Tussen hete lucht en vriesdrogen:

optimalisatie naar product kwaliteit

HighTech meets Agro – Droogtechnieken 11 Mei 2016

Erik Esveld

Wageningen Food & Biobased Research

• Contract Research, University, Top Institute Food & Nutrition,

• Engineering new food processes

for conservation, drying and structuring

• R&D Focus at process – product interaction

• Modelling (heat and mass transfer – reactions – physical transformations)

• Experimenting

Erik.Esveld@wur.nl

Scientist Food Process Engineering

Drying mild and functional

Focus on process-quality relationships during drying

• Minimalization of thermal impact

● Fast or Cold

Structure formation

● Expansion / Porosity control

● Agglomeration

● Rehydration / Instantiation properties

Flavour / colour

● Formation e.g. Maillard reaction

● Encapsulation - Flavour retention

Pasteurization

Keep the flavour and texture

Look for intermediate methods and processes

Very low wet bulb temperature drying

● Dry air or cold product

Remove heat transfer barrier with MW

Remove mass transfer barrier

● Vacuum

● Freeze drying – Frozen Porous product

Freeze drying

Best quality

Slow

Expensive

High temp. water removal

Fast and cheap

Chemical degradation of product

Loss of volatiles

Reduction of shrinkage with microwave

Hot air drying CHEAPFreeze Drying EXPENSIVE

Microwave - Air Microwave Vacuum

Dryer classification on different criteria

Criterion Types

mode of operation batch, continuous

physical form of feed liquid, paste-like, powder, chips, continuous

sheet,

state of material in the dryer stationary, moving, agitated, vibrated,

converged, dispersed, fluidised

heat input convection, conduction, radiation,

electromagnetic field, combination of various

modes, adiabatic/non-adiabatic

operating pressure vacuum, atmospheric, high pressure

drying medium hot-air, superheated steam, flue gases

drying temperature below triple point, below boiling point, at boiling

point

time variation of heat input continuous, intermittent

number of stages single, multiple

Quality changes in foods during drying

Physical Chemical Biochemical

shrinkage loss of chemical activity degradation of cellular structure and biomolecules

loss of density decomposition of some chemical constituents

oxidation of liquids

alteration of shape, size, porosity

denaturation of proteins

crystallisation enzymatic browning

change of solubility Maillard reaction

loss of aroma, organoleptic properties

loss of vitamins

reduced rehydration

Relative rates for quality deteriorative reactions

Water-activity determines:

● germination of spores

● growth of micro-organisms

● prevention of all kind of

chemical and biochemical

reactions

microbial decay aw > 0.7

oxidation, non enzymatic

browning & enzymatic

reactions are prevailing at

lower aw

Normal approach for the development of a drying process

Specify design problem

Collect data on process/feed/product

quality requirements etc.,

select suitable dryer types

Obtain sorption & drying kinetics data

Heat/mass balances/drying times;scale up

Experience, tests

Dimension dryer

Estimate costs

Evaluate safety aspects

Pilot tests

Literature and/or tests

Product related

Product related

Dryer related

Material characterisation during drying

0 2 4 6 8 10

0

0.2

0.4

0.6

0.8

1

Shrinkage c

oeff

icie

nt

microwave convective

Moisture content (g H2O/g d.m.)

0.7

0 2 4 6 8 10 12

0

0.1

0.2

0.3

0.4

0.5

0.6

Moisture content (g H2O/g d.m.)

Open p

ore

poro

sity (

-)

microwave convective

Rehydration time (min)

0 10 20 30 40 50

0

1

2

3

4

5

Mois

ture

conte

nt

(gm

ois

ture

/g d

.m.)

microwave convective

Shrinkage Open-pore porosity Rehydration

hot airmicrowave-vacuum

SEM pictures of dried vegetable

material

fresh mushroom

highly porous porous non-porous

WUR X-ray tomography facility

CAT-AgroFood microCT centre starting 2013 at FBR

Assessable to all interested users

Two X-ray sources to have both:

