Yrjö H. Roos

Melting and glass transitions of low molecular weight carbohydrates

Abstract Glass-transition temperatures at various water contents (Tg) and in maximally freeze-concentrated solutions (T′g), fusion temperatures (melting points, Tf), and heats of fusion (ΔHf) were determined for pentoses, hexoses, disaccharides, and alditols, using differential scanning calorimetry. The Tg and T′g values of the anhydrous compounds increased in the order pentoses

Characterization of food polymers using state diagrams

Abstract Food polymers often exist in an amorphous nonequilibrium state, which is formed in processes in which temperature is decreased below melting point or water is removed by evaporation or by ice formation. Amorphous materials have a glass transition temperature ( T g ) below which they are in a glassy state. Above T g , they exist as liquid-like ‘rubbers’. Most food polymers are thermoplastic and subject to water plasticization. Above T g various properties of the materials are changed. The most important changes are an exponential increase of molecular mobility and decrease of viscosity, which govern time-dependent structural transformations such as stickiness and collapse in food processing and storage. Above T g , molecular mobility improves diffusion, which affects crystallization, reaction rates, and food deterioration. At low temperatures the physical state is important to frozen food stability. T g values can be used to establish state diagrams, which describe the effect of composition on stability and show temperature and moisture effects on viscosity, structure, and crystallization. State diagrams may also show the formation of the amorphous state and describe various temperature-, moisture-, and time-dependent phenomena.

Influence of trehalose and moisture content on survival of Lactobacillus salivarius subjected to freeze-drying and storage

Solutions of sucrose, trehalose, skim milk, sucrose+skim milk, trehalose+skim milk, sucrose+trehalose or sucrose+trehalose+skim milk were used as protective media for Lactobacillus salivarius subsp. Salivarius (UCC 500) in freeze-drying and the ability of these materials to stabilize the bacteria during subsequent storage was determined. A decrease of about 99% or more in the viable population was observed when no protective solute (distilled water only) was used. Trehalose greatly enhanced survival rate when used alone or together with other protective materials. Trehalose+sucrose in addition to skim milk were the most efficient materials giving a survival rate of 83–85% immediately after freeze-drying and enhanced stability during subsequent storage. The ability of Lactobacillus salivarius subsp. Salivarius for utilization of trehalose as a sole source of carbon was examined. This strain could ferment trehalose and generated a significant drop in pH from initial 6.8 to 4.8. The results however suggested that the capacity of Lactobacillus salivarius subsp. Salivarius to metabolize trehalose did not contribute to resistance to freeze-drying. The effect of water on survival rate was also examined. The results showed that a residual water content of 2.8 to about 5.6% improved survival rate during storage.

Glass Transition Temperature and Its Relevance in Food Processing

Amorphous, noncrystalline solids are typical of low water content and frozen foods. Solids in these foods, e.g., confectionary, dehydrated foods, cereal foods, and frozen foods, often form nonequilibrium glass-like structures. The glassy state of the solids forms during food processing in a reversible glass transition. Vitrification can occur in numerous glassy states that exhibit various relaxations around the glass transition. The success of freeze drying, spray drying, and extrusion and the stability of dehydrated foods against flow, collapse, and crystallization is based on the control of the glassy state during the dehydration process and storage. Encapsulation processes often use glass-forming materials to entrap dispersed components or improve retention of volatiles. Plasticization of the noncrystalline structures by temperature or water reduce relaxation times exponentially above the glass transition, which results in rapid deterioration. Critical values for water activity and water content express the level of water plasticization leading to glass transition in food storage.

Thermal analysis, state transitions and food quality

Thermal properties of food systems are important in understanding relationships between food properties and changes in food quality. Concentrated food systems (low-moisture and frozen foods) are seldom in an equilibrium state and they tend to form amorphous, non-crystalline structures. Several glass transition-related changes in such foods affect stability, e.g., stickiness and caking of powders, crispness of snack foods and breakfast cereals, crystallisation of amorphous sugars, recrystallisation of gelatinised starch, ice formation and recrystallisation in frozen foods and rates of non-enzymatic browning and enzymatic reactions. Relationships between glass transition, water plasticisation and relaxation times can be shown in state diagrams. State diagrams are useful as stability or quality maps and in the control of rates of changes in food processing and storage.

