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STICKINESS DURING SPRAY DRYING Dr Bhesh Bhandari SPRAY DRYING RESEARCH GROUP School of Land and Food Sciences andamp; School of Engineering The University of Queensland AUSTRALIA

Spray Drying Research Group: 

Spray Drying Research Group Prediction of glass transition temperature of model mixtures- relevant to sugar-rich foods such as fruit juice, honey Design of static and dynamic stickiness testing devices for food powders In-situ stickiness measurement of droplets Drying kinetics and dryer design for sticky materials Current research activities

My other research activities: 

My other research activities Structural relaxation of dried food materials Application of ultrasound in food processing- meat tenderisation, homogenisation, encapsulation Development of microencapsulation process for food flavours, probiotics, vitamins… Water activity prediction (flavour powders, IMF) Extrusion and stability of microencapsulated flavours Ultrasound spectroscopy in non-invasive characterisation of food materials (gelation, composition, texture etc..)

Spray drying: 

Spray drying Most common process to convert liquid to solid Large throughput- capacity several tonnes per hour- (15 tonnes per hour- New Zealand) Produce free flowing, fine to granulated powders Low thermal effect on materials during drying Versatile in use- ceramic or milk

A typical two-stage spray dryer: 

A typical two-stage spray dryer Source: Dairy Processing Handbook. Published by Tetra Pak Processing Systems AB, S-221 86 Lund, Sweden. pg. 369.


Source: Dairy Processing Handbook. Published by Tetra Pak Processing Systems AB, S-221 86 Lund, Sweden. pg. 370. FILTERMAT DRYER


Stickiness issues during spray drying Stickiness on the drier wall (spray drying) Wet and plastic appearance Agglomeration and clumping in packing container Operational problems Losses


Sticky product Hot air Non-sticky product


Products exhibiting stickiness during drying Products with high amount of sugars or organic acids Fruit juices/pieces/purees/leathers Honey Molasses Whey (acid or sweet) High DE maltodextrins (DEandgt;30) Pure sugars- glucose, sucrose, fructose High acid foods High fat foods


Major factors causing stickiness High hygroscopicity High solubility Low melting point temperature Low glass transition temperature (related to thermoplasticity)


Glass Transition Approach Recent approach to describe stickiness Applied to spray drying


Physical properties of sugars and stickiness behaviour


What is a glass transition? Amorphous non-aligned molecular structure very hygroscopic go through glass transition predominant in dried food Crystalline aligned molecular structure non hygroscopic no glass transition Physical states of dried solid materials


Semi-crystalline solid Liquid solution Grinding Extrusion cooking Thermal melting andamp; cooling Rapid water removal- drying Rapid cooling below Tg water andlt;-135oC honey andlt;-45oC Amorphous solid (glass) Crystalline solid


Property of an amorphous solid


_______________________________________________________ Food materials T g ( o C ) abc _______________________________________________________ Fructose 14 Glucose 31 Galactose 32 Sucrose 62 Maltose 87 Lactose 101 Citric acid 6 Tartaric acid 18 Malic acid -21 Lactic acid -60 Maltodextrins DE d 36 (MW=550) 100 DE 25 (MW=720) 121 DE 20 (MW=900) 141 DE 10 (MW=1800) 160 DE 5 (MW=3600) 188 Starch 243 e Ice-cream f -34.3 Honey g -42 to -51 Glass transition temperature of various food materials


General concepts Product above glass transition temperature (Tg) exhibits stickiness Shorter chain molecules- low glass transition temperature Tg of monosaccharidesandlt;Tg of disaccharides Water depresses the Tg significantly Tg of amorphous solid water is -135oC For a complex food system, Tg is a function of weight fraction of each component and their Tgs’- but the relationship is not linear


Spray drying of sticky product some guideline Drying below the glass transition temperature (often not feasible) Mild drying temperature conditions Increasing the Tg by adding high molecular weight materials (such as maltodextrins)- a predictive approach needed according to the composition Immediate cooling of the product below its Tg Appropriate drier design to suit the sticky product

Spray drying of honey: 

Spray drying of honey Honey composition Glucose Fructose (Sucrose, Maltose) Impossible to spray dry due to low Tg (andlt;20oC)

Spray drying of honey: 

Spray drying of honey Tg curve Moisture Tg 20oC 50oC

Spray drying of whey: 

Spray drying of whey Whey contains lactose Lactose Tg is sufficiently high (101oC) Not difficult to spray dry Hygroscopic- crystallisation- caking problem during storage

Spray drying of whey: 

Spray drying of whey Moisture Tg 101oC Tg curve

Spray drying of acid and hydrolysed whey: 

Spray drying of acid and hydrolysed whey Presence of lactic acid Tg of lactic acid -60oC Dramatic reduction on Tg of whey Problem of stickiness Hydrolysed whey Lactose  glucose (Tg=31oC) + galactose (Tg=32oC) Difficult to spray dry


Spray drying of hydrolised whey Moisture Tg 101oC Tg curve- lactose 32oC Tg curve- hydrolysed lactose Hydrolysis

Empirical approach- Index method: 

Empirical approach- Index method Index assigned for each components of food (Tin/Tout=160oC/60oC) Possible to dry

Overall index value to determine drying aid: 

Overall index value to determine drying aid Xi=fractional weight of a component i (eg maltodextrin sucrose, glucose..) ai=index value assigned for that particular component and Y= overall index

Predicted and experimental determined recoveries for model mixtures : 

Predicted and experimental determined recoveries for model mixtures

Weighted average drying index values for honey and pineapple juice : 

Weighted average drying index values for honey and pineapple juice


Experimental recoveries during the spray drying of honey and pineapple juice at various proportions with maltodextrin


Conclusions Stickiness is related to the material property It can be correlated to glass transition temperature An empirical approach can be used to optimise the processing condition- however the Tg concept can be more appropriate Drying parameters and drier design influence the stickiness property of droplet Further research is needed to correlate the stickiness property with the Tg, drying parameters, drying kinetics, evolution of surface property of droplets

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