Food Drying PDF

FOOD DRYING/DEHYDRATION Prof. Sabah Mounir  Outlines Introduction  Definition & Principle of Drying/Dehydration  Goal of Drying/Dehydration  Types of Water present in Food  Concept & Importance of Water Activity  Natural & Artificial Drying  Mechanism of Drying  Drying Rate & Factor...

FOOD DRYING/DEHYDRATION Prof. Sabah Mounir  Outlines Introduction  Definition & Principle of Drying/Dehydration  Goal of Drying/Dehydration  Types of Water present in Food  Concept & Importance of Water Activity  Natural & Artificial Drying  Mechanism of Drying  Drying Rate & Factors affecting the Drying Rate  Problems associated with traditional drying process  Quality of dried Food  Different Techniques of Drying Introduction  Drying is the oldest method of food preservation; the sun, the wind, and a smoky fire were used to remove water from fruits, vegetables, grains, herbs, and meat  Classification of the methods of food preservation 1- Heating/thermal processing: pasteurization , sterilization, drying, & canning 2- Low temperature: cooling & freezing 3- Reduction of water activity (aw): drying  different techniques 4- Non-thermal processing: high pressure processing (HPP), pulsed electrical field (PEF), ultrasound (US), irradiation 5- Addition of chemicals: sugar, salt (NaCl), citric acid, antibiotics , natural or synthetic antioxidants 6- control storage and packaging Introduction  Drying is an important unit operation, can occur in conjunction with other processes (baking of bread or bakery products, frying)  Dried foods can be stored for long periods without deterioration occurring  During drying, the water content of food is reduced to critical level below which microorganisms can not grow  Microorganisms which cause food spoilage and decay are unable to grow and multiply in the absence of sufficient water and many of the enzymes which promote undesired changes in the chemical composition of the food cannot function without water Definition of Drying or Dehydration  A process of removing water/moisture from food by evaporation due to a simultaneous heat and mass transfer. Heat is transferred from the surrounding environment to evaporate the surface moisture. Principle of Drying  the removal of moisture from the food involved 2 basic phenomenon: 1- vaporization of moisture from the external surface of the food material 2- the migration of moisture from the interior of food material to its external surface as a result of internal diffusion , cell contraction, & vapor pressure gradient  As a result, the water activity is reduced  extending the shelf life or the storage period of dried product without the need for refrigerated storage.  e.g. liquid milk is highly perishable, whereas milk powders are more stable & easy for preservation and handling. Goal of Drying or Dehydration  Preservation of food  extending its shelf life (storage period)  Decreasing the weight and bulk of food  decreasing the costs of transportation, handling, and storage  Easy and economic mean to save the surplus food to a later  minimizing the post harvest loses  overcoming the seasonality problem  making the food materials available over the year  Production of convenience products  instant coffee, milk powder, & instant mash potatoes  Providing important modern marketing requirements Water in food Free-water Bound-water Held inside cells Is a part of molecule structure; sugar Maintains properties of free water Does not retain properties of free water May be removed by applying Reduced mobility pressure, heat  water activity  Water activity (aw) is the amount of free water or available water in a product as opposed to bound water  aw is the measure of the energy status of the water in a system !!!  aw is a principle of thermodynamics & follows the same rules  a higher aw means more energy & the water can do work  such as microbial growth, moisture migration, or chemical and physical reactions.  water at higher aw has more energy than the water at lower aw Definition of water activity  aw is the ratio of the vapor pressure of water in a food at a specified temperature to the vapor pressure of pure water at same temperature  water activity  Water activity (aw) is one of the most critical factors in determining the quality & safety of food  affects the shelf life, safety, texture, flavor, color  While temperature, pH, & other factors can influence if and how fast microorganisms will grow in a product, water activity may be the most important factor in controlling spoilage & deterioration of food  Water activity (not water content) determines the lower limit of available water for microbial growth  By measuring water activity, it is possible to predict which microorganisms will & will not be potential sources of spoilage Bacteria XXX below 0.90 Molds XXX below 0.80  Water activity plays a significant role in determining the activity of enzymes  Solvent & free Strongly Less strongly water Bound water Bound water Multilayers & capillary Monolayer adsorbed water Drying vs Dehydration Drying Dehydration Natural process: open air, shade, Artificial process: oven drying, solar drum-drying, spray-drying, freeze-drying, etc Removing of water by non Removing of water under conventional energy sources; sun & controlled conditions; wind  no control of climatic temperature, relative humidity of conditions air and air velocity Contamination from insect, dust, Sanitary conditions are controlled birds, rain Poor quality of dried product Good quality of dried product More space is required for drying Less space is required  occurred Availability of sun within a drying chamber (dryer) Less expensive  energy sources More expensive  high energy are natural, sun or wind consumption  energy-intensive Oven-dryer (tray-dryer) Spray-dryer Freeze-dryer Rotary drum-dryer Preservation Effects of Drying  Dried/dehydrated foods have lower water activity (aw)  0.2 to 0.6  This aw is