Phase and state transition effects on dielectric, mechanical, and thermal properties of polyols

The present study investigated the glass transition, crystallisation and melting behaviour of erythritol, xylitol, and glucitol (sorbitol) using dielectric analysis (DEA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Sorbitol and xylitol were plasticised by water and their glass transition temperatures decreased when water content was increased. Erythritol crystallised rapidly, and its water plasticisation behaviour could not be determined. Melting of the crystalline polyols occurred at their specific melting temperatures. Melts of erythritol and xylitol crystallised on recooling and no glass transition was apparent on reheating. Quench cooled sorbitol melt remained amorphous and showed a glass transition on reheating. Glass transition and crystallisation were apparent in the DSC thermogram and the dielectric and the dynamic mechanical spectra of mixtures of amorphous and crystalline xylitol.

Stability-related transitions of amorphous foods

Abstract The processing and storage of foods result in changes due to physical processes and chemical reactions. The rates of these changes depend on many factors, but the mobility of the reactants and of other significant food components is often a major factor controlling these rates. Recently, it has been recognized that the glass transitions of food components are important in controlling mobility. This recognition led to the development of “state diagrams” in which glass transition temperatures (Tg) of major food constituents provide a boundary between regions of low mobility (glasses) and increasing mobility (“rubbers”). This approach has been particularly useful in predicting rates of shrinkage, recrystallization, textural changes and volatile (flavor) loss in carbohydrate-based foods. Work on chemical reactions (non-enzymic browning and lipid oxidation) has also demonstrated the profound effect of glass transitions on the rates of these reactions. The common unifying factor is the effect of glass transition on the diffusivities of important reactants in the materials which undergo the transition.

The physical state of amorphous corn starch and its impact on crystallization

Abstract Crystallization of amorphous polymers is affected by their physical state and molecular mobility. In the present study, crystallization behaviour of amorphous corn starch was related to its physical state and glass transition. Amorphous corn starch was produced by freeze-drying a gelatinized 5% (w/w) starch suspension. State diagram and sorption properties were determined to characterize the material. Amorphous corn starch samples containing 60, 70, and 80% solids were stored at various temperatures that gave various temperature differences between storage temperature and glass transition temperature ( T − T g ). The melting behaviour of crystallites formed was determined using differential scanning calorimetry. Crystallization in starch samples occurred with a rate that was dependent on storage temperature, water content, and T − T g . In addition, the melting temperature and the extent of crystallization were affected by storage temperature, water content, and T − T g . The data reported allow the prediction of crystallization in starch, as a physical state-dependent phenomenon and, therefore, stability of starch-containing products during storage.

Use and application of gelatin as potential biodegradable packaging materials for food products

The manufacture and potential application of biodegradable films for food application has gained increased interest as alternatives to conventional food packaging polymers due to the sustainable nature associated with their availability, broad and abundant source range, compostability, environmentally-friendly image, compatibility with foodstuffs and food application, etc. Gelatin is one such material and is a unique and popularly used hydrocolloid by the food industry today due to its inherent characteristics, thereby potentially offering a wide range of further and unique industrial applications. Gelatin from different sources have different physical and chemical properties as they contain different amino acid contents which are responsible for the varying characteristics observed upon utilization in food systems and when being utilized more specifically, in the manufacture of films. Packaging films can be successfully produced from all gelatin sources and the behaviour and characteristics of gelatin-based films can be altered through the incorporation of other food ingredients to produce composite films possessing enhanced physical and mechanical properties. This review will present the current situation with respect to gelatin usage as a packaging source material and the challenges that remain in order to move the manufacture of gelatin-based films nearer to commercial reality.

Factors affecting crystallization and crystallization kinetics in amorphous corn starch

Abstract The levelling-off extent of crystallization and crystallization behaviour in corn starch were studied using an X-ray diffraction technique, taking into account the glass transition temperature range of amorphous corn starch. Amorphous corn starch samples at 60, 70, or 80% solids were stored at various temperatures giving various temperature differences between storage temperature and glass transition temperature ( T − T g ). Corn starch was observed to crystallize into the same crystal form, independent of water content and storage temperature and, therefore, of the T − T g . The Avrami equation was found to be useful in modelling of crystallization kinetics in starch. The data obtained can be used in prediction of stability during storage of starch-containing products with various water contents.