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What’s glass transition temperature?


Frozen programs inside the meals trade rely strongly on the glass transition temperature (Tg‘) with a view to keep their high quality in fresh-like circumstances and broaden storage life. On this put up, we’ll 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, nevertheless, this phenomenon relied on winter frosts. Farmers, fishermen, and trappers would grains and produce in unheated buildings through the winter season. These days, nevertheless, the freezing approach itself, identical to the frozen meals market, is growing to turn into quicker, extra environment friendly and cheaper. 

Freezing can broaden 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. 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%.  It is very important remember the fact that the glass transition phenomenon is a second-order thermodynamic transition. Which means that the amorphous materials beneath evaluation modifications its bodily state and never its bodily part. Principally, whereas a crystalline stable has a long-range molecular order, an amorphous stable lack long-range molecular order however incorporates short-range molecular order. 

Differences between amorphous and crystalline solids
Variations between amorphous and crystalline solids. PhotoCredit: SolutionPharmacy.

Fascinating, however how does this incidence really translate to the meals sector? Essentially, 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.

Throughout 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, that means that the pace 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 isn’t low sufficient to succeed in values under the glass transition temperature (Tg‘), the molecules inside the meals matrix will expertise translational mobility as a consequence 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 (Tg‘), there’s additionally the top level of freezing or onset of melting (Tm‘), an much more essential worth when referring to the soundness 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 succeed in a temperature under the Tm of the meals being saved, you’re in massive hassle, expensive 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 larger charges of diffusion-controlled reactions inside the meals matrix. The chance of the frozen meals matrices spoiling would then be excessive as a consequence of poor freezing not reaching decrease temperatures.

Subsequently, Tg‘ and Tm‘ could be employed to precisely establish the soundness of meals at numerous freezing and frozen storage circumstances.

A state diagram is often utilised by meals scientists for mapping the bodily states of meals by plotting the temperature as a perform of the quantity of solids or water content material at fixed stress. This diagram would often 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.

State diagram of a sucrose-water mixture.
A state diagram of a sucrose-water combination. PhotoCredit: Roos & Karel (1991a).

It’s noteworthy to say that each Tg‘ and the Tm‘ values aren’t the identical for all meals programs. These values fluctuate strongly relying on the kind of amorphous meals elements current and their molecular weight. Furthermore, consideration needs to 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 lowered the free quantity, that means that each Tgand the Tmare 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 needs to be extremely conscious of the glass transition temperature of the meals with a excessive water content material that they work with. These values could be precisely used as an indicator to establish stability losses throughout meals manufacturing advert storage. Equally, numerous characterization strategies could be employed when the glass transitions in frozen programs aren’t recognized. 

The glass transition temperature in meals programs 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 modifications 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 might be explaining how every of those works, however once more, they simply monitor bodily modifications in amorphous meals supplies at completely different temperatures.

Meals are anticipated to point out the best stability within the glassy state (achieved by temperatures under Tg‘) because of the low fee 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 need to study extra about these ideas and the present information on the glass transition temperature of assorted meals, I strongly suggest studying the evaluation articles displayed within the supply supplies under.

 

SOURCES:

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