Food Engineering

Module 5. Food processing equipments and unit operations

Lesson 42

DEHULLING

42.1 Introduction

The purpose of hulling machine is to remove husk from any grain or seeds with minimum damage to the bran layer and as far as possible not to break the grains. Due to surface characteristics of different types of grains it is necessary to apply friction to the grain to remove the husk. Therefore, during hulling, certain percentage of broken cannot be avoided. In this respect the construction of machine, it’s precision adjustment and the operation govern the optimum performance and the efficiency of the machine and best quality of grain production.

42.2 Selection of a Huller

Factors to be considered in the selection of a huller are

· Capacity

· Energy requirement

· Efficiency of husk removal

· Extent of physical damage to beans

· Separation of loose husk

· Durability

· Ease of maintenance

42.3 Method of Milling/ Dehusking

(1) Wet milling method

This method of processing is being adopted by comparatively smaller units. This method is practiced in southern india. The milling quality of this method was found to be good and on an average 72% recovery of grains

(1) Dry milling method

This method of milling of pulses is mostly aodpted in the northern and central India. It varies widely from region to region. Dhal from dry milling method is said to have better cooking quality a wet milling.

42.4 Equipment for Dehulling

a) Engle berg Huller

The most common machine used for paddy hulling in India is “Engle berg” huller. The working element of this machine is ribbed cast iron roller. The roller rotates on axis inside a large concentric cylinder. On the inner cast iron roller, spiral ribbed strip are mounted to ¼ part of the length and on remaining ¾ part 4 to 6 straight ribbed strips are mounted. The roller is rotated at 600 to 900 rpm. Paddy is fed into the hopper and due to rotational direction of flutes, it is moved around the cylinder and finally towards the outlet. Friction between the grain and the steel parts of the huller causes the husk and bran to be scraped off. The huller does the job of husking and bran removal simultaneously.

b) Under Runner Disc Huller

The Under Runner disk huller consist of two horizontal cast iron disks partly covered with an abrasive layer preferably of energy. The top disk is fixed with the body of machines, while the bottom disk rotates. The rotating disk is vertically adjustable by which clearance between the two disks is adjusted. As per the variety and the condition of grains, the clearance is decided. The condition of abrasive coating on the disks also affect the clearance. During hulling there is wear of abrasive coating and is not uniform over the entire surface of coating. Hulling is mainly concentrated at the center, therefore, after some time a ridge is formed at the outer ring of coating.

c) Rubber–Roll Sheller

Rubber-roll sheller consists of two rubber rolls rotating in opposite direction at different speed. A feeder feeds uniormly to the machine. Feed is fed in thin layer between the rotating roll by the feeder. One of the roll is fixed while the other is adjustable to obtain desirable clearance between them. Speed of the rolls develop a shearing force on grain surface resulting in the opening and breaking of husk.

d) Husk Separator

This machine is required to blow away husk from the mixture of shelled grains, husk and unshelled grain obtained from huller/sheller. In the first stage the husk, brokens, germ and bran must be separated which is accomplished by the husk separator. It is a simple machine having a fan and an arrangement to distribute the product of sheller uniformly on an oscillating sieve with fine perforation. This is done to ensure that air flows across uniformly and blow away the husk, the broken, germs and bran are separated through perforation while the immature grains are also blown away by fan.

e) Popular Industrialised Dehulling System

i) Hot Dehulling

The most popular dehulling system to meet today’s high protein/low fiber meal market is Crown’s Hot Dehulling System. This system will produce the meal quality for 12 months of the year, even with hard to dehull new crop beans while maintaining hull fats of less than 1%. When processing high moisture beans, up to 13.5% moisture, directly from the field or within a couple of weeks of harvest, the Crown Hot Dehulling system will produce the high protein/low fiber meal as per the market requirements. With the Hot Dehulling system there is no need to install expensive, high maintenance Grain Dryers, Tempering Silo’s, or Rotary Conditioners. In a standard conventional dehulling system the beans are heated twice, but in the Hot Dehulling system the beans are heated only once. A Vertical Seed Conditioner - ‘VSC’ is used to slowly heat the beans to 140°F [60°C] and dry the beans upto more than 2% moisture level. This heating and drying process results in very consistent conditioned beans in only 30 minutes. The patented Jet Dryer is then used for the final heating and drying.

ii)Warm Dehulling

The Warm Dehulling System is designed for areas of the world where imported beans are processed. With imported beans, the beans are harvested two to three months prior to processing and are naturally conditioned during the shipping process. There are only two changes from Hot Dehulling system to Crown’s Warm Dehulling System. One is a slightly larger Crown VSC installed to heat the beans to proper flaking temperatures, 155°F [70°C] along with drying more than 2%. The other change is the Jet Dryer is no longer required, reducing installation costs. The remainder of the dehulling system is very similar to the Hot Dehulling system. The low fiber/high protein meal is still accomplished while maintaining hull fats close to 1%. The warm system is designed so the customer can upgrade to a complete hot Dehulling system in the future.

iii) Cold Dehulling

A Cold Dehulling System if plants have an existing preparation system that includes grain dryers and conditioners and want to add dehulling to produce a lower fiber meal. The cold system utilizes existing drying and conditioning equipment and installs the very efficient, patented Aspirators to separate the free hulls and meats after the cracker and conditioner. Again, we design this system so it can easily be upgraded to a complete Hot Dehulling system in the future. With any of the systems you choose, Crown’s experienced technical expertise will provide you with the best solution for your plant layout.