not enough (free water) for:  microorganisms growth  needs aw > 0.93 specially bacteria  Yeast  needs aw > 0.88  Mold  needs aw > 0.80  Enzymatic reactions  chemical reactions  Maillard reaction (non-enzymatic browning need aw > 0.30 Not destroyed  resume their growth after the rehydration of dried food 1 Air characteristics 2 Concentration of water at the surface 1 2 1 Drying Rate 1st critical point Moisture content (Kg H2O/Kg db) Drying process can be divided into B C 3 periods A 1- Initial adjustment period 2nd critical point Known as heating up period (A-B) D  The food material gets heat from hot air, increasing its temperature Eqlibrium moisture content  the drying has not started E  But, evaporation of water that already present on the material’s surface (Starting accessibility) Time (hr) 2- Constant drying rate period (B-C)  The temperature of food material & drying rate remain constant  Evaporation of the moisture present on the material’s surface  The moisture evaporated from the surface is replaced by water diffusion from the core of the material at rate equal to the rate of evaporation  The moisture at the end of this period (point C) is referred to as critical moisture content (CMC)  dry spots start appearing & drying rate starts falling Drying Rate 1st critical point Moisture content (Kg H2O/Kg db) Drying process can be divided into B C 3 periods A 3- Falling drying rate period (C-E) 2nd critical point D 3-1- First falling drying rate period (C-D)  known as unsaturated surface period Eqlibrium moisture content E  During this period the rate of evaporation is insufficient to saturate the air in contact with the surface  because Time (hr) the surface water is no longer replaced at a rate required to maintain continuous film (water film) on the surface  Point D is referred to as the second critical point  At this point, the film of surface water has completely evaporated  Between points C and D, the number and area of dry spots continue to increase, and the rate of drying falls steadily. Drying Rate 1st critical point Moisture content (Kg H2O/Kg db) B C Drying process can be divided into 3 periods A Internal 2nd critical point diffusion  limiting 3-2- Second falling drying rate period (D-E) phenomenon D  At the end of the first falling rate period, Eqlibrium moisture content the rate of drying falls even more rapidly  The level of vaporization moves from E the surface into the core of the food material (Paradoxical drying phase)  opposite Time (hr) direction of water diffusion  So, the vapor reaches the surface by molecular diffusion through the material  Point (E) is referred to as the equilibrium moisture content (at which material is neither gaining or losing moisture  Beyond this point drying is not possible as remaining moisture is bound moisture  Drying rate here becomes zero Molecular diffusion: is the thermal motion of molecules at temperatures above absolute zero (-273°C). The rate of this movement is a function of temperature, & viscosity How to predict the drying time during the constant rate period ??!!.. mc  moisture removal rate (kg water/ (s. kg dry solids)) w0  initial moisture content (kg water/ kg dry solids) wc  critical moisture content (kg water/ kg dry solids) tc  time for constant rate period (s) How to predict the drying time ??!! 1- Rate of heat transfer: q = h A (Ta – Ts) q  rate of heat transfer (W) h  convective heat transfer coefficient (W/m2 °C) A  surface area of product (m2) Ta  heated air temperature (°C) Ts  product surface temperature (°C) Noting that: during constant rate, the product temperature will remain at the wet bulb temperature of the air Film of water present on the surface of sample 2- Rate of water vapor transfer:. Km  mass transfer coefficient (m/s) A  surface area of product (m2) Mw  Molecular weight of water P  Atmospheric pressure (KPa) R  Universal gas constant (4314.41 m3 Pa/kg ml k) TA  Absolute temperature (K) Ws Humidity ratio at product surface (kg of water/kg of dry air) Wa  humidity ratio of air (kg of water/ kg of dry air).. Thermal Energy Transfer. q = mc HL q  is the amount of heat needed to evaporate the water from the sample surface  because a film of water is always present on the sample surface HL latent heat of vaporization at wet bulb temperature (J/kg water).. q = mc HL.  tc  time for constant rate period (s) HL latent heat of vaporization at wet bulb temperature (J/kg water) w0  initial moisture content (kg water/ kg dry solids) wc  critical moisture content (kg water/ kg dry solids) h  convective heat transfer coefficient (W/m2 °C) A  surface area of product (m2) Ta  heated air temperature (°C) Ts  product surface temperature (°C)  Factors affecting the drying rate 1- Drying Temperature  This varies with food & the method of drying  greater the temperature difference between heating medium and food , greater the rate of heat transfer  greater the rate of drying 2- Relative Humidity of air  lower the relative humidity of air, more rapid the rate moisture transportation (external moisture transfer)  greater the rate of drying 3- Velocity of air  higher the velocity of air, more efficient the process of drying 4- Drying Time  Drying time depends upon the type of food being dried, its moisture content and temperature, air characteristics 5- Specific surface area  smaller pieces  more rapid the rate of moisture removal 6- Tray load  overloading of trays  less circulation of air  slowdown the rate of moisture removal Overloading Problems associated with classical hot air drying cell contraction Shrinkage  water removal Reduce the rate of drying to produce food with dry surface and a Case Hardening moist interior poor Texture reconstitution/reh compactness ydration properties changes :color, flavor, nutritive long time of value Overheating drying  Quality of dried Food  Products undergo changes during drying that reduce their quality compared to fresh