8 attributes, taste was the strongest attribute for both instant tea and green tea granules produced,
and strength was the weakest attribute. Sinija, 2011.
C. FREEZE CONCENTRATION
Concentration of fluid foods by freezing involves lowering the temperature of the product in a sufficiently controlled manner to partially freeze the product, resulting in a slurry of ice
crystals in a fluid concentrate. If formed under the appropriate conditions, these ice crystals will be very pure. That is, very little product will be incorporated within the ice crystals. The ice
crystals are then removed in some way with a minimum of liquid carryover, resulting in a concentrated product. The basic components of a freeze concentration system, as shown in
Figure 4.
Freeze concentration is appliable to many food concentration, such as citrus fruit juices, vinegar, coffee, tea, sugar syrups, dairy product, and aroma extract. The major advantage of
using a freeze concentration process as opposed to evaporation or reverse osmosis are related to the low temperature operation suitable for sensitive food products without the loss of product
quality. In addition, the solid-liquid separation in freeze concentration results in no losses of the more volatile flavors and aromas, as occur in evaporation. The disadvantages of freeze
concentration compared to evaporation and reverse osmosis have include higher capital cost, higher operating costs, and excessive loss of product during the ice separation Hartel, 1992.
D. SPRAY DRYING
Spray drying is one-step continuous processing operation that can transform feed from a fluid state into a dried form by spraying the feed into a hot drying medium. The product can be a
single particle or agglomerates. The feed can be a solution, paste, or a suspension. This process has become one of the most important methods for drying liquid foods to powder form. The
principal of spray drying as shown in Figure 5. Feed
Crystal Nucleation Crystal Growth
Separation Crystal Slurry
Concentrate Ice
Figure 4. Schematic of a basic freeze-concentration process Hartel, 1992
9 The main advantages of spray drying are the following:
• Product properties and quality are more effectively controlled • Heat-sensitive foods, biologic products, and pharmaceuticals can be dried at atmospheric
pressure and low temperatures. Sometimes inert atmosphere is employed. • Spray drying permits high tonnage production in continuous operation and relatively simple
equipment • The product comes into contact with the equipment surfaces in an anhydrous condition, thus
simplifying corrosion problems and selection of material of construction • Spray drying produces relatively uniform, spherical particles with nearly the same proportion
of nonvolatile compounds as in the liqiud feed. The principal disadvantages of spray drying are as follows:
• Spray drying generally fails if a high bulk density product is required • In general it is not flexible. A unit designed for fine atomization may not be able to produce a
coarse product, and vice versa. • For given capacity, evaporation rates larger than other types of dryers are generally required
due to high liquid content requirement. The feed must be pumpable. Pumping power requirement is high
• There is a high initial investment compared to other types of continuous dryers. • Product recovery and dust collection increases the cost of drying Xin and Mujumdar, 2010
Spray drying consist of four process stages: 1. Atomization of feed into a spray
The formation of spray and the contacting of the spray with air, are the characteristic features of spray drying. The selection and operation of the atomizer is of supreme importance in achieving
economic production of top quality products. The selection of the atomizer type depend upon the nature of the feed and desire characteristics of the dried product. In all atomizer types, increased
amounts of energy available for liquid atomization result in sprays having samLler droplet sizes. If, the available atomization energy is held constant but the feed rate is increased, sprays having
Figure 5. Spray Dryer. 1, feed reservoir; 2, feed pump; 3, product feed pipeline; 4, atomizer; 5, drying chamber; 6, air fan; 7, air heater; 8, hot air duct; 9, a mixture of dried product and air-
carrying duct; 10, cyclone separator; 11, heavy powder falling down; 12, product tank; 13,exhaust air Sharma et al, 2000.
10 larger droplet sizes will result. Rotary atomizers are used to produce a fine to medium coarse
product mean size 30-130 µm, while nozzle atomizers are used to produce a coarse product mean size 120-250 µm.
2. Spray-air contact mixing and flow Product and air pass through the dryer in co-current flow, they pass through the dryer in the
same direction. This arrangement is widely used, especially if heat-sensitive products are involved. Spray evaporation is rapid, the drying air cools accordingly, and evaporation times are short. The
product is not subject to heat degradation.
3. Drying of spray moisture volatiles evaporation As soon as droplet of the spray come into contact with the drying air, evaporation takes place
from saturated vapour film which is quickly established at the droplet surface. The temperature at the droplet surface approximate to the wet-bulb temperature of the drying air. A substantial part of
the droplet evaporation takes place when the droplet surfaces are saturated and cool. Drying chamber design and air flow rate provide a droplet residence time in the chamber, so that the
desired droplet moisture removal is completed and product removed from dryer before product temperatures can rise to the outlet drying air temperature of the chamber. Hence, there is little
likehood of heat damage to the product.
4. Separation of dried product from the air Total recovery of dried product takes place in the separation equipment. This system places
great importance on the separation efficiency of the equipment. Separation of dried product from the air influences powder properties by virtue of mechanical handling involved during the
separation stage. Axcessive mechanical handling can produce powders having a high percentage of fines. Master, 1991
There are many variables in spray dryer that give an effect on powder product, such as inlet temperature, feed solid content, drying temperature difference, and feed temperature. Increase of
inlet temperature can decrease the heat requirement of the dryer for producing a given product rate because product dried quickly. Increase in feed solids for a given production rate from 50 to
60 reduces the heat load by nearly 50. Spray drying is an expensive method of evaporating volatiles and thus to obtain optimum heat utilization condition the spray dryer should always fed
with the maximum solids feedstock possible. The higher the temperature difference ie. Inlet drying air temperature minus outlet drying air temperature, the lower the heat requirement to
produce a unit weight of product of constant residual moisture content from a constant solid feedstock. Feed temperature, particularly in existing plants, can also be optimized. Increasing feed
temperature reduces the heat required to produce a unit weight of dried product. Preheating of feed is normally carried out to reduce feed viscosities, thereby improving atomization performance and
to present feed crystallization that can cause atomizer blokage Master, 1991.
11
III. RESEARCH METHODOLOGY
