STICKINESS DURING SPRAY DRYING

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STICKINESS DURING SPRAY DRYING

• Dr Bhesh Bhandari
• SPRAY DRYING RESEARCH GROUP
• School of Land and Food Sciences
• School of Engineering
• The University of Queensland
• AUSTRALIA

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Spray Drying Research Group
Current research activities

• 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

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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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FILTERMAT DRYER
Source Dairy Processing Handbook. Published by
Tetra Pak Processing Systems AB, S-221 86 Lund,
Sweden. pg. 370.
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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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Hot air
Sticky product
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 (DEgt30)
• 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
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What is a glass transition?
Physical states of dried solid materials

• Amorphous
• non-aligned molecular structure
• very hygroscopic
• go through glass transition
• predominant in dried food
• Crystalline
• aligned molecular structure
• non hygroscopic
• no glass transition

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Liquid solution
Crystalline solid

Semi-crystalline solid
Grinding
Rapid water
Rapid cooling below
Tg
Extrusion cooking
removal- drying
water lt-135oC honey lt-45oC
Thermal melting cooling
Amorphous solid (glass)
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Property of an amorphous solid
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Glass transition temperature of various food
materials
__________________________________________________
_____
o
abc
Food materials
T
(
C
)
g
__________________________________________________
_____
Fructose

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Glucose
31
Galactose
32
Sucrose
62
Maltose
87
Lactose
101
Citric acid
6
Tartaric acid
18
Malic acid
-21
Lactic acid
-60
Maltodextrins
d
DE
36
(MW550)
100
DE 25 (MW720)
121
DE 20 (MW900)
141
DE 10 (MW1800)
160
DE 5 (MW3600)
188
e
Starch
243
f
Ice-cream
-34.3
g
Honey
-42 to -51
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General concepts

• Product above glass transition temperature (Tg)
  exhibits stickiness
• Shorter chain molecules- low glass transition
  temperature
• Tg of monosaccharidesltTg 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 composition
• Glucose
• Fructose
• (Sucrose, Maltose)
• Impossible to spray dry due to low Tg (lt20oC)

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Spray drying of honey
50oC
Tg
20oC
Tg curve
Moisture
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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
101oC
Tg
Tg curve
Moisture
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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 (Tg31oC) galactose (Tg32oC)
  
• Difficult to spray dry

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Spray drying of hydrolised whey
101oC
Tg
Tg curve- lactose
32oC
Hydrolysis
Tg curve- hydrolysed lactose
Moisture
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Empirical approach- Index method

• Index assigned for each components of food
• (Tin/Tout160oC/60oC)

Possible to dry
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Overall index value to determine drying aid

• Xifractional weight of a component i (eg
  maltodextrin sucrose, glucose..)
• aiindex value assigned for that particular
  component and
• Y overall index

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Predicted and experimental determined recoveries
for model mixtures
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Weighted average drying index values for honey
and pineapple juice
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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