Tony Howes

Implication of glass transition for the drying and stability of dried foods

Dry products obtained from most of the common drying processes are predominantly in a glassy amorphous form. The mobility of the solid matrix in this state is highly limited. For the product to be stable for long periods of storage, this physical state should not alter with time. When the temperature is above glass transition temperature (T-g) an amorphous solid exists in a rubbery state. In this state, the molecular mobility of the matrix and the reactants are accelerated, which results in an increased rate of physicochemical changes in dried products, such as sticking, collapse, caking, agglomeration, crystallisation, loss of volatiles, browning and oxidation. These changes play an important role in the ability to both process and store dried food products. This paper reviews the importance of T-g in relation to drying process and dried foods

Problems associated with spray drying of sugar rich foods

ABSTRACT Stickiness is a major reason that limits the spray drying of various sugar-rich food products.Higher hygrmopicity of amorphous powder, increase in solubility of sugars with temperature, and lower melting point and glass transition temperature, contribute to the aickiness problem. So far, the glass transition temperature has been widely accepted as a ben indicator for stickiness. There are various manawm that have been applied to spray dry such product. Some of them are the addition of drying aids, modilication of drier design and use of mild drying temperature conditions. This review paper highlights the major research works that deal with the stickiness property of sugar-rich foods.

STICKINESS IN FOODS: A REVIEW OF MECHANISMS AND TEST METHODS

Problems associated with the stickiness of food in processing and storage practices along with its causative factors are outlined. Fundamental mechanisms that explain why and how food products become sticky are discussed. Methods currently in use for characterizing and overcoming stickiness problems in food processing and storage operations are described. The use of glass transition temperature-based model, which provides a rational basis for understanding and characterizing the stickiness of many food products, is highlighted.

Hydrocolloid Gel Particles: Formation, Characterization, and Application

Hydrocolloid gel particles of micron and sub-micron size are particularly attractive for use in many applications in the food, agricultural, pharmaceutical, and chemical industries, due to their biocompatibility, perception as “natural” materials, and soft-solid texture. Industrial applications for such particles include uses as texturizers in confectionery and cosmetic products, slow-release encapsulation agents for flavors, nutrients, and pharmaceutical products, and thickeners in soups and sauces. Properties such as particle size, hardness, shape, texture, and molecular release rates can be important for individual applications. In addition, product formats will determine specific needs for physical form (e.g. dry or wet) and compatibility with other components. The diverse range of potential applications for hydrocolloid gel particles provide a driver for understanding-led tailoring of raw material and process conditions. This review introduces some of the materials that are used to form hydrocolloid gel particles and the corresponding gel formation mechanisms. One issue of importance in the production of hydrocolloid gel particles is the control of particle properties, such as release profiles, strength, and detectability within products. An alternative technique to traditional methods of hydrocolloid gel particle production is evaluated and a model for control of particle size, and subsequently other particle properties, is proposed. Key properties of hydrocolloid gel particles are identified and characterization methods for evaluating these properties are described.

A SEMI-EMPIRICAL APPROACH TO OPTIMISE THE QUANTITY OF DRYING AIDS REQUIRED TO SPRAY DRY SUGAR-RICH FOODS

ABSTRACT A semi-empirical linear equation has been developed to optimise the amount of maltodextrin additive (DE 6) required to successfully spray dry a sugar-rich product on the basis of its composition. Based on spray drying experiments, drying index values for individual sugars (sucrose, glucose, fructose) and citric acid were determined, and using these index values an equation for model mixtures of these components was established. This equation has been tested with two sugar-rich natural products, pineapple juice and honey. The relationship was found to be valid for these products.

Glass Transition Behavior of Spray Dried Orange Juice Powder Measured by Differential Scanning Calorimetry (DSC) and Thermal Mechanical Compression Test (TMCT)

Spray drying behavior of orange juice concentrate with various levels of maltodextrin (DE 6) was studied. Five combinations of orange juice concentrate and maltodextrin (25:75, 30:70, 35:65, 40:60, and 50:50) were spray dried at 160 and 65°C inlet and outlet temperatures, respectively. The product recovered with 50% maltodextrin concentration was sticky and only 20% powder was recovered. The recovery of orange juice powder increased as the amount of maltodextrin in powders increased. The particle size and bulk density remained almost the same in all except in 50% maltodextrin powder which was slightly larger and more dense. The moisture content of spray dried powders was high and desiccated before measuring glass transition temperature. The anhydrous spray dried powders showed increased Tg values with increasing maltodextrin concentration, from 66°C in 50% maltodextrin to 97°C in 75% maltodextrin containing powders. The glass rubber transition (Tg-r) values of all the products measured using novel Thermal Mechanical Compression Test (TMCT) were higher than Tg values measured by DSC; the difference in values increased with increase in maltodextrin concentration.

