What is glass transition temperature?

Frozen methods inside the meals business rely strongly on the glass transition temperature (Tg‘) to be able to preserve their high quality in fresh-like circumstances and broaden storage life. On this publish, we’ll find out how this worth is essential to meals scientists and the way it permits the identification of the steadiness of meals high quality throughout processing and storage.

Freezing of meals has been in use by humanity for hundreds of years. Initially, nevertheless, this phenomenon relied on winter frosts. Farmers, fishermen, and trappers would grains and produce in unheated buildings throughout the winter season. These days, nevertheless, the freezing method itself, similar to the frozen meals market, is creating to change into quicker, extra environment friendly and more cost effective. 

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

The glass transition temperature (Tg‘) 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 remember the fact that the glass transition phenomenon is a second-order thermodynamic transition. Which means the amorphous materials beneath evaluation adjustments its bodily state and never its bodily section. Principally, whereas a crystalline stable has a long-range molecular order, an amorphous stable lack long-range molecular order however comprises short-range molecular order. 

Fascinating, 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 lowering the molecular mobility.

In the course of the glassy state, the viscosities of amorphous supplies are assumed to indicate very excessive values that vary between 1012 to 1014 N-s/m2, that 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 will not be low sufficient to achieve values beneath the glass transition temperature (Tg‘), 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.

Aside from the idea of glass transition temperature (Tg‘), there may be additionally the top level of freezing or onset of melting (Tm‘), an much more essential worth when referring to the steadiness of meals. This worth (Tm‘) is all the time higher by way of temperature if in contrast with Tg‘. If you’re a meals producer, and your freezers don’t cool sufficient to achieve a temperature beneath the Tm‘ of the meals being saved, you’re in large hassle, pricey reader. At temperatures higher than Tm‘, 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.

Subsequently, Tg‘ and Tm‘ will be employed to precisely determine the steadiness 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 normally show the glass transition curve (which pertains to Tg‘ the worth), a freezing/melting curve (associated to the Tm‘ 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 Tg‘ and the Tm‘ values should not the identical for all meals methods. These values fluctuate strongly relying on the kind of amorphous meals elements current and their molecular weight. Furthermore, consideration must be paid to the presence and quantity of plasticizers. As an illustration, the upper the molecular weight of meals biopolymers (e.g., inulin) inside the meals, the extra diminished the free quantity, that means that each Tg‘and the Tm‘ are elevated. Nonetheless, when the quantity of plasticizer will increase results in a discount in Tg‘ and Tm‘ values. Meals researchers, and particularly meals scientists that carry out meals high quality and security supervision duties must 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 should 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. Based mostly 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 usually reported to find out the glass transition temperature. In future blogposts, I can be explaining how every of those works, however once more, they simply monitor bodily adjustments in amorphous meals supplies at completely different temperatures.

Meals are anticipated to indicate the best stability within the glassy state (achieved by temperatures beneath Tg‘) because of the low charge of physicochemical deterioration achieved by the diminished molecular mobility. On this blogpost, I shortly 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 need to study extra about these ideas and the present knowledge on the glass transition temperature of varied meals, I strongly suggest studying the evaluate articles displayed within the supply supplies beneath.

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