Please enable JavaScript.
Coggle requires JavaScript to display documents.
Week 13: Evaporation & Dehydration (Dehydration; physical removal of…
Week 13: Evaporation & Dehydration
Evaporation (aka Concentration)
:heats liquid foods to boiling point to remove water as vapor. The end product is still a liquid :check:
3 Benefits
Reduce Storage costs
Increase self-life (inhibits microbial deterioration and prevent deteriorative reactions)
Reduce Transportation costs
Principle: the temperature in foods is raised to its boiling point through the transfer of sensible heat from steam to the foods. Steam used in evaporation supply the latent heat of vaporization to form bubbles of water vapor
3 Factors affecting rate of heat evaporation
Fouling and deposits on the heat transfer surfaces
Temperature difference between the steam and boiling liquid
Higher temperature difference can be achieved by increasing the temperature of steam or reducing the boiling temperature of liquid foods.
Boundary stationary layer
3 Common Evaporators and their Principles
(for condensed milk) Rising film: Vertical tubes heated from environment using condensing steam
While heating the liquid at the bottom of tube, bubbles and slugs of liquid rise to the top at high velocity.
(bubbles.. liquid) When leaving the top, they hit baffles to break any foam
Mostly 'once through'. Recirculation can be used
High heat transfer coefficient
(Suitable for heat-sensitive products (eg orange juice)) Falling film: Vertical tubes with liquid flowing down due to gravity
Properties
Can handle more viscous products than rising film evaporators
Complex design for even distribution of liquid flowing down
Distributors/ spray nozzles can be added
Short residence time (20-30s)
Pan evaporator: Heats product in a steam jacketed vessel/ vessel with steam coils
(Vessel) Exposed to atmosphere/ linked to vacuum (boil liquid below its atmospheric boiling point)
(steam) Condenses inside jacket/coil
(heats product) Via convection
Huge residence duration
Low processing capacity
Small heat transfer
2 possible add-ons
Coils to rotate to increase speed of heating
Paddles to agitate
5 Effects on Food
Loss of volatile and heat sensitive components (e.g. aroma compounds)
Loss of nutrients (e.g. vitamins)
Reduction in water activity
Darkening of color (e.g. maillard reaction)
Low product yield if foaming occurs
Dehydration; physical removal of water in foods by evaporation through application of heat under controlled conditions. :check:
4 ways water moves to surface
:check:
Liquid movement by capillary forces
Diffusion of liquids, caused by differences in the concentration of solutes in different regions of food
Diffusion of liquids which are absorbed in layers at the surface of solid components of the foods
Water vapor diffusion in air spaces within the food caused by vapor pressure gradients
Stages of Dehydration :check:
represented by Drying Rate Curve
in 4 stages
Initial unsteady condition (inital adjustment period)
Constant rate stage
First falling rate stage
Second falling rate stage
Factors Affecting Dehydration :check:
Processing conditions
Relative Humidity:
The smaller the relative humidity of air, the bigger the driving force for transfer of moisture to the drying air.
Air velocity:
Higher speed of air blow increase the moisture migration from surface to drying air. Hence, shorten constant rate period.
Pressure
: At lower pressure (vacuum), vapor pressure of water in air is reduced and drying occurs more rapidly during constant rate period.
Temperature
: Higher temperature leads to higher evaporation rate and higher diffusion rate to surface
Properties of foods
cellular structure,
: Generally, natural tissues have cellular stucture with moisture between and within cells. When animal/plant is killed or cooked, cells become permeable to moisture. Hence, it is easier to dry them provided cooking does not cause excessive toughing.
type and concentration of solutes
; Foods with high sugar or other lower molecular weight solutes will dry slower as these solutes interact with water and influence the water mobility, water activity and product viscosity.
constituent orientation
: In emulsion, if water is emulsified whereby it is coated with oil i.e. oil is the continous phase the dehydration will be much slower than if water is the continous phase.
product surface
: The distance water molecules travel within food products. The greater the distance, the longer the dehydration time.
5 Dehydration Systems & Their Principles :check:
(for dry liquid food & slurries) Spray dryer: Relatively short. Dry relatively heat-sensitive materials
Cabinet dryer:
Hot air: Can hold relatively large amount of food. Used for slices/ diced fruits & vegetables
Vacuum: Gives better quality than air-dried. Costly
Freeze dryer: Better quality. Heat damage is relatively low. May damage structure, leading to poor texture. May denature proteins from changing pH and concentration of solutes. Prone to oxidation & fragile.Most expensive
Tunnel dryer
Countercurrent: Usually used to dry sliced/diced fruits & vegetables
Concurrent
Fluidized Bed Dryer: Cannot be sticky/ Sensitive to mechanical damage. Relatively short drying time with very close contact between heated air & particles (High heat transfer)
5 Effects of Dehydration on Food Quality :check:
Texture : Formation of hard skin
Flavor and aroma : Loss of volatile compounds during high temperature heating or high velocity of air
Color : Surface reflectivity changes, pigment loss or browning
Nutritive value : Loss of nutritive value due to heat
Rehydration : Foods dried under optimum conditions rehydrate more rapidly and completely than poorly dried foods. However, rehydration will not be the same due to coagulation of protein, change of structure and cell membrane.
