DRYING/DEHYDRATION

Why drying/ concentrate food materials?

Preservation (reduce aw)

Obtain specific attributes (flavor, texture etc)

Reduce bulk and transport costs

Enable formulation of specific products

Basic methods of drying

Hot air - convection

Hotsurface - conduction

Radiation energy - MW or dielectric source

Freeze drying - moisture is frozen and sublimed under heat at vacuum

Some definitions

% Moisture content (wet basis): MCwb = Mw/(Ms+ MW) x 100

% Moisture content (dry basis): MCdb= Mw/Ms x 100

Exercises - Moisture content Calculations:

The wet basis moisture content of fresh apple is 85%. What is its dry basis moisture content? How much water must be removed (per kg of fresh apple) to reduce the moisture content to 15% (wet basis)? What is the new dry basis moisture content?

Drying efficiency

Drying efficiency is defined as: Amount of latent heat required to evaporate the water

from the product per unit energy input to achieve the drying (expressed as a percentage)

Drying using heated air

Heated ambient air directs into a product chamber:

Heated air discharges

Moisture evaporates moisture humid air to + Product from the product the atmosphere

The product chamber: cabinet, tunnel, belt or spray dryers

Heat source : Heat exchanger, gas, electricity or solar

Mechanism of drying

Hot air flow: heat is transferred to the food surface

Creates a region of low Pvon surface

High vapor Pv in the centre of the moist food

dPv/dx gradient provides MC driving force

Moist interior of food ( vapor pressure) Surface of food (vapor pressure) Establishes gradient

Water moves to the surface by following mechanisms

I. Capillary flow: Liquid movement by capillary

action

II. Liquid diffusion:Diffusion of liquids due to solutes

concentration gradient in the food

III. Surface diffusion: Adsorption of diffused liquid at

the surface of food

IV. Vapour diffusion:Diffusion of water vapor within

the air space of the food by dPv/dx gradient V. Thermal diffusion: Diffusion of thermal energy into the food

VI. Hydrodynamic flow

Stages of drying

Drying time (h)

Moisture content % (W)

Drying Rate (dw/ dt)

Moisture content % (W)

Initial stage (A-B)

Settling down period: System comes to equilibrium (surface heat up to the wet bulb)

Constant rate period (stage B-C)

The surface of the food remains saturated

Rate of water diffusion from inside = Rate of evaporation

H2O evaporates depends on the rate of heat transfer

Rate of heat transfer is controlled by

Temperature of the air Temperature of the food Velocity of the air

Surface area of the food Surface heat transfer

coefficients

Constant food temperature

Rate of heat transfer = Rate of mass transfer

Rate of drying during the constant rate period is increased by;

Increasing drying air temperature

Decreasing drying air humidity

Increasing drying air velocity

Falling rate period (stage C-D)

Rate of water diffuses < Rate of evaporation

from inside the food from the surface

Dry food Surface Rate of evaporation T of the food rises

Longest part of the drying cycle

Critical moisture content: moisture content at point C,

Transition from constant rate falling rate

Uncontrolled drying Controlled drying

EFFECT OF FOOD PROPERTIES

Food properties effect on the drying process: Food properties changed quality of the final product affect

Factors that influence food quality during drying

Physical Chemical Nutritional Microbial Rehydration Browning Vitamin loss Bacteria

Solubility Lipid oxidation Protein denaturation

Fungi

Texture Color change Yeast

Aroma Gelatinization

Constituent orientation

Foods are heterogeneous

* Water escapesrapidly from muscle fibers than fat The orientation of layers affect on drying rate

* O/W emulsion dries more quickly than W/O emulsion

Solute concentration

High Solutes in solution => high boiling point => concentrate foods dry slowly

Binding of water Free water easy

to remove Water in colloidal

gels difficult to remove

Starch, pectin, gums etc.