High Resolution 600 nm

Large Sample Size 30 cm

Root system of rice plant

Internal effects with MW drying

Porosity Example Mechanism Effect

Very open Sugar cube Internal heat internal drying

No drying front High drying rate

Moderate BakingWood

Positive temperaturegradient drives liquid and vapor outwards

Less pore collapseLonger CRPShorter FRP

Closed surface

Pre-driedvegetables

Strong internal pressure

Expansion

Not al all Gum Just heatingInternal diffusion

Only startup enhancement

Addition of microwave field for expansion

Additional microwave heating

Larger heat transfer

Faster expansion

Lower air temperature

Results in

Improved expansion

Extended product range

● Less starch content

● More sugar content

● Larger Volumes

Elimination of post drying

Microwave addition for hot air expansion

Volume expansion increases with microwave power density

More vapour pressure

Bulk volume nearly double

● Compared to hot air alone

Other, less obvious products can be

expanded or puffed as well

Vegetables pieces

Fruits pieces

High proteins products

Microgolf-vacuum drying

Slow conductive heat transfer

● Heat from microwaves

Pressure = vapor pressure

● Drying temperature set

● No external mass transport

limitation

● Easy creation of internal gas voids

Microwave heat = evaporation

● No microwave, no drying

● At end point no air cooling

Vapor convectionMicrowave Vapor convectionMicrowave

Microwave

Vapor diffusion

Heat diffusionMicrowave

Vapor diffusion

Heat diffusion

Vapor diffusion

Heat diffusion

Vapor diffusion

Heat diffusion

Microwave vacuum drying

Microwave air drying

Air drying

https://vimeo.com/user38515234/enwave

Microwave Air - Drying

Wet material is above wet bulb temperature

Can even be warmer than surrounding air

Drying continues in saturated air

Microwave heated particles work as ‘in situ heater’

At end point less microwave absorption

But not zero, air works as coolant

And distributes the heat

Vapor convectionMicrowave Vapor convectionMicrowave

Microwave

Vapor diffusion

Heat diffusionMicrowave

Vapor diffusion

Heat diffusion

Vapor diffusion

Heat diffusion

Vapor diffusion

Heat diffusion

Microwave vacuum drying

Microwave air drying

Air drying

Example: MW drying of sugar cubes

Slow IR heating replaced by microwaves

Much faster reduced floor space

Better control of cube hardness

Reduced peak electricity use

GEA: Elba sugar cubing line from Aquarius

Energy reduction with MW-RF drying ?

Same evaporation heat needs to be supplied:

From electricity with ~ 60% efficiency.

No primairy energy reduction with just replacement of combustion based indirect heating.

But the process will be intensified:

Compact / fast Minimised heat loss to surrounding

Inhanced internal transport CRP to lower MC

Higher T and RH of exhaust air better heat recovery

Literature claims for drying are 30-50% efficiency increase

In-line monitoring of moisture

Moisture content : Numbers tell the tale

● Drying process ? End MC key parameter

● Product quality depends or moisture content range

● (Half) product performance

● Microbial stability

● Process performance – Heterogeneity – Over processing

● Variable feed Process control

● Moisture content is a commercial specification

Why measure in-line ?

● Fast continuous monitoring as opposed on off-line sampling and analysis

● Continuous quality monitoring

● Understanding of the process variability

● Direct feedback to process control

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Tales: In-line monitoring of moisture critical

Agrifirm Cattle Feed : pellets via extrusion and dryer:

● MC content range must be controlled with ± 0.5%

● Solution: microwave moisture measurement, NIR did not work due to large

recipe variation.

2011 DDS Nutreco : Fish Feed MC critical for optimal buoyancy

● “Online NIR measurement proved to be a wise investment”

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Technologies for moisture measurement

Air humidity

Equilibration temperature

● Psychrometer wet and dry bulb temp.

● Dew point Surface condensation

X LiCl sensor Temp for which RH = 11%

Material change via absorption

● Hygroscopic Polymers

● Sorption relates to RH

● Porous Metal Oxide sensors

● Relates to vapour pressure

Direct physical determination

● Acoustic velocity

● IR laser absorption

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Product moisture content

In-line spectroscopic methods

● Near IR reflectance

● HF capacitance

● Microwave resonator

● Microwave transmission

X Low resolution NMR

X Neutron scatter

Reference laboratory method

X Gravimetric (drying)

● Stove, Microwave, IR

X Karl Fisher Titration

X Spectroscopic laboratory

Product moisture content

In-line moisture measurement of the product is far more challenging.

And far more relevant with respect to feed-back control to the drying process

For choice of technology the product morphology and transport is leading

● Powder - fluidized or on a belt

● Sheets , board - with systematic variation over the width

● Granular, flakes - free flowing in duct or on a conveyor

● Fibres, Tubular, Sticks

● Boxes bundles - One by one

In-line measurement always needs product specific calibration.

