Significance of Glass Transition Temperature of Food Material in Selecting Drying Condition: An In-Depth Analysis

ABSTRACT

Drying is a complicated phenomenon involving a combination of transport, deformation, and chemical kinetics. It is an energy intensive lengthy process and results in deterioration of food quality. The glass transition temperature (GTT) significantly affects the internal mass transfer mechanism and hence significantly affects the drying kinetics. Moreover, the rheological and transport characteristics of food materials are remarkably impacted by GTT, which has an influence on the energy consumption and quality of food products during drying. Similarly, molecular weight and drying conditions also affect the GTT. This comprehensive review uncovers the fundamental understanding of GTT and demonstrates its crucial relationship with physio-structural and transport properties of food items. It has been demonstrated that a clear understanding of the glass transition temperature may help in determining appropriate drying conditions while ensuring great food quality.

KEYWORDS:

• Drying
• glass transition temperature
• food quality
• drying kinetics
• rheological and transport properties

Introduction

The glass transition phenomenon (GTP) is a thermodynamic phase transition from brittle glassy to flexible rubbery^[1].^[2,3] This second-order phase transition is generally viewed as a continuous phase transition, as contrasted to a first-order transition such as melting. At the glass transition zone, there is no discontinuity in thermodynamic variables like volume, enthalpy, or free energy. However, because of the coefficient of expansion and heat capacity shift by a few degrees in the glass transition zone, chemical reactions, and molecule mobility slowdown while increasing above the glass transition region.^[4]

The GTP plays a vital role when food products are under thermal treatment such as drying, frying, or cooking. This controls crucial phenomena, such as case hardening, cell collapse, oxidation reactions, non-enzymatic browning, and microbiological stability.^[5] Moreover, the physio-chemical properties of food items, such as Young's modulus, coefficient of thermal expansion, and specific heat capacity, are significantly altered by the GTP. On the other hand, T_g can be influenced by a multitude of factors, which include moisture content, processing conditions, composition, and the presence of plasticizers in food.

Drying is one of the most energy-intensive industrial processes, accounting for almost 15% of the total energy use.^[6] Depending on the drying procedure, 1-kg moisture loss from food during drying consumes 14.53 MJ to 90 MJ energy.^[7] As a result, research into drying energy efficiency is required for sustainability in the food drying business. The purpose of this study is to investigate the link between glass transition temperature and drying process parameters to increase energy efficiency. This paper summarizes the impacts of glass transition temperature on the thermo-chemical characteristics and heating parameters of biological materials, as well as presenting a critical assessment of the elements that influence T_g.

Section 2 describes the established theory and mathematical model relating to the glass transition temperature. Section 3 briefly discusses various methods for determining the glass transition temperature. Section 4 focuses on how process variables and food qualities impact the glass transition temperature. Finally, sections 5 and 6 examine the potential influence of the glass transition on properties and quality of food materials during drying.

Theory and mathematical modeling of glass transition

To determine the glass transition temperature in materials and to explain the effect of various factors on the glass transition temperature, several mathematical models have been developed. It is worth mentioning that all the mathematical models are empirical, and therefore they cannot be applied universally. However, these models demonstrate the important relationship between different influential parameters and glass transition temperature. Commonly used glass transition temperature mathematical models are discussed below.

The Gordon-Taylor model can be used to determine the glass transition temperature of composite food materials. This takes into consideration the glass transition temperatures of individual components and the mole fraction of the components.^[8]

(1) $T_g=\frac{w_1{T}_{\mathit{g}1}+w_2{T}_{\mathit{g}2}\mathit{k}}{w_1+\mathit{k}{w}_2}$(1)

where w=weight fraction concentration in the mixture and k= a parameter that depends on the component’s thermal expansion coefficient when it transitions from a glassy to a rubbery state during the glass transition.

This model can be used to predict the value of T_g in a composite mixture, predict the effect of moisture content on Tg, explain the effect of plasticizer on T_g, and can be applied to a polymer blend. However, it requires a lot of experimental data and is not suitable for a mixture in which any component has a negligible T_g response^[9–11]

The Couchman-Karasz model assumes that the entropy of mixing is a continuous function in the glass transition region.^[9]

(2) $ln\mathit{\hspace{0.17em}}{T}_{\mathit{gm}}=\frac{x_1\mathrm{\Delta }{C}_{p_1}ln{T}_{\mathit{g}1}+x_2\mathrm{\Delta }{C}_{P_2}ln{T}_{\mathit{g}2}}{x_1\mathrm{\Delta }{C}_{P_1}+x_2\mathrm{\Delta }{C}_{P_2}}$(2)

where x= mole fraction concentration and ∆C_P = heat capacity.

