WET CLEANING METHODS
1. SOAKING a)
Simplest in wet cleaning and mostly applied to root vegetables which are heavily contaminated with earth.
b)
Vegetables/fruits are held in tanks of water.
c)
The food may be gently stirred with slow moving paddles or tumbled gently in a perforated drum which is partially submerged in the water.
d)
Compressed air is blown into the tank to agitate the water.
e)
Heavy contaminants sink to the bottom and are removed through a grill. Light contaminants rise to the top and removed over a wire or pipe.
f)
Detergents are added to the water to assist in removing pesticide, herbicide and fertilizer residues.
g)
Chlorinated water is used to decrease bacterial, higher load of bacteria may need more chlorine.
h)
Heated water may be used, provided it does not speed up spoilage.
2. SPRAYING a.
Widely used to remove soil.
b. Pressure of the spray should be as high as possible without damaging the food.
c.
Belt washer: i. food is conveyed on perforated belt beneath the spray. ii.
Roller conveyors may be used for near-spherical items, so that they may spin, exposing all the surface to spray.
d. Drum washers i. food is tumbled gently in a cylinder made of metal rods (rod washer) or wires. ii.
the food is retained in the cylinder while contaminants pass out between the rods or wires.
iii. Water is sprayed over the food from a pipe located centrally in the cylinder.
3. FLOATATION WASHING a.
Food suspended in water is made to flow down an inclined chute, passing over a number of adjustable weirs.
b.
Heavy contaminant will be trapped behind the weirs.
c.
Food and light contaminants flow over the weirs and over a vibrating screen, while the light contaminants are washed away with water sprays.
d.
Applications: remove stones, dirt and plant debris from beans, peas, dried fruits at rates of 3 tonnes/hr. water flow rate : 200-500 L/min.
4. FROTH FLOTATION a)
Food particles such as peas are introduced into a mineral O/W emulsion.
b)
Air is blown through the liquid and a layer of froth is formed on the top.
c)
The peas sink to the bottom and light debris rises to the top and floats on the froth. This is skimmed off.
d)
Peas are washed before proceeding to the next stage of the process.
5. ULTRASONIC CLEANING a) Use of ultrasonic waves at frequencies of 20-100kHz. b) The liquid is exposed to high energy ultrasonic waves at 16kHz. c)
Fluctuation in pressure will cause bubbles to expand and contract e.g. Cavitation.
d) Cavitation occurs at the interface between liquids and thus one phase will become dispersed in one another.
f)
E.g. wedge resonator, the liquid is pumped (50-200 psi) through a nozzle and hit the vibrating plate
g) The blade resonates and imparts wave of ultrasonic frequency (18 – 30kHz) to the liquid. The disperse phase droplet size is about 1-2 mm.