Sterilizing is a process where the fruit bunches are cooked under steam pressure. Cooking is normally carried out under 3 bar pressure (45 pounds per square inch – psi) for approximately 90 minutes. The sterilization serves several purposes:
- Heat treatment destroys oil-splitting enzymes and arrests hydrolysis and auto-oxidation and also kill the yeast in raw material.
- For large-scale installations, where bunches are cooked whole, the wet heat weakens the fruit stem and makes it easy to remove the fruit from bunches on shaking or tumbling in the threshing machine.
- Heat helps to solidify proteins in which the oil-bearing cells are microscopically dispersed. The protein solidification (coagulation) allows the oil-bearing cells to come together and flow more easily on application of pressure.
- Fruit cooking weakens the pulp structure, softening it and making it easier to detach the fibrous material and its contents during the digestion process. The high heat is enough to partially disrupt the oil-containing cells in the mesocarp and permits oil to be released more readily.
- The moisture introduced by the steam acts chemically to break down gums and resins. The gums and resins cause the oil to foam during frying. Some of the gums and resins are soluble in water. Others can be made soluble in water, when broken down by wet steam (hydrolysis), so that they can be removed during oil clarification. Starches present in the fruit are hydrolyzed and removed in this way.
- When high-pressure steam is used for sterilization, the heat causes the moisture in the nuts to expand. When the pressure is reduced the contraction of the nut leads to the detachment of the kernel from the shell wall, thus loosening the kernels within their shells. The detachment of the kernel from the shell wall greatly facilitates later nut cracking operations.
Operated in single-peak, double-peak or triple-peak cycles as batch process. When disposed in a horizontal position, the cylindrical vessel sterilizer has fairly good disposition because the oil palm fresh fruit bunches placed in cages with a low stacking height are more uniformly spread out in this position across the length of the elongated vessel. Thus, when pressurized steam is injected into the interior of the horizontally positioned cylindrical vessel, the steam can reach out to different directions and corners of the contents within the cages thereby helping treatment of the fruit bunches. Due to the low stacking height of the fruit bunches in the cages, condensate drains out freely from the fruit bunch stack facilitating heat penetration.
Vertical sterilizers are available in several variations where fruit bunches are placed directly into a vertical vessel without steel cages. They operate at 4 bar abs steam pressure in single-peak, double-peak or triple-peak cycles as batch process. The vertical sterilizer is considered for use in palm oil mills where space saving is important. Vertical Sterilization System enables a “one way traffic” Fresh Fruit Bunches (FFB) handling system instead of the conventional fruit cages double-handling arrangement. FFB is fed into the Vertical Sterilizer using the robust scrapper bar conveyers at control of a push button. All these handling is completed using only maximum two operators. Operation cost saving is proven to be halved compares to conventional cages system.
Blow-off Silencer & Blow Down Chamber
However, during sterilization it is important to ensure evacuation of air from the sterilizer. Air not only acts as a barrier to heat transfer, but oil oxidation increases considerably at high temperatures; hence oxidation risks are high during sterilization. Over-sterilization can also lead to poor bleach ability of the resultant oil. Sterilization is also the chief factor responsible for the discolouration of palm kernels, leading to poor bleach ability of the extracted oil and reduction of the protein value of the press cake. The silencer system design of sterilization tank vapor emission can reduce noise pollution.
Vent silencers are to be applied behind safety valves, start-up valves, control valves and by-pass valves. The vent silencer, or blow-off silencer, is developed to reduce noise during the venting of steam and gasses. The principle is based on a combination of controlled expansion and sound absorption. In the expansion part, the venting pressure is reduced to practically atmospheric conditions. During this process, a natural transformation takes place from a low frequency to a high frequency sound. Subsequently, the remaining high-frequency sound is reduced to the required level in the absorption part.
The sterilizer blow down chamber will be provided and installed to receive water from all sterilizer and discharged into sterilizer condensate pit.