To predict T_g, this mathematical model takes only a modest amount of experimental data. Only ideal mixing systems with no component variability operate well with this equation. This model explains the influence of plasticizers on T_g, aids in understanding the effect of molecular weight on T_g, and has been used in multi-component systems, such as water, glucose, and sucrose, as well as ternary, quaternary, and higher-order systems^[9–12]

The William-Landel-Ferry (WLF) model is useful for viscous and viscoelastic food materials .This model considers viscosity is the main influential factor of molecule motion.^[13]

(3) $log\frac{\mu }{{\mu }_g}=\frac{-C_1\left(\mathit{T}-T_g\right)}{C_2+\left(\mathit{T}-T_g\right)}$(3)

where µ= viscosity; µ_g= viscosity at T_g; C₁  = 17.44 & C₂  = 51.6 K are universal constants.

The influence of drying conditions and water content on transformation kinetics and rheological properties is not explained by this model. However, it explains the dependency of viscosity on temperature above Tg, and it applies roughly over the temperature range from T_g to T_g+100 K. It also explains why chemical reactions are slow below Tg^[9,10,14,15]

In the context of cooling processes, the Kwish model can give insight into how the cooling rate affects the glass transition temperature of food materials.^[16]

(4) ${T_g}^{m_i\mathit{x}}=\frac{x_1{T}_{\mathit{g}1}+\mathit{k}{x}_2{T}_{\mathit{g}2}}{x_1+\mathit{k}{x}_2}+\mathit{q}{x}_1{x}_2$(4)

where k& q are the fitting parameters that depend on the intermolecular interaction between the components of the polymer mixture and x= mole fraction.

This model is used to examine water solute interaction and explains the effect of hydrogen bonding on the glass transition temperatures of polymers^[16,17]

Pinal’s Prediction model considers the effect of entropy on the glass transition temperature of a mixing. The model demonstrated that the T_g is shifted as a result of gaining entropy.^[10]

(5) $\mathit{Ln}\left(T_g\right)=\left(\frac{\begin{array}{c}{x}_1\mathrm{\Delta }{C}_{P_1}ln\left(T_{\mathit{g}1}\right)+\\ x_2\mathrm{\Delta C}{p}_{2}ln\left(T_{\mathit{g}2}\right)\end{array}}{x_1\mathrm{\Delta }{C}_{P_1}+x_2\mathrm{\Delta }{C}_{P_2}}\right)-\left(\frac{\mathrm{\Delta }{s}_m}{{\mathrm{\Delta }}_{C_P}}\right)$(5)

where x= mole fraction; ∆C_P = heat capacity difference between the liquid and crystalline form.

∆s_m = configurational entropy of mixing.

This model helps to explain how the entropy of mixing affects the T_g of the amorphous mixtures.^[9,10]

The Mandelkern, Martin, and Quinn Equation model considers chemical structure.^[18,19]

(6) $\frac{1}{T_g}=\frac{1}{x_1+\mathit{R}{x}_2}\left(\frac{x_1}{T_{\mathit{g}1}}+\frac{x_2}{T_{\mathit{g}2}}\right)$(6)

where R is a parameter that considers volume–temperature coefficients of the liquid state of homopolymers.

All the above-mentioned mathematical models for determining the glass transition temperature are empirical models. Prior to applying any of these models to food materials, experimental validation is crucial to ensure the accuracy of the model.

Glass transition temperature measurement techniques

Since the glass transition does not involve latent heat, the second-order phase change is identified by observing changes in a variety of properties, including dielectric properties like the dielectric constant, mechanical properties, such as viscosity, and thermodynamics characteristics like heat capacity, free volume, enthalpy, and thermal expansion coefficient. There are the ways to calculate the glass transition temperature based on observational properties. Individual methods detect glass transition temperature depending on the glass transition’s distinct features in response to varied conditions. Different methods are helpful in measuring the Tg of various materials. As illustrated in Fig. 1, the main methods for evaluating glass transition including thermal, thermo-mechanical, volumetric, and spectroscopic approaches.

Figure 1. Glass transition temperature measurement techniques.

In the thermo-mechanical approach, thermal and mechanical properties of materials are used to determine the value of Tg. Many instruments are used including TMCT, DMA, TMA, OSF, SMP and TMCT to evaluate T_g using the measured thermo-mechanical properties of material. Some devices such as DSC and TSDC use only the thermal characteristics of the material to determine the Tg value of a given material.

Unlike the mentioned approach, the volumetric-based approach quantifies interaction energies of polar and nonpolar molecules within the polymers. This estimation method is sensitive to the properties of the bulk polymer above the glass transition temperature (T_g) and used to determine interactions at polymer surfaces below T_g. Two significant boundaries in IGC are the retention volume and peak width or peak broadening. The starting point of retention volumes and peak broadening is broadly known at above and close to the glass transition temperature (T_g).