Cellular structure

Moisture evaporation depends on cellular structure

Blanched or cooked tissues become more permeable

Cell wall structure

Shrinkage

Cubes become smaller and concave surfaces

In rapidly dried product; Surface becomes dry and rigid

Inside spits, voids, and honeycomb Reduces the bulk density

Textural changes of apple cubes Fresh, Convection drying (CD) and vacuum microwave drying (HPP-VMFD)

Shrinkage of carrot cubes dehydrated by freeze-drying (FD), vacuum-microwave drying (VMD) and convective drying (CD).

Textural effects

Textural loss of blanched food Gelatinization of starch

Crystallization of cellulose Rehydration: slowly absorbs the water and firm

texture

MEATS: Aggregation and denaturation of proteins and

loss WHC => caused toughening

POWDER: Lower the ρ

Structure of beetroot samples dehydrated by freeze-drying (FD), vacuum-microwave drying (VMD) and convective drying (CD).

Case hardening H2O

Drying H2O+solids Solid

Migration H2O + solid Dry food

Water moves to the surface with dissolved solids

On Surface glassy layer “Case hardening”

Case hardening + shrinkage effectsblock pores

low drying

Concentration gradient => interior or surface

Solute migrated to the surface

OR

Solute migrated to the center

Thermo-plasticity

Food make softer by heating – thermoplastic food

i.e.: high in sugar no structure => fruit juices Heat Heat cooling

Sugars soften melt to a sticky crystalline/amorphous glass form

(Brittle and easily removable)

Porosity

Higher porosity food: easier to dry

Porous structures: limited heat transfer due to insulating

Porosity enhances: solubility and increases volume

Disadvantages: increased bulk density and faster degradation

Porosity enhancing: increase PV within the product Whipping or foaming a liquid product

Different porosity and food structure

Color

Changes surface reflectivity =>surface color change

Carotenoid and chlorophyll loss due to oxidation Oxidation and residual enzyme activity: browning

Flavor and aroma

Volatiles loss with drying: Temperature, solids concentration, volatiles Pv and solubility of volatiles

Concentration effects: Increase flavor characteristics dried products >fresh product

Oxidation of lipids in dried milk: rancid flavor Oxidation of unsaturated FAs in fruits & vegetables

produce H2O2 : form off flavor

Oxygen scavengers or inert atmospheres: Other methods to retain flavors:

Recovery and return of volatiles Mixed with flavor fixing compound Addition of enzymes or activation of naturally enzymes

Nutritive value

Solubility of vitamins varies with drying Riboflavin precipitates@ high moisture contents Ascorbic acid remain soluble @ low moisture Losses of the more heat sensitive vitamins Vit C and thiamine

Proteins are generally not affected “but” milk proteins denatured during drum drying Lysine loss in milk powder

Rehydration

Rehydration is not the reverse => Texture change Solute migration Volatile loss

Heat reduces: degree of hydration of starch

Elasticity of cell walls Coagulates proteins to reduce WHC

DEHYDRATION OF FRUITS

Fruit drying:sun drying, Kiln drying and tunnel drying etc.

Fruit powder: Added corn syrups and vacuum or spray drying Eg:

Mango, Jackfruit etc.

Sortedwashpeel trimmed Chemical pretreatment Sulfuring (KMS)

Drying Blanching Acidification (citric acid) Salting (NaCl) etc.

Osmotic dehydration (salt or sugar) Grapes

Lye peeling sulfuring sun-drying Lye peeling tomato, peach, strawberry, potato etc.

