Studies of Reaction Kinetics in Relation to the Tg′ of Polymers in Frozen Model Systems

Abstract

The rates of diffusion-controlled processes in a frozen system can be influenced by the presence of glassy states. One characteristic of cryostabilization by this mechanism is a change in the temperature dependence of reaction rates at the Ts' of the system. The cryostabilization behavior of solutes such as maltodextrin, carboxymethylcellulose (CMC), and sucrose was studied. Three different model reaction systems (enzyme hydrolysis, protein aggregation, and non-enzymatic oxidation) were used. Maltadextrin had a consistent pattern of cryostabilization behavior at temperatures ranging from -3°C to -20°C for all three model systems. Significant retardation effects were evident in the temperature range corresponding to its glassy states. Sucrose did not show a stabilizing effect in the non-proteinaceous model system (the non-enzymatic oxidation reaction). This could partly be due to the absence of the glassy state, since the storage temperatures were above its Ts'. However, in the protein aggregation model system, sucrose was an excellent stabilizer in protecting actomyosin from aggregation. This may be explained by a "solute exclusion" mechanism. CMC did not show any stabilizing effect in the protein aggregation and non-enzymatic oxidation model systems studied, even though it has a Ts' as high as that of maltodextrin. These results demonstrated that although the presence of a glassy state may well l1ave a retarding effect on the rates of diffusion processes, just knowing the Ts' of a polymer is not sufficient for prediction of its stabilization effect in a frozen system.

Biological glasses: nature's way to preserve life

“…when the pollen is in a glassy state. Similar results have been shown with food systems and pharmaceuticals where degradative reactions still occur below T g and conditions for maximum storage stability are considerably less than T g (Lim and Reid, 1991;Nelson and Labuza, 1994;Hancock et al, 1995;Chang et al, 1996). In line with glassy concepts used in the food and pharmaceutical literature, we conclude that ageing reactions in pollen can occur in the glass, albeit at lower rates compared with ageing rates above T g (Fig.…”

“…We subsequently attempted to relate the storage behaviour of pollen to cytoplasmic glass formation that has a comparable dependence of water and temperature. The presence of glasses has profound beneficial effects on the stability of various food materials (Lim and Reid, 1991;Karmas et al, 1992;Roos, 1995 and references therein) and pharmaceutical biomolecules (Chang et al, 1996). Intracellular glasses in seeds are hypothesised to play a role in storage stability (Burke, 1986;Leopold et al, 1994;Maki et al, 1994).…”

“…One of the most studied functions of glasses is maintaining the structural and functional integrity of macromolecules (for reviews see Slade and Levine, 1993;Roos, 1995). Many model systems have shown that glasses retain the activity of enzymes and conformation of proteins or retards crystallisation due to their high viscosity (Lim and Reid, 1991;Roos and Karel, 1991;Karmas et al, 1992;Karel et al, 1993;Slade and Levine, 1993;Bell and Hageman, 1996;Chang et al, 1996;Duddu and Dal Monte, 1997;Hancock and Zografi, 1997;Sun, 1997a;Crowe et al, 1998;. It has also been postulated that glasses are capable of maintaining stability of membranes.…”

Glass Transition of Freeze-concentrated Aqueous Solution of Ascorbic Acid as Studied by Alternating Differential Scanning Calorimetry

“…The retention of ascorbic acid in frozen foods is known to be highly dependent on the storage temperature and, as has been shown in model systems, also on the presence of glass-forming substances (1). The glassy state in foods has been receiving increasing attention since Levine and Slade (2) focused on glassy state theory to explain properties of dried and frozen foods.…”

The effect of starch gelatinization and solute concentrations onTg? of starch model system

“…6 For example, when a higher molecular weight solute (maltodextrin) was added to a sucrose solution, the T g ' of the sucrose solution was increased from À33°C to À13°C. 7 In contrast, the addition of lower MW solutes, would lower the T g ' of a product.…”