Moreover, many spectroscopic methods including NMR, PALS, and DRS are used to determine T_g of different materials. The reader interested in more insight on the determination procedure of T_g by using these techniques can consult numerous other sources available, such as those listed in the references. The different glass transition measuring techniques are summarized in Table 1.

Table 1. Comparisons among different measurement techniques of glass transition.

Group	Technique	Observational Properties	Compatible for materials	Ref.
Thermo-mechanical	Thermal Mechanical Compression Test (TMCT)	Linear displacement and strain Force dissipation.	Skim milk powder	^[Citation20]
	Dynamic Mechanical Analyzer (DMA)	Varying stress on dynamic moduli	Ribbon-shaped spaghetti, LaTeX film (Two different styrene/butadiene latices).	^[Citation21,Citation22]
	Thermo-mechanical Analysis (TMA)/Dilatometry (DIL)	Viscoelastic properties (storage/loss moduli).	Skim milk powder, Single-Crystal C6o, POLY-VINYL ACETATE.	^[Citation20,Citation23,Citation24]
	Oscillatory Squeezing Flow (OSF)/Broadband frequency squeezing flow	Stiffness and viscoelastic properties (loss and storage modulus).	Pharmaceutical powders	^[Citation25]
	Scanning Probe Microscopy (SPM)/Atomic Force Microscopy (AFM)	Relaxation (change in resonance frequency).	Hexadecane, Octadecane, Oligoethylene, Octacaine, Latex films (Two different styrene-butadiene copolymers).	^[Citation22,Citation26]
Spectroscopic	Positron Annihilation Lifetime Spectroscopy (PALS)	Relaxation and lifetime of positron and positronium.	polystyrene (PS), Acrylonitrile butadiene rubber (NBR), polystyrene, and poly (p-methyl styrene).	^[Citation27–30]
	Nuclear Magnetic Resonance (NMR)	Molecular mobility Diffusion	Amylopectin, Spray-dried lactose, maltodextrin, atactic polypropylene, and cis-1,4-polyisoprene.	^[Citation27, Citation29, Citation31–33]
	Dielectric Relaxation Spectroscopy (DRS)	Spin-spin and spin-lattice relaxation times	Crystalline Forms of the Drug Substance (SSR), Two potential lyophilized formulations of a chimeric monoclonal antibody, one with sucrose and the other with trehalose as a bulking ingredient,^[Citation34] D-sorbitol (biochemical grade) and Maltitol, polyethylene terephthalate (PET) films, D-Glucose, D-fructose, D-mannose, L-rhamnose monohydrate, D-xylose, D-glucitol, maltose monohydrate, and malt triose	^[Citation34–38]
Thermal	Differential Scanning Calorimetry (DSC)	Heat flow rate Heat capacity	Latex films, Amylopectin, Ribbon shaped spaghetti, apple, strawberry, blackberry, raspberry, peach, polyethylene glycol (PEG), lactose, Hydroxypropyl methylcellulose-HPMC (E4M Prem), ICI poly-ethylene terephthalate (PET), Poly-ethy1ene terephthalate, Poly-methy1 methacrylate (PMMA) and poly-styrene-co-acrylonitrile (SAN), poly-methyl methacrylate and poly-epichlorohydrin.	^[Citation21, Citation31, Citation39–44]
	Thermally Stimulated Depolarization Current (TSDC)	Dynamics and molecular motions/relaxations in semi-crystalline and amorphous polymers as well as pharmaceutical powders.	Drug substance LAB687, purity 99.9% by HPLC and polymer Kollidon 30 powder (polyvinylpyrrolidone PVPK-30), α-D-Glucose, polyacrylate (PAr), Crystalline Forms of the Drug Substance (SSR), Epoxy resin.	^[Citation35, Citation45–48]
Volumetric	Inverse Gas Chromatography (IGC)	Gas retention time and volume	PVC, Styrene-butadiene (% of styrene), PMMA, PNIPAM, PS, Styrene-butadiene, PSNM, PSNMP, PSMAM, PSMMA, PAN, PVC non plasticized, Plasticized, PSMA, PGME, PGME-en, PDDMA, PANSMAMPS, P (MHPMA-co VP), Ardel®D-100, lactose, PMMA and PS beads.	^[Citation3, Citation49–52]

It is clear from Table 1 that diverse methods are available to determine T_g for different food materials. However, a single method is not applicable to all materials for measuring T_g. Moreover, each method has its specific advantages and disadvantages. For example, dielectric relaxation spectroscopy (DRS) is less frequently used for determining T_g of food materials.

Factors affecting glass transition temperature

The phase change from glassy to rubbery associates the value of T_g of a food material is influenced by several factors including composition, chemical structure, thermo-physical properties, and even drying conditions. T_g values of food materials vary from that of water (−135^οC) to those of polysaccharides (approximately 243^οC).