material 1- Texture  Rupture, crack, compress and permanently distort the rigid cell  change texture appearance  given the food a shrunken appearance  Rehydrated product absorbs water more slowly and does not regain the firm texture of fresh food  Drying pieces of meat  severe changes in texture  aggregation and denaturation of proteins and loss of water holding capacity  Quality of dried Food 2- Flavor & aroma  Heat not only vaporizes water during drying but also causes loss of volatile components  Volatile loss depends on temperature, moisture content of the food, & their solubility in water  The open porous structure of dried food allows access of oxygen  which is a second important t cause of aroma loss  due to the oxidation of volatile components and lipids during storage  These changes can be reduced by using vacuum, low storage temperature, preservation of natural antioxidants or addition synthetic antioxidants  Quality of dried Food 3- Color  Causes of color loss or change in dried foods. Drying changes the surface characteristics of a food and alter the reflectivity of surface  Fruits & vegetables: 1- chemical changes to carotenoid and chlorophyll pigments  caused by heat and oxidation during drying 2- Enzymatic browning by polyphenoloxidase  Prevented by treatment of fruits with ascorbic acid or sulfur dioxide Sulfuring causes bleaching of anthocyanin and the residual sulfur dioxide is also health problem Types of drying techniques used for Food Drying  1- Cabinet Drying  Cabinet dryers are usually small, insulated units with a heater, circulating fan, and shelves to hold the product being dried  is a convective oven with thermostatic setting of different  temperatures  heated air is forced over multiple trays  Energy efficiency can be obtained if some of heated air is recirculated  Energy saving of 50 % or more can be achieved with recirculation  Suitable for drying of fruits, vegetables (food in solid form)  2- Tunnel Drying  Tunnel dryers are a large scale modification of he cabinet dryer concept  The drying chamber is a tunnel with multiple carts containing trays  New carts of un-dried product (fresh) are loaded at one end of the tunnel and the dried products are removed from the other end  Air for drying is circulated by fan  Air flow in this type of dryers may be parallel (Co-current) or counter (Counter current) to the movement of carts in the tunnel  3- Rotating –drum-drying  Large rotating drums are used for drying slurries (liquid with a high solids content  viscous liquid  A thin film of the slurry is deposited on the bottom of a rotating drum as it passes through the slurry  The slowly rotating drum is heated by steam (120- 155 °C) and sometimes held under vacuum  The dried product is scraped from the drum with knife (doctor blade) before the one full circle rotation  The dryer may be one rotating drum or double drums which rotate n an opposite direction 4- Spray-drying  Spray-drying is used to dry liquid (milk, fruit juices). The product to be dried is sprayed into stream of heated air  The powder particles are small and move easily within an air stream  Air flow may be co-current or coutner current  The two major operations of concern in spray-drying are: 1- To optimize drying, droplets should be small and uniform in size  so special procedures must be used to ensure that atomization is satisfactory 2- Collection of the dried powder also required special techniques Single-orifice Nozzle Multi-orifice Nozzle  5- Freeze-drying  Freeze-drying  known as Lyophilization  It is a process in which food material is frozen then a high- pressure vacuum is applied to sublimate the water in the form of vapor  Coffee was first freeze dried food product Principle of Freeze- Drying  Based on sublimation, where water passes directly from solid state (ice) to the vapor state without passing through the liquid state  Sublimation of water can take place at pressure and temperature below triple point  4.579 mm of Hg and 0.0099 °C Triple point of water ? !!!  In thermodynamics, the triple point of water is the temperature and pressure at which the three phases (gas, liquid, and solid) of water coexist in thermodynamic equilibrium  Steps of Freeze-drying 1- Freezing: the product is firstly frozen at temperature (-50 and -80°C) 2- Primary Drying: the pressure is lowered and enough heat is supplied to the product for the sublimation of ice  this phase may be slow because, if too much heat is added, the product’s structure could be altered 3- Secondary Drying: to remove unfrozen water molecules, so the temperature is increased higher than the primary drying phase  to breakdown any physico-chemical interactions that have formed between the water molecules and the frozen material  1 3 The main components of Freeze- dryer 4 1- Refrigeration System: to cool  the condenser located inside the freeze dryer  the shelves in the product 2 chamber for the freezing of the product 2- Vacuum System: consists of a separate vacuum pump connected to an airtight condenser and attached product chamber  the condenser located inside the freeze dryer  the shelves in the product chamber for the freezing of the product  1 3 The main components of Freeze- dryer 4 3- Product Chamber  a large chamber with a system of shelves on which to place the product 2 4- Condenser  to attract the vapor being sublimated off of the product. The sublimated ice accumulates in the condenser.  The temperature of the condenser has to be below the temperature of the product 5- Control system  include temperature & pressure sensing ability to monitor the process  Advantages of Freeze Drying  The physical structure of the food is not altered  Food retains much of its color, shape, and flavor  Rapid and easy reconstitution Disadvantages of Freeze Drying  High costs  limited to dry biological materials (lactic acid bacteria, bakery yeast) and pharmaceutical application Time consuming