1. 75oC-105 oC hot lye solution 2. Conc. 8-15% NaOH/KOH solution (or may use

40% alkaline solution) 3. 2-5 min. 4. Washing in scrubber

Fruit Puree

Pulping pretreatments blanching drum drying

Vegetables Same as fruits

i. Shredded - cabbage Sliced or diced – carrot, beet or potato etc.

ii. Blanched @ hot water 90 oC, steam, MW or IR

1-3 min.: leafy vegetable

2-8 min.: peas, sweet corn, beans

3-6 min.: Potato carrot

iii. Drying temperature:

48-60 oC vegetable fruits

70 oC Corn, beans

iv. Final MC.: 3-4% leafy vegetable, black tea

6-10% fruits and vegetables v. Potato: Added CaCl2 to blanching water

Reduce heat damage Non-enzymatic browning

Dehydration of fish

Larger fish – clean, split and salted Hot or cold smoke – artificial drying or sun drying

Dehydration of milk

Whole milk or non-fat skim milk Drum drying or spray drying Low heat spray dried milk – better flavor

color

Dehydration Eggs

Whole egg powder

Egg yolk powder

Egg white powder

Lowering glucose by enzyme or fermentation

Egg white spray dried, cabinet dried or

tunnel dried Whole eggs spray dried

Properties of dried egg powder 1. High solubility 2. Mild order 3. Low MOs 4. Uniform and proper heating 5. High form stability

FOOD CONCENTRATION

Products Concentrate > 65% TSS

Added sugars

Added acids

Mild heat treatments

For Fruit jellies and fruit preserves

Sweeten condensed milk

HIGH SOLID-HIGH ACID FOODS

Jelly, Jam, fruit preserves (i.e.: chutney), marmalade and fruit butter

No MOs

Hermetic sealing

But some yeast and mold can growth

Jelly

Semisolid food: sugar and fruit juice

TSS 65% + Flavor, color, pectin and acid

Fruit gel: pectin, acid, sugar and water

Filtered fruit juice, no fruit pieces or

insoluble solids

Consistence is high – high jell strength

On real strawberry jam

Jam

Product containing both soluble and

insoluble fruit

Large pieces of fruit Fruit pulp + sugar => 45% of fruit + 55% of sugar

(45 kg : 55 kg)

Less consistency –low jell strength

Marmalade

From Citrus fruit – jelly like high viscous liquid product

Citrus juice + citrus peel + sugar High pectin 30% in citrus peel

Concentrate to achieve gel structure

Fruit butter

Smooth, semisolid fruit mixture with no fruit pieces or peel

With spices Similar to the Jam and Jelly Made from pureed fruit Thicker than jam and jelly Low added sugar and no pectin Can be enhanced by adding spices Shorter shelf-life about 6 months

Pectins Pectins are in the peels and cores Boiling releases the pectin from the fruit Long pectin chains forms intermolecular

interactions

This network forms at the ‘setting point’ of jam => approximately @ 104 ˚C During cooling gel network ‘traps’ the water

=> jam setting

High pectins: apples, peel of citrus fruits

graphs

Low pectin: strawberries

For low or higher pectin in fruits -

commercial pectin must be added Pectin is classified according to the degree

of methoxylation (DM)

DM => percentage of esterified galacturonic acid units

DM effect on the properties of pectin =>

solubility and the gel forming characteristics

DM = 50% methoxylation

HM Pectin High methoxyl (HM) pectin > 50% DM

Forming gels in aqueous systems High TSS

Low pH values

LM Pectins Modification of the extraction

process, or continued acid treatment

LM< 50% methoxyl groups DM

Gels in the presence of bivalent Ca++

Low solids content

Wide pH range

Parameters HM pectin LM pectin

DM > 50% < 50%

TSS (%) ≥ 65% 10 - 70%

pH 2.0 - 3.8 2.6 - 7.0

Bivalent ions, Ca++ (mg/g)

>15

Sugar

Vital for the flavour and jam set

Pectin set – enhances the pectin’s gel-

forming capability by absorbing

moisture

Preservative

Sugar content of jam should be

between 65-69%

Finished products should contain non-

crystallizing sugar ie.: glucose or

fructose

Prevent sucrose crystals

Inverts some of the sugar to help

prevent from crystallization during

storage Sucrose + Acid heat Glucose + Fructose

Acids

COOH in the pectin ionized=> negative

charges COO- repulse the ionized

molecules

Avoid ionization @ pH = 2.8-3.3 => COOH

Fruits acids – citric acid, malic acid

and tartaric acid

Lower the pH naturally by lemon juice

Ionization

Inverts some of the sugar to help prevent crystallization during storage.