Compositions

Composition of food including water, sugar, and fat content can significantly affect glass transition temperature. Depending on which solvents are present in the material, the glass transition temperature is impacted differently. Therefore, T_g of a pure polymer is not the same as the T_g of a composite.^[53] It can be said that glass transition temperature of a food material is significantly affected by its composition and accompanying microstructure.^[54] The modified Gibbs-DiMarzio law explains how it changes with the chemical composition.

(7) $\mathrm{Tg}(\mathit{\hspace{0.17em}}<\mathit{r}>)=\frac{\mathrm{Tg}\left(<\mathit{r}>=2.0\right)}{1-\mathit{\beta }\left(<\mathit{r}>-2.0\right)}$(7)

where β is a parameter fitted from the experimental data, <r> is the average coordination number defined as, <r≥${\sum }_{r=0}^{\mathrm{Zmax}}\mathit{rx}$r, where r is the maximum number of covalent bonds that an atom can generate. The occurrence of each sort of atom in the glass is represented by X_r and the maximum coordination among all-atom species is Z_max.^[55]

Moisture content

In most food materials, there are two forms of water: free and bound water.^[56] In the literature, the overall water content is used to connect T_g with moisture content. The influence of moisture content on the glass transition temperature T_g is explained by the Gordon–Taylor equation shown below^[8]:

(8) $T_g=\frac{w_1{T}_{g_1}+\mathit{k}{w}_2{T}_{g_2}}{w_1+\mathit{k}{w}_2}$(8)

where $w_1\mathit{and}{w}_2$ are the weight fraction of solids and water, respectively. $T_{g_1}\mathit{and}{T}_{g_2}$ are the glass transition temperature of bone-dry solid and water (−135^οC), respectively.^[57] Whereas k is the fitting parameter.

The glass transition temperature (T_g) rises as the moisture content decreases, as shown in Fig. 2.^[59,60] Many researchers have tried to figure out why this happens, and their findings can be summarized as follows.

Figure 2. Effect of moisture content on glass transition temperature^[58].

1. Plasticizing effect of water: Water functions as a plasticizer when moisture content increases, it lowers the glass transition temperature.^[61] When moisture level rises, the water functions as a plasticizer.^[62,63] When water is used as a plasticizer, the inter-and intra-macromolecular forces are reduced, lowering T_g.^[11,58,64]

2. Weakening intermolecular hydrogen bonds: Due to the attractive force of the water molecule bond, water weakens intermolecular hydrogen bonds, allowing chain mobility and lowering T_g.^[58]

3. Increase in mobility: Moisture occupies the free volume within the polymer chain, causing it to become more mobile, and eventually lowering the T_g.^[65]

4. Decrease in viscosity: The viscosity increases when the moisture content decreases as migration of moisture causes increase of solute density.

In general, the glass transition temperature of any food material increases with the decrease of moisture content. Therefore, the amount of water is vital in thermo-physical aspects of food materials during drying.

Cellulose, hemicellulose, lignin, and pectin

Cellulose, hemicellulose, lignin, and pectin are the fundamental building blocks of plant cell wall. Therefore, their proportion in food significantly affects the overall T_g of food materials as the T_g of those individual components are different than each other. Table 2 shows the glass transition temperature of building blocks of plant cells.

Table 2. Glass transition temperature of fundamental building blocks of plant cell wall.

Components	Tg	Ref.
Cellulose	From 200 to 220°C (No water content) From −60 to −100°C (30% water content)	^[Citation66]
Hemicellulose	40°C	^[Citation67]
Lignin	50 to 100°C	^[Citation67,Citation68]
Pectin	16.8 to−24.6°C	^[Citation69]

It is hypothesized that the effective glass transition temperature of plant-based food materials can be estimated from the proportion of the basic components including water, cellulose, hemicellulose, lignin, and pectin. Due to the plasticizing nature of food material, individual building blocks can affect the overall T_g. Hemicellulose and pectin can affect the value of T_g as these have plasticizing properties.^[65–69]

Plasticizer

The presence of plasticizers improves the softness, flexibility, and extensibility of food materials.^[4] The impact of plasticizer is explained by two mechanisms: the plasticizer molecule reduces the attractive force between the molecules of the substance and increases the space between them.^[9,70]

The glass transition temperature is inversely related to plasticization, as shown in Fig. 3.^[72]

Figure 3. Effect of plasticizer on the glass transition temperature^[71].

