Spray Dryer
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Transcript of Spray Dryer
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STICKINESS DURING SPRAY DRYING
Dr Bhesh BhandariSPRAY DRYING RESEARCH GROUPSchool of Land and Food Sciences &
School of Engineering The University of Queensland
AUSTRALIA
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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
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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..)
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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
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A typical two-stage
spray dryer
Source: Dairy Processing Handbook. Published by Tetra Pak Processing Systems AB, S-221 86 Lund, Sweden. pg. 369.
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Source: Dairy Processing Handbook. Published by Tetra Pak Processing Systems AB, S-221 86 Lund, Sweden. pg. 370.
FILTERMAT DRYER
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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
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Stickyproduct
Hot air
Non-sticky product
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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 (DE>30)– Pure sugars- glucose, sucrose, fructose– High acid foods
• High fat foods
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Major factors causing stickiness
• High hygroscopicity
• High solubility
• Low melting point temperature
• Low glass transition temperature
(related to thermoplasticity)
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Glass Transition Approach
•Recent approach to describe stickiness
•Applied to spray drying
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Physical properties of sugars and stickiness
behaviour
Sugars Hygroscopicity Melting point Approx solubility
in H20
Tg Stickiness
(relative) (oC) 60oC (%,w/w) (oC) (relative)
Lactose + 223 35 101 +
Maltose ++ 165 52 87 ++
Sucrose +++ 186 71 62 +++
Glucose +++++ 146 72 31 +++++
Fructose ++++++ 105 89 5 ++++++
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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
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Semi-crystalline solid Liquid solution
GrindingExtrusion cookingThermal melting & cooling
Rapid waterremoval- drying
Rapid cooling below Tg
water <-135oChoney <-45oC
Amorphous solid(glass)
Crystalline solid
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Solid Liquid
Glass transition
Stickiness
Property of an amorphous solid
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_______________________________________________________Food materials Tg (
oC)abc
_______________________________________________________Fructose 14Glucose 31Galactose 32Sucrose 62Maltose 87Lactose 101Citric acid 6Tartaric acid 18Malic acid -21Lactic acid -60Maltodextrins
DEd 36 (MW=550) 100 DE 25 (MW=720) 121DE 20 (MW=900) 141DE 10 (MW=1800) 160DE 5 (MW=3600) 188
Starch 243e
Ice-cream f -34.3Honey g -42 to -51
Glass transition temperature of various food materials
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General concepts• Product above glass transition temperature (Tg) exhibits
stickiness
• Shorter chain molecules- low glass transition temperature• Tg of monosaccharides<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
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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
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Spray drying of honey
• Honey compositionGlucose
Fructose
(Sucrose, Maltose)
• Impossible to spray dry due to low Tg (<20oC)
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Spray drying of honey
Tg curveStickiness curve
Moisture
Tg
Drying
10-20oC
Particle temperature
20oC
Stickiness curve
50oC
Tg curve after maltodextrin addition
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Spray drying of whey
• Whey contains lactose
• Lactose Tg is sufficiently high (101oC)
• Not difficult to spray dry
• Hygroscopic- crystallisation- caking problem during storage
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Spray drying of whey
Moisture
Tg
Drying
Particle temperature
Stickiness curve
101oC
Tg curve
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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
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Spray drying of hydrolised whey
Moisture
Tg
Particle temperature
101oC
Tg curve- lactose32oC
Tg curve- hydrolysed lactose
Hydrolysis
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Empirical approach- Index method
• Index assigned for each components of food (Tin/Tout=160oC/60oC)
0
1
Difficult to dryEasy to dry
(+1)
Maltodextrin lactose maltose sucrose glucose fructose citric acid
Possible to dry
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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
a X Yi
n
ii 1
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Predicted and experimental determined
recoveries for model mixtures
Weight fraction
Source Sucrose Glucose Fructose Malto-
dextrin
Overall
index (Y)
Recovery
(%)
Experimental 0.20 0.2 0.2 0.4 0.97 28
0.183 0.183 0.183 0.450 1.02 56
Predicted 0.188 0.188 0.188 0.435 1.00 50
Experimental 0.34 0.34 0 0.32 0.98 25
0.315 0.315 0 0.370 1.02 51
Predicted 0.327 0.327 0 0.347 1.00 50
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Weighted average drying index values for honey and pineapple juice
Honey Pineapple juice
Components* Drying index (ai)
Wt. fraction (Xi)
ai Xi Wt. fraction (Xi)
ai Xi
Fructose 0.27 0.553 0.149 0.210 0.057
Glucose 0.51 0.414 0.211 0.320 0.163
Maltose** 1.00 0.034 0.034 - -
Sucrose 0.85 0.002 0.002 0.440 0.374
Citric acid -0.40 - - 0.035 -0.014
Weighted average index (ai Xi) 0.396 0.580
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Honey:Malto* Overall index
Recovery %
Pineapple: Malto*
Overall index
Recovery %
0.47:0.53 1.03 56.5 0.50:0.50 1.09 58.5
0.50:0.50 1.00 55.0 0.59:0.41 1.00 50.0**
0.53:0.47 0.96 48.0 0.60:0.40 0.99 53.0
0.55:0.45 0.94 20.3 0.75:0.25 0.84 45.0
Experimental recoveries during the spray drying of honey and pineapple juice at various proportions with maltodextrin
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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