting (T_m‘), an much more essential worth when referring to the soundness of meals. This worth (T_m‘) is all the time larger when it comes to temperature if in contrast with T_g‘. In case you are a meals producer, and your freezers don’t cool sufficient to achieve a temperature under the T_m‘ of the meals being saved, you’re in massive bother, expensive reader. At temperatures larger than T_m‘, the molecular mobility is elevated considerably to the purpose the place plasticization of melting of ice happens, leading to greater charges of diffusion-controlled reactions inside the meals matrix. The chance of the frozen meals matrices spoiling would then be excessive on account of poor freezing not reaching decrease temperatures.

Due to this fact, T_g‘ and T_m‘ will be employed to precisely determine the soundness of meals at varied freezing and frozen storage circumstances.

A state diagram is usually utilised by meals scientists for mapping the bodily states of meals by plotting the temperature as a operate of the quantity of solids or water content material at fixed strain. This diagram would often show the glass transition curve (which pertains to T_g‘ the worth), a freezing/melting curve (associated to the T_m‘ worth) and the situation required to realize maximal-freeze focus, that’s the lowest temperature required to make sure that freezing is achieved when the weigh fraction of solids is comparatively excessive or saturated.

It’s noteworthy to say that each T_g‘ and the T_m‘ values are usually not the identical for all meals methods. These values fluctuate strongly relying on the kind of amorphous meals parts current and their molecular weight. Furthermore, consideration ought to be paid to the presence and quantity of plasticizers. As an example, the upper the molecular weight of meals biopolymers (e.g., inulin) inside the meals, the extra decreased the free quantity, which means that each T_g‘and the T_m‘ are elevated. Nevertheless, when the quantity of plasticizer will increase results in a discount in T_g‘ and T_m‘ values. Meals researchers, and particularly meals scientists that carry out meals high quality and security supervision duties ought to be extremely conscious of the glass transition temperature of the meals with a excessive water content material that they work with. These values will be precisely used as an indicator to determine stability losses throughout meals manufacturing advert storage. Equally, varied characterization strategies will be employed when the glass transitions in frozen methods are usually not recognized. 

The glass transition temperature in meals methods produces variations within the bodily properties and behavior of the supplies. These variations are associated to the warmth capability, deformation, stability, conductivity, particular quantity and electrical resistance. Primarily based on this, meals researchers can monitor these adjustments to additional decide the glass transition temperature. Differential scanning calorimetry (DSC), dynamic mechanical evaluation (DMA), thermomechanical evaluation (TMA), nuclear magnetic resonance (NMR), in addition to different rising applied sciences are characterisation strategies typically reported to find out the glass transition temperature. In future blogposts, I will likely be explaining how every of those works, however once more, they only monitor bodily adjustments in amorphous meals supplies at completely different temperatures.

Meals are anticipated to point out the very best stability within the glassy state (achieved by temperatures under T_g‘) because of the low price of physicochemical deterioration achieved by the diminished molecular mobility. On this blogpost, I rapidly reviewed the fundamentals to know the thermal transition ideas and the way pivotal these are for meals preservation and stability of meals with excessive water content material. To those that wish to study extra about these ideas and the present information on the glass transition temperature of assorted meals, I strongly suggest studying the assessment articles displayed within the supply supplies under.

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