When a plasticizer is applied, it gets stuck between the polymer chains and keeps them apart. As a result, plasticizer lowers inter- and intra-macromolecular interactions, allowing segmental mobility and lowering internal friction in polymers.^[11,64] Besides that, plasticizers reduce mechanical properties, increasing workability, flexibility, and distensibility.^[64] As a result, low-temperature polymer-chain movement is feasible, resulting in a drop in a polymer’s T_g.^[73] Water is the most effective plasticizer for food and biological materials out of all of the plasticizers.^[64,74]

Molecular weight

The glass transition temperature of a polymer is directly related to its molecular weight. The relationship between molecular weight (MW) and T_g may be simply understood using the Fix-Flory equation, which is,

(9) $\mathrm{Tg}=\mathrm{Tg}\left(\mathrm{\infty }\right)\mathit{\hspace{0.17em}}-\frac{B}{\mathit{MW}}$(9)

where Tg (∞) is the glass transition temperature for an ideal chain of infinity length and B is a polymer-specific constant. The glass transition temperature rises with the increase of molecular weight (MW), as shown by Equation (9)(9) $\mathrm{Tg}=\mathrm{Tg}\left(\mathrm{\infty }\right)\mathit{\hspace{0.17em}}-\frac{B}{\mathit{MW}}$(9) .^[57,75,76] T_g values increased with increasing molecular weight as the increased size of molecules causes more points of engagement with the adjacent molecules as demonstrated in Table 3.^[47,76,77]

Table 3. Effect of Molecular Weight on T_g^[25].

Molecular Weight, Mw (g/mol)	Glass Transition Temperature, Tg
1000	38
10000	92
100000	104
1000000	105

The specific volume reduces as the molecular weight increases.^[25] According to the free volume theory,^[78–80] this decrease in specific volume reduces chain mobility, causing T_g to rise.^[81] Plasticizer reduces with increasing molecular weight, and the decreased effect of plasticizer raises T_g.^[9]

Physical properties

Physical properties of food significantly affect the value of T_g. The effects of different physical properties of T_g have been discussed in the following section.^[81]

Porosity

Porosity refers to the presence of free spaces within a total volume of food samples. Porous materials have a porosity greater than 0.4, while nonporous materials have a porosity of less than 0.25.^[82] Porosity affects the physical, mechanical, and textural properties of food.^[82,83] The glass transition temperature of food is affected by porosity, and it lowers as porosity increases.^[84,85] Higher porosity causes less restriction of solid food matrix or polymer chains. According to Fig. 4 samples with higher porosity had a wider T_g range than samples with lower porosity^[83]..

Figure 4. (A) Correlation between porosity and Tg in dehydrated plant-base sample,^[84] (b) contribution of porosity on the change of T_g.

When porosity increases, free space also increases, which eventually decreases T_g. Porosity is inversely related to thermal conductivity of multiphase materials such as plant-based food materials, higher porosity levels result in lower T_g.^[83]

Flexibility

Flexibility is defined as the ability to twist without breaking.^[83,86] The glass transition temperature is influenced by the flexibility of the polymer’s main chain.^[87] Flexibility of the material allows the polymer chain to move more easily. As a result, the flexibility is inversely proportional to the glass transition temperature. On the other hand, the T_g of polymers rises as the chain stiffness rises.^[88] The glass transition temperature (T_g) increases as the chain stiffness increases.^[89]

Free volume at microlevel

In the context of amorphous polymers at microlevel, the term “free volume” refers to the empty space in the polymer chain, which is not occupied by atoms or molecules. Free volume is the amount of internal space available within a polymer matrix.^[90] The glass transition temperature (T_g) rises as the free volume decreases,^[75] as shown in Fig. 4 (b). More free volume is associated with a short polymer chain. As a result, the shorter chain’s glass transition temperature is lower than the longer chains.^[23] Free volume diminishes as pressure rises, resulting in a high glass transition temperature (T_g).^[73] When the polymer is heated above T_g, separated polymer chains create more free volume.^[91]

A cross-link is a bond that connects two polymer chains. Because of the polymer’s limited chain mobility, the glass transition temperature is directly proportional to the degree of cross-linking. Increased cross-linking reduces polymer-chain mobility and free volume, increasing T_g.^[73] Furthermore, chain length and branching have a big impact on the glass transition temperature. When polymers have more branching and chain ends, they have more free volume, which lowers T_g. Glass transition temperature rises in larger groupings where branches hinder rotations. The glass transition temperature of polyethylene decreased as the degree of branching increased.^[92] As a result, branching lowers the temperature of the glass transition.^[93]

Drying conditions

The glass transition temperature (T_g) is one of the most important techniques for quantifying water mobility in biological materials and managing product shelf life. Temperature and moisture content influence the mechanical and other barrier qualities of biological materials.^[94] A rubbery state exists in dried biological products when they are above the glass transition temperature, which accelerates the molecular mobility of the product, resulting in a faster rate of physiochemical change in the dried product.^[39] Drying conditions including drying temperature and heat transfer rate affect changes in the morphology and water distribution, which eventually affect the glass transition temperature. Therefore, a sample with the same moisture content but at different drying temperatures and heat transfer rates can have different glass transition temperatures.

