All natural flavoring and coloring materials require a size reduction. Even when they are used as such, size reduction ensures proper distribution in the manufactured or cooked food products. For the preparation of the extractive, size reduction is needed so that access to the inside of the cells becomes easier.
Impact, attrition, shearing, and compression are the forces that can be used for size reduction of agricultural products. For products that are brittle, such as black pepper and nutmeg, compressive strength is very useful for size reduction. With a sheath-like material, as in the case of red chili, attrition and a shearing action will be useful. For tough and hard products, such as ginger, coffee, turmeric, and chicory, a combination of these forces will be needed. Jaw and gyratory crushers, which are very useful in the mineral industry, are not very useful in the food industry. Similarly, ball mills are not usually applicable for powdering dry plant products. If cryogenic grinding is possible, it is superior to normal grinding for preserving the sensory properties and flavor attributes in pepper (Liu et al., 2013). When a mixture of enzymes, cellulase, P-glucanase, pectinase, and xylanase was used as a pretreatment, the yield of the oil was found to be higher, with an increase in the major components in the case of black pepper and cardamom. This was attributed to destruction of the cell walls, making it easier for the essential oil to be released (Chandran et al., 2012). Similar results were obtained when cloves were treated with pectinase, amylase, lignocellulase, and cellu- lase. A mixture of these gave an essential oil yield of 18.82% compared with a yield of 10.1% for untreated cloves (Amudan et al., 2011).
In this process, swinging hammer-like heads attached to a disk rotate at high speed inside a metal case. The material is pulverized due to the impact of the swinging hammers, which rotate inside the casing with only a small clearance. The escape opening in the bottom of the mill can be fitted with screens with different sieve sizes. The material will be subjected to this impact until the size of the particles is small enough to pass through the selected sieve.
For tough products the hammer mill is a very efficient type of size-reducing equipment. The major disadvantages are the possible production of high heat levels and difficulty in regulating the particle size.
Fixed Head Mills
Here, instead of a swinging hammer arrangement, fixed stout projections rotate against a fixed casing with a narrow clearance. A modification in the design is a moving disk with two or three rows of thick pins, which move inside rows of fixed pins with a narrow clearance. This type of mill is also called a pin disk mill. Since the grinding is achieved by pins that are well distributed, less heat develops.
In this type of mill, material is ground between two circular plates, one of which is moving. Material passing through will be ground, primarily through shearing and attrition actions. The surface of the plates can be made suitably rough. Although in the plate mill, plates are placed vertically, it is similar to hand-rotated stone mills where the movement is horizontal. Because of the high friction, the amount of heat developed will normally be high.
The material is passed through two heavy rollers rotating in opposite directions. The roller mill is used in wheat flour milling. Modifications could be the use of two rollers moving at different speeds, grooved instead of smooth rollers, two pairs of rollers placed one below the other, and so on. Here, the main force acting will be compression, with some degree of shearing, especially when rollers move at different speeds. Heat development will usually be low.
In the case of black pepper, passage through two sets of rollers will flatten each berry into a flake. Rather than size reduction, it is the squeezing effect that opens up the cells that is more important.
Plant products that are large and tough, such as rhizomes of ginger, turmeric, and galangal, need a preliminary size reduction in a hammer mill or pin disk mill before passing through the rollers.
For spices, passage through a roller mill will help to break up the cells without an increase in temperature. With impact mills, heat is generated. Higher yields of volatile oils are obtained in the case of celery seed (Sowbhagya et al., 2007) and cumin seed (Sowbhagya et al., 2008) when a roller mill is used instead of a hammer mill.
In this case the thin blade rotates at high speed; also the clearance between the blade and the outer case is greater. This type of mill is useful for fresh undried plant materials and is somewhat similar to a kitchen mixer. For slicing, sharp thin knives rotate such that the plant product is kept somewhat perpendicular to the motion of the blades.
Amudan, R.; Kamat, D.V.; and Kamat, S.D. 2011. Enzyme-assisted extraction of essential oils from Syzygium aromaticum. S. Asian J. Exp. Biol. 1 (6), 248-254.
Chandran, J.; Padmakumari Amma, K.P.; Nirmala, M.; Purushothaman, J.; and Nisha, P. 2012. Effect of enzyme assisted extraction on quality and yield of volatile oil from black pepper and cardamom. Food Sci. Biotechnol. 21 (6), 1611-1617.
Liu, H.; Zeng, F.; Wang, Q.; Ou, S.; Tan, L.; and Gu, F. 2013. The effect of cryogenic grinding and hammer milling on the flavor quality of ground pepper (Piper nigrum. L.). Food Chem. 141 (4), 3402-3408. Sowbhagya, H.B.; Sampathu, S.R.; and Krishnamurthy, N. 2007. Evaluation of size reduction on the yield and quality of celery seed oil. J. Food Eng. 80, 1255-1260.
Sowbhagya, H.B.; Sathyendra, R.; and Krishnamurthy, N. 2008. Evaluation of size reduction and expansion on the yield and quality of cumin (Cuminum cyminum) seed oil. J. Food Eng. 84, 595-600.