Stickiness of food powders has been an issue in the food industry, especially when products contain high sugar content. Characterization of the stickiness behaviour in food powders provides useful information for designing processing, transport, handling and storage operations in order to minimize the effect of stickiness during powder production. There are existing techniques for this purpose, however several disadvantages have been reported. Therefore, it was the objective of this study to develop new and simple instrumental techniques which can measure and simulate the stickiness behaviour of powders in real situations.
Two new instrumental techniques for measurement of stickiness in food powders have been developed. These were a mechanical technique for measuring the surface stickiness of powders based on mechanical changes due to glass-rubber transition, and a dynamic technique for measuring particles' cohesive and adhesive behaviours in a cyclone of controlled temperature and humidity.
The Thermal Mechanical Compression Test (TMCT) device consisting of a thermally controlled sample cell attached to a texture analyser was developed and tested. This cell allowed heating from ambient temperatures to 300°C. It was designed to measure a glass-rubber transition based on mechanical property. The TMCT was carried out in creep mode where the sample was subjected to a constant compression force and probe displacement was measured as a function of temperature. The displacement versus temperature data was plotted and the glass-rubber transition temperature (Tgg-r) was determined by linear regression in the region of increasing displacement.
The influences of compression, heating rate and sample size on the Tg-r were investigated using compression force, heating rate and sample size ranges of 9.81-49.05 N, 5-50°C/min and 0.5-2.0 g, respectively. Standard procedure for the Tg-r analysis was established with the use of maltodextrin as a standard material and skim milk powder (3.06% moisture dry basis) was used as a model for food powders. There was no significant difference in the thermal expansion, hence the Tg-r, values, obtained using the compression forces between 19.62-49.05 N and heating rates between 20- 50°C/min. Sample sizes between 0.5-1.0 g gave the results closest to the Tg, determined by DSC. These test conditions were recommended for the Tg-r analysis of food powders.
The TMCT technique was validated and compared with the standard DSC and DMTA/TMA techniques. The skim milk powder having moisture content in the range of 2 to 5% dry basis were analysed using all three techniques. The Tg-r values were close to the Tg obtained by DSC while the Tonset obtained by DMT /TMA was much higher. Therefore, the TMCT technique was valid for prediction of the glass-rubber transition analysis of food powders.
A Cyclone Stickiness Test (CST) device was designed to measure the surface stickiness of powder particles in a vortex air stream, simulating real situations during spray drying and pneumatic transport of powders. It consists of an air heater, spraying chamber and a cyclone test chamber. Humidified air at different temperatures and relative humidity was generated by spraying of water (0-0.162 g/s) into heated air stream (120-300°C and total air flow rate of 0.00608 m3/s) in the spraying chamber. Particle cohesion and adhesion were observed when the powder particles found to be in a sticky condition inside the cyclone test chamber. Surface moisture contents of the particle at the sticky conditions were determined using the BET model with adsorption data obtained at 20- 50oC. The sticky temperatures were compared to Tg, measured by DSC. The experimental sticky temperatures were found at 11.4 oC (on average) above the Tg, compared to the literature value of 23.3°C above the Tg for skim milk powder.
Both TMCT and CST techniques were validated for characterization of stickiness behaviour of whey, honey and fruit juice powders. The Tg-rfor of whey powder having various moisture contents were analysed and compared with the Tg measured by DSC. The Tg-r for the original spray dried honey and fruit juice powders were analysed by TMCT and compared to the Tg values. The equilibrium surface moisture was determined using the BET model for whey powder and the Oswin equation for honey and whey powders based on the sorption isotherm conducted at 20 to 50°C. Tg of the samples obtained from the sorption isotherm experiment were analysed using a DSC. The sticky points of those three powder samples were determined and the Tg at equilibrium surface moisture content was predicted using the Gordon-Taylor equation. The powders were found to exhibit both cohesive and adhesive behaviour at the sticky points and became sticky at lower RH when tested at higher temperature. The sticky RH values for honey and apple juice powder were much lower than those for whey powder due to their higher sugar contents. The results were compared to those obtained by DSC and DMTA/TMA and literature values.
The TMCT technique was also applied for the melting point analysis of sucrose and glucose crystals and the results were not significantly different from the values obtained by the standard DSC and DMTA/TMA techniques.
This study showed that the newly developed techniques (TMCT and CST) were valid for characterization of stickiness in food powders and they have a potential of characterization of a wide range of food powders.