Heating rate

The glass transition temperature of a dried product is affected by its heating rate, and it rises as the heating rates rise.^[95,96] When calculating the glass transition temperature of processed or stored biological materials and pharmaceuticals, the effect of heating and cooling rate is critical.^[25,97] This factor is important in the case of volumetric heating such as microwave heating, which causes a nonuniform temperature distribution. The following equation shows a high degree of dependency of the glass transition temperature (T_g) on heating $\left|\mathit{q}\right|$ under specified conditions.

(10) $\frac{\mathit{d}ln\left|q\right|}{\mathit{d}{T}_g}=\frac{\mathrm{\Delta }{h}^{\ast }}{\mathit{R}{T}_r^2}$(10)

Where T_r, is a temperature in the middle of the transition range, $\mathrm{\Delta }\mathit{h}$* is the activation enthalpy for the relaxation time controlling the structural enthalpy or volume relaxation, and R is the ideal gas constant.^[98]

Figure 5 illustrates how the heating rates affect the glass transition temperature. The molecular motions involved in structural relaxation are generally thought to be rough, similar to those involved in viscous flow.^[98] Another study discovered that the heating and cooling speeds of dried products affect the beginning and end temperatures of the glass transition, and enthalpy^[99]

Figure 5. Effect of drying temperature and heat transfer rate on T_g during drying.

Drying temperature

Unit operations in food processing include freezing, dehydration, flaking, sheeting, baking, and drying process.^[100–102] The conditions need to be maintained carefully during drying. Fig. 5 demonstrated how the glass transition temperature (T_g) changes due to drying temperature. This is because the higher the unit’s operating temperature,^[103] the less time it takes to reach a higher glass transition temperature (T_g).^[104] Amorphous supercooled liquid with exceptionally high viscosity is defined as glass in this^[105] context (between 10¹⁰ and 10¹³ Pa.s).^[75,106] In certain circumstances, the drying temperature is higher than the glass transition temperature (T_g), and raising the temperature or moisture content causes the system to shift from a glassy to a rubbery state.^[107]

From the above discussion, a combination of factors affects the value of T_g. Eventually, there is not always a clear correlation between a given characteristic and the glass transition temperature (TG), as other factors usually have an impact.

Effect of Tg on food properties

Drying kinetics is affected by both drying conditions and food properties. Transport and physical properties and morphology are the main properties that significantly influence the drying kinetics of food materials. It is well established that these properties evolve spontaneously during heat and mass transfer. GTT plays an important role in determining the nature of the evolution of these properties during drying, and this is an area that requires extensive research. In the following section, we present evidence of the effects of T_g on different properties of food materials, which eventually affect drying kinetics.

Mechanical properties and relaxation times

Mechanical properties of food materials are important in both energy consumption in food drying and quality aspects of food. Young’s modulus and stress relaxation are the most important mechanical properties of food materials. The role of T_g on these mechanical properties is described below.

Young’s modulus

Glass transition temperature rises with stiffness in polymers and plant-based biomaterials.^[88] The two main mechanisms that cause a material’s Young’s modulus to increase are crystallization and the transformation to the glassy phase.^[108] The material’s flexibility affects the reduction in Young’s modulus.^[109] When a food substance transitions from brittle to rubbery at the glass transition, its mechanical properties also change.^[110] When the material is heated above the glass transition temperature, however, the amplitude of the effect may decrease by four to five orders. Glass transition occurs around −20°C in a starch–glucose water film, resulting in a modulus reduction of more than 10³ Nm^−2.[111]

Stress relaxation

Relaxation times are the rates at which mechanical properties change. Creep and stress relaxation are time-dependent mechanical qualities. The material’s viscoelastic or viscoplastic behavior manifests itself in a variety of ways, including creep under constant load, time-dependent creep rupture, stress relaxation under constant deformation, and time-dependent deformation recovery after load removal.^[77,112] Under applied stress, creep is a time-dependent extension. Stress relaxation is the complementary effect under continual expansion. The duration and rate of loading have a significant impact on polymeric material deformation, which becomes increasingly important as the temperature approaches the glass transition point.^[77,113,114]

Morphology change

Food morphology has an impact on food production and transformation. All food products have three morphological characteristics.^[115] The first is the geometric, topological, and symmetry levels of describable configuration. The glass transition temperature (T_g) affects food texture as well as dry food storage stability.^[116] The T_g of amorphous food systems regulates changes in food structure and texture. Amorphous structures are stable below T_g in the solid glassy state.^[117] The solid-state is changed into a supercooled liquid state around and above T_g with time-dependent flow.^[116,118] Dehydrated, glassy meals, for example, have a firm, brittle consistency, but at T_g, the materials may flow or become mushy.^[116] Understanding the glass transition phenomenon can reveal the causes of the cohesiveness of many essential powders, and it has an impact on new product development attributes such as wettability and solubility.^[119]