● Absolute accuracy is ultimately limited by off-line (gravimetric) moisture determination.

● All technology suppliers claim that their in-line technology shows less variance than the control measurement.

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NIR reflectance for moisture measurement

NIR spectrometers can utilise whole

spectrum to distinguish

● Moisture, Fat , Proteins etc.

● Requires extensive PLS calibration

● Expensive, laborious

For moisture determination only few

selected bands are measured (simpler)

● Water (1940nm or 1420nm) with respect Background (e.g. 1800 and 2200)

● Rotating filter wheel

● Up to 40 cm from product layer

● Can measure though sapphire window

● Fluidized bed

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NIR reflection calibration and application

Reference signal corrects for

reflection intensity differences

Moisture calibration is required for

● Variation in fat, protein, starch

● Colour differences are problematic

Achievable accuracy 0.2 – 2 %MC

● related to calibration range

Application area’s include

● Paper, cardboard

● Powders, Flakes, Noodles

● Mud, Clay, Sand, Woodchips

Investment ~ 15-20 k€

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Bakers

yeast

calibra

tion

Application o

n C

risps

Dielectric sensing of product moisture content

Water has a high (80) dielectric constant in contrast to dry matter (4-10)

● Dielectric polarisation of matter strongly influenced by moisture content

● Also energy loss due to dipolar losses (GHz)and ionic conduction (MHz)

Electric field probes many centimeters in the product

● Depending on construction of the sensor

● Depending on moisture (max 25%) and wavelength ( 3-5 cm@3GHz / 1m@1MHz )

● Wavelength must be larger than particle size to prevent internal resonance.

Each product composition needs separate calibration

due to differences in

● Dielectric constant dry matter (Take care : TiO2 , Organic solvents)

● Water binding / ionic conductivity influence dielectric response versus MC

● Air content (Porosity - Bulk density - Packing - Compression)

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O

H

H

Moisture content in granular flows

Low frequency (1MHz) method for using fin electrode

● Mounted in by-pass chute ( < 2.5 cm/s)

● Temperature correction (they all do)

● No density correction (2% of range)

● Used extensively grain dryersbut also soybeans, feed, etc.

● Cheap 3k€ including control

Microwave Hydro-probe

● Mounted in product flow

● Or stirred trough product

● Or mounted at the wall

● 25 measurements / second (all do)

● No density correction

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Moisture content of powders in dryer

High frequency capacitance measurement with automatic sampling cup

● Specially design for in fluidised bed

● Drying - Granulation

● Temperature up to 120°C

● No density correction

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Microwave resonance sensors

Planar or tubular -> close contact with material under measurement

● Material influences coupling between emitting and probing microwave resonators

Notably TEWS reports a large variety of applications and setups.

● Wheat Flour, Coffee, Herbs, Noodles

● Board , Fibers, Pellets, Tobacco

● Washing powder, Tablets, Capsules

● Paper, Tissue, Yarn, Adhesive films

Density corrected, Tmax 125°C

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Vibroconveyor

Slidingblock above

conveyor belt

Conveyortransfer point

BypassDrop shaft,

silo, chute

Fork sensorover web

Moisture of inhomogeneous media

Measurement of inhomogeneous, moist bulk goods is challenging

● Peat, clay, sand, bark, broken wood, grid, compost

● Moisture content per mass and density must be known for commercial specification

● Packing / Density variable and not linear with moisture content.

Solution INADCO:

● Take a sample of known volume

● Measure mass

● Measure volumetric moisture content

Also as weight belt integration with laser volume detection

3x Innovator award

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Conclusions overview in-line moisture

measurement techniques

For air moisture measurement:

Huge variation in operational principle and complexity

Measuring in hot moist industrial air dryer environment requires either high temperature sensor or a good (low pressure) sampling system

With regard to in-line in product measurement

Choice between NIR or Dielectric is a choice between non-contact surface or close-by volumetric measurement

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● NIR moisture sensors from different suppliers seem all similar in operation

● Price 15-20k€

● Versatile but with limited accuracy due to principle

● Sensitive to composition and colour variation

● Microwave sensors come in many geometrical variations for different materials and processes

● Price 3-30k€

● Density variations

● Prevent them (?)

● Correct them (expensive)

● Measure it (discontinuous)

Further information

Contact

Erik EsveldWageningen UR Food & Biobased ResearchT: + 31 317 480125E: erik.esveld@wur.nl

Website NWGD

Participation‘Grip op Drogers’

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