0.5% 1% 1.5% Optimum

64% 67% 71% (Depend on type of pectin) Weak jelly Crystals

Optimum

pH

Hard jelly Optimum No jelly

Pectin Pectin-H2O

Network

JELLY STRENGTH

Continuity of Structure Rigidity of Structure

Concentration of Pectin Acidity

Conc. of Sugar

2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6

Methyl group

Tropical Fruit (i.e: Sapodilla)

Pears, Apple, Guava, citrus fruits

contain large amount of pectin

Cherry, grapes, strawberries contain

small amount of pectin

Typical levels of pectin in plants

Fruits Pectin (fresh wt%) Apples 1–1.5% Apricots 1% Cherries 0.4% Oranges 0.5–3.5% Carrots 1.4% Citrus peels 30%

Sugar content (TSS%) and pH of fruits

COMPOSITION

TSS 66-69%

Moisture 31-34%

Titratable acidity 0.3-1.1%

Crude pectin 0.5-1.5%

Ash 0.1-0.5%

Tamarind physico-chemical properties

Food Colloidal System

Functions of Emulsifiers Application Examples

Surface Active Ability

Emulsification W/O Margarine, Butter, Butter Cream O/W Ice cream, Cream, Milk Drink

Dispersion Chocolate, Cocoa, Peanut Butter

Foaming Cake, Desserts

Anti-foaming/ Defoaming

Tofu, Fermentation Industry, Jam

Wetting Powdered Foods, Chewing Gum

Solubilization Flavors Cleaning Cleaning Agent for Food Industry

Starch Complex Forming Ability

Protection of Starch Granule

Instant Mashed Potato

Anti staling Bread, Cakes

Prevention of Sticking

Pasta, Noodles, Rice

Prevention of Gelatinization

Flour Paste, Desserts

Modifying Ability for Oils and Fats

Crystal Modification Margarine, Shortening, Chocolate Creaming Ability Margarine, Shortening

Water-Holding Ability Margarine, Shortening

Others Coating Agent, Lubricant Agent Protein Modifying Ability

Gluten Modification Dough Conditioner

Others Tofu, Frozen Surimi

Others Antibacterial and Anti-Fungal

Plasticizing Anti-Oxidation

HEATED AIR DRYING EQUIPMENT

Convection heating method of drying

Bin or silo dryer Cabinet, tray, or compartment dryer Tunnel dryer Belt or conveyor dryer Fluidized bed dryer Pneumatic dryer Rotary dryer Spray dryer

Conduction heating methods of drying

Drum (or roller) dryer Vacuum drying

Radiation heating

method

Microwave IR Other dielectric

source

Freeze drying (lyophilisation)

Absolute pressure < 4.58 torr (610.5 Pa), ice sublimes directly to vapor

Foods often dried in two stages A. sublimation to approximately 15% MC (wb) B. desorption to around 2% MC

Psychrometrics

Study of interaction of temperature and the humidity of air

Capacity of air to remove moisture from a food depends on: The temperature of the air Water vapor already removed Temp + RH = evaporation rate

Moisture content (Absolute RH) Mass of water per mass of food

% RH Partial pressure of water vapor in the air to that of saturated water vapor at the same temperature

Equilibrium moisture content Moisture of food in equilibrium with drying air

Dry bulb temperature Temperature of air read by a thermometer

Wet bulb temperature Temperature read by a thermometer which has its bulb surrounded by a wet cloth

Dew point Point of saturation (100% RH)

The difference between the wb and db temperature is

used to find the RH of air on a psychrometric chart

Hygroscopic Partial pressure of water vapor varies with moisture content

Non-hygroscopic Constant water vapor pressure at different moisture contents