Shrinkage occurs when water migrates from the intercellular space of the food material to the surrounding environment during drying, reducing the volume of the material.^[120] By influencing the mechanical and textural aspects of food, shrinkage has a significant impact on the quality of the dried product.^[121]

Molecular diffusivity decreases dramatically below the glass transition temperature due to high viscosity and as a result, water and other organic components cannot travel freely,^[122,123] resulting in a stiff product.^[124,125] The mobility rate of the solid matrix is directly related to the temperature difference between the product and its glass transition temperature for a certain moisture content.^[126] Drying at a temperature above the glass transition temperature keeps the product rubbery, allowing for a larger shrinkage rate due to the mobile solid matrix.

Transport properties

Food solids are plasticized by water and other low molecular weight hydrophilic solvent components. Water is a mobility enhancer because its low molecular weight results in a significant improvement in mobility, lowering local viscosity and increasing free volume. The glass transition temperature is reduced as a result of plasticization, and this is related to the moisture concentration.^[39]

Diffusivity

In the drying phenomena, moisture diffusion is crucial.^[127] Diffusivity has an impact on food reactions, flavor loss, and a variety of other alterations. The dependence of diffusivity in amorphous materials can be demonstrated in two ways, the first being the use of the Arrhenius equation.

(11) $\mathit{D}=D_0{e}^{-\frac{E_D}{\mathit{RT}}}$(11)

Where E_D is the activation energy for diffusion (Jkg⁻¹K⁻¹), D is the diffusivity at temperature T (m²s⁻¹), D₀ is the pre-exponential factor, R is the gas constant and T is the absolute temperature (K).

The activation energy differential above and below the glass transition temperature is determined by diffusant size and amorphous matrix packing density.^[128] The diffusion coefficient of the polymer chain decreased when Tg was raised as shown in Fig. 6.^[129] This trend is generally observed for homogeneous medium, which relates to Fikcian diffusion.

Figure 6. Different zone of diffusion, function of the temperature (K) and penetrant concentration (kg/m³).

On the other hand, the effect of glass transition temperature on diffusion is not straightforward for viscoelastic materials, such as plant-based food materials. Besides Tg, diffusivity depends on solid matrix composition, morphology, and water distribution within the food sample.^[130]

Permeability

The capacity of a material to enable vapor to pass through it is measured by its vapor permeability. The glass transition temperature (T_g) and the transport properties of dense membranes have a decent relationship.^[131] The permeability of gas and vapor is much higher in the rubbery state than in the glassy state.^[132] The double mode sorption and mobility model predicts a reduction in permeability with increasing pressure in glassy polymers, as demonstrated by CO₂ in polycarbonate.^[133] Permeability decreases when polymers become more crystalline below T_g. Permeability increases as polymers become more amorphous above T_g. Permeability is calculated by multiplying the diffusion and solubility coefficients.^[134]

From the above discussion, it is demonstrated that glass transition temperature has significant effect on different aspects of food properties and qualities. Contribution of glass transition temperature in reaction kinetics, transport phenomena, and deformation may be vital as T_g significantly affects thermo-physical properties of food materials. It can be concluded that T_g should be taken into consideration during the selection of drying conditions.

Effect of Tg on food quality

There are many factors that directly affect both food quality and energy consumption of food drying. T_g of the food materials directly and indirectly contributes to changing of food quality and energy consumption in food drying. In the following sections, a detailed explanation of the influence of T_g on energy consumption and food quality is presented. As discussed in the earlier section, T_g significantly affects physio-chemical, mechanical, and transport properties; eventually, T_g has both direct and indirect influences on drying energy consumption and quality of the food.

The quality of food is affected when it is dried at a temperature greater than its glass transition temperature (T_g).^[135] Glass transitions can cause physical and chemical changes in food during processing and storage.^[136] After drying, many dried foods become glassy, and this physical state has an impact on chemical alterations. Water content has a considerable effect on raising the reaction rate, which is equivalent to the effect of increasing temperature.^[31]

Sensory properties

People base their food purchases on a variety of sensory factors.^[137] The five major sensory qualities of food are appearance, texture, fragrance, taste, and irritation.^[138] For example, crispiness is an essential quality for food with low-moisture quantity such as cereal. Crispiness can be affected by both moisture content and T_g. Therefore, storage at a temperature lower than T_g can significantly affect the crispness.

Colour

The colour of the dried food product is frequently used by the consumer to choose or reject it. As a result, colour is a significant quality characteristic of dried foods. Chemical and biological interactions modify the colour of food throughout the drying process and non-enzymatic browning degrades food quality.