The simulation of chaotic mixing and dispersion for periodic flows in baffled channels

Abstract We report numerically generated flow visualisation simulations for the flow of an incompressible Newtonian fluid within a two-dimensional channel which can contain periodic baffles. For unsteady flows in this geometry a regime of chaotic advection is observed when baffles are present. The unsteadiness takes one of two forms: a “natural” unsteadiness caused by a symmetry breaking instability of the flow, or a “forced” unsteadiness generated by applying an oscillatory component to the flow. This chaotic advection is shown to provide an efficient mixing mechanism and has a number of applications in the process industry. Enhanced transverse mixing is observed which results in increased transfer properties, reduced fouling rates and, in some circumstances, a reduction in axial dispersion, as recently experimentally reported in the literature.

Surface Stickiness of Drops of Carbohydrate and Organic Acid Solutions During Convective Drying: Experiments and Modeling

Abstract Drying kinetics of low molecular weight sugars such as fructose, glucose, sucrose and organic acid such as citric acid and high molecular weight carbohydrate such as maltodextrin (DE 6) were determined experimentally using single drop drying experiments as well as predicted numerically by solving the mass and heat transfer equations. The predicted moisture and temperature histories agreed with the experimental ones within 6% average relative (absolute) error and average difference of ± 1°C, respectively. The stickiness histories of these drops were determined experimentally and predicted numerically based on the glass transition temperature (T g ) of surface layer. The model predicted the experimental observations with good accuracy. A nonsticky regime for these materials during spray drying is proposed by simulating a drop, initially 120 µm in diameter, in a spray drying environment.

Relating the Stickiness Property of Foods Undergoing Drying and Dried Products to their Surface Energetics

Abstract Stickiness is a common problem encountered in food handling and processing, and also during consumption. Stickiness is observed as adhesion of the food to processing equipment surfaces or cohesion within the food particulate or mass. An important operation where this undesirable behavior of food is manifested is drying. This occurs particularly during drying of high-sugar and high-fat foods. To date, the stickiness of foods during drying or dried powder has been investigated in relation to their viscous and glass transition properties. The importance of contact surface energy of the equipment has been ignored in many analyses, despite the fact that some drying operations have reported using low-energy contact surfaces in drying equipment to avoid the problems caused by stickiness. This review discusses the fundamentals of adhesion and cohesion mechanisms and relates these phenomena to drying and dried products.

Evaluating strategies for overcoming overheating problems during solid-state fermentation in packed bed bioreactors

Normal operation of packed bed bioreactors used for solid-state fermentation involves a static bed aerated from the bottom throughout the fermentation. This leads to axial temperature profiles with the highest temperature, sometimes over 20 degrees C higher than the inlet air temperature, occurring at the top of the bed. An axial heat transfer model was used to explore two strategies designed to prevent the temperature reaching undesirable levels, namely periodic reversal of the direction of airflow, and periodic mixing. Simulations were done for the growth of Aspergillus niger on a starchy substrate. The bed was assumed to be wide enough such that the radial heat transfer could be ignored. With hourly air reversal or mixing events, the maximum temperature predicted during the fermentation is higher than the maximum temperature predicted for normal operation. Increasing the frequency of air reversals leads to temperatures close to the maximum for growth in the middle of the bed. However, for a 0.345 m high bed and a superficial air velocity of 0.0236 m s(-1), mixing 10-60 times per hour can lead to lower maximum temperatures in the column compared to normal operation, because the frequent mixing distributes the benefits of the effective cooling at the bottom of the column throughout the bed. The degree of reduction in maximum temperature increases with increasing specific growth rate of the microorganism. Modelling is a useful tool in guiding experimental programs for the design and scale-up of bioreactors for solid-state fermentation, since it can be used to identify promising strategies and eliminate unfruitful strategies