T_g affects the physical state of the food matrix and eventually influences the perception of food color.

Figure 7 shows the impact of Tg on browning reaction. From this figure, it is clear that below the glass transition temperature, slow browning occurs, leading light color of the dried food.^[14]

Figure 7. Relation between browning (absorbance at 420 nm) reaction and T-T_g(K)^[14] .

Deformation

Deformation takes place during drying as food undergoes cell collapse and tissue shrinkage. There is a subtle distinction between shrinkage and collapse; shrinkage refers to a reversible reduction in the volume of the food sample, whereas collapse signifies an irreversible breakdown of the structure at either the cellular or tissue level. Due to transport processes, structural changes in the cellular level occur during the drying period.^[139] The evolution of porosity and shrinkage during drying has a considerable impact on the transport process as well as several quality aspects. As a result, accurate structural deformation prediction is required for proper characterization of transport processes and quality aspects. Numerous empirical and theoretical approaches to deformation during drying are available in the literature. Some literature shows significant effect of T_g on the trend of deformation. Recently, Joardder and Karim^[140] developed a deformation prediction model using T_g based shrinkage velocity equation as mentioned below:

(12) $v_s=\frac{{\mathit{D}}_{\mathit{eff}}{\rho }_w\left(\mathit{T}-T_g\right)}{2\mathit{l}{\rho }_p\left(T_0-T_{\mathit{g}0}\right)}$(12)

Where v_s is the shrinkage velocity in ms⁻¹ and l is the half thickness of the sample. D_off=Effective gas diffusivity. T and T₀ are instantaneous and initial temperature of the sample (K), respectively. T_g , T_g0 are instantaneous and initial glass transition temperature of the sample (K); ${\rho }_w,{\rho }_p$= density of water and solid materials, respectively.

Texture

Food texture is one type of feature that can be detected with the touch of the mouth or hand. The crispiness is a prominent textural attribute for dried food products including crackers, popcorn, potato chips, and breakfast cereals.^[141,142] T_g can affect the texture of dried food significantly. Crispness is linked to a brittle, low-density cellular structure that produces a high-pitched noise when broken. The decline in rigidity modulus associated with the glass rubber transition in polymers was blamed for the loss of brittleness as shown in Fig. 8.

Figure 8. Glass transition and crispness as a function of water content^[142].

Some food items, such as potato chips, with higher glass transition temperature offers crispier texture at room temperature. On the other hand, foods having lower T_g means that it will transition to rubbery state from glassy at lower temperature.

Flowability

Flowability is an important property of fruits and vegetable powder. Sticky and its consequential caking effect is observed in the powder at low flowability. T_g of the powder materials is the key factor that determines the degree of flowability of the powder material. If the T_g of the powder material is less than the ambient temperature, the caking effect takes place.^[143] Therefore, storing powder of fruits and vegetables below the T_g is recommended to prevent the caking effect.

Food stability

During food processing and storage, several variables, including the amount of water, the composition of the food, the preservation method used, the P-H level of the food a, and processing variables like temperature and pressure, can strongly influence microbial death, growth dynamics, and deteriorate physical, chemical, and biological changes. As a result, it might be challenging to assess the stability of food ingredients. Water activity is a concept for determining the stability of food, it was put forth in the 1950s. The glass transition concept, however, was put forth in the 1980s as a result of the water activity theory’s shortcomings.^[143]

Since diffusion-controlled reactions are relatively slow at or below T_g, food is most stable at or below the glass transition point. Diffusion-controlled reaction kinetics significantly rise at a temperature above in T_g. The stability of food will eventually be affected by slight changes in storage temperature near the T_g, determining whether a product will be stable for tens or hundreds of days. Food with a higher T_g is more stable as they are less susceptible to moisture absorption and other degradation. Furthermore, glassy state is more stable in response to reactive species. For instance, glucose crystals can remain stable for months in dry air at room temperature.^[5]

Conclusion

Understanding the mechanics involved in drying necessitates the knowledge of some critical parameters including the glass transition temperature and moisture content. This review reveals how Tg significantly impacts the transport and physio-structural properties of food products. Modifications to transport and other features then influence drying kinetics. The understanding of Tg can be used to choose the ideal drying conditions to minimize energy usage and quality changes during drying. On the other hand, the composition, water content, and process conditions are all factors that affect Tg. Furthermore, it is crucial to have a thorough understanding of the relationships between T_g and changes in food properties. Taking all of these into consideration, knowledge of T_g fresh and dried food can offer important insight for designing a sustainable drying system and prediction of dried food qualities. Further investigation of these connections will be beneficial in identifying the ideal drying conditions that ensure high food quality while using the least amount of energy.^[71,144]

Disclosure statement

The authors do not have any conflicts of interest.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.