Size reduction is a unit operation involving such activities as cutting, slicing, milling or pulping of food. It requires energy input to overcome a breaking stress. When this happens the food will break along lines of weakness resulting in release of energy and sound. So think of the last time you ate an apple, that sound that you heard is energy being given off. Where enough energy is not added to the material for breaking, it returns to its original size. This is a characteristic of elasticity. Therefore to break materials, the energy input must overcome both the elastic stress limit and breaking stress.
The three types of forces involved in size reduction of solids are compression, impact and shear. Compression is a grinding force, like when you pulverize cereal in a mortar. Impact occurs when material is thrown against a surface causing it to disintegrate. For example, if you take an egg and throw it against a wall causing it to splatter, that’s an example of impact. Shear is a slicing force where you have one surface sliding over another. The typical example of this is the use of knives to slice food. For example, slicing bread. All of these forces normally exists together although one usually predominates. For example, when you slice bread, the primary force is shear, but there is also compression as the knife is forced downward on the bread and impact, as the knife is first brought down to make contact with the surface of the bread.
Equipment used in size reduction of solids are generally placed into three categories:
- Cutting, slicing, dicing, mincing, shredding and flaking equipment
- Pulping equipment
- Milling
Size reduction of meat, fresh fruits and vegetables is generally done using equipment from the first two categories while grains, pulses, spices and other low-moisture foods are processed by milling.
Energy Requirement
The amount of energy applied in milling will depend on the friability of the food (ability to fracture), moisture content and heat sensitivity. A food material’s friability will be affected by its internal structure. For example, fibrous roots such as ginger will be harder to pulp compared to carrots which is less fibrous and much softer. Dehydrated cooked beans containing gelatinized starch will be much easier to mill compared to uncooked beans that is crystalline and tough. In general, harder foods will have fewer lines of weakness and will therefore need longer residence time, larger machines, or more abrasiveness. This will in turn use up more energy.
Reduction of energy consumption may require adjustment in moisture for some dried foods. For example, wheat is conditioned by adjusting the moisture content to a higher moisture. This results in a softening of the endosperm, making it easier to be extracted with little bran contamination. Of course there has to be a delicate balance since too much moisture could make a sticky mess rather than efficient grinding and separation. On the other hand, grinding at loo low a moisture content can create excessive dust and fire risks.
Heat-sensitive foods such as spices which are highly aromatic and can lose flavor during processing, should be heated under cool conditions. This may involve pumping a coolant through the mill to remove heat as it is generated.
Determination of energy required in size reduction is generally done using either Kick’s law or Rittinger’s law. Kicks’s law is used for coarse grinding when the size ratio of original material to end materials is 8:1. Bond’s law and Retinger’s law are used for intermediate and fine grinding respectively, where ratios approach or exceed 100:1. These are described in Fellows (2009). Kick’s Law says:
E = Kkln(d1/d2)
Where, E (Wh-1kg-1) is energy required; Kk is Kick’s constant; d1 is initial particle size, and d2 is end product particle size
Effect of Size Reduction
Advantages: Size reduction provide great benefits. In the process, foods are reduced to a size that is convenient, preferred or necessary for processing. For example, materials can be more easily transported at a lower cost due to reduction in bulk density. Efficiency in mixing is improved along with heat transfer, and extraction of food components such as flavors and oils. Consumers experience improved sensory attributes, especially texture and appearance. For example, a customer may prefer finely milled corn to make their porridge compared to corn grits; or turkey slices in their sandwich instead of minced turkey.
Disadvantages: Size reduction, for example milling, may cause loss of volatiles, especially when working with heat sensitive foods like spices. In wheat milling, important food components may be lost due to separation of the bran and aleurone layer which contains proteins, vitamins and minerals. For this reason wheat flour is fortified to replace loss of micronutrients. The increased surface area resulting from size reduction could lead to increased oxidation and loss of some nutrients such as vitamin A and essential fatty acids. Enzymes may also be activated, leading to faster spoilage and reduction in shelf life. The use of dull knives could lead to damage of cell structure and increased juice-drainage and nutrient loss in fruits and vegetables. Size reduction does not improve food safety but may increase risk of food contamination due to opening surface area. Skin, husk and bran surfaces of food that are highly contaminated could cause contamination of the internal food matrix. Like any other process, size reduction on unsanitized surfaces will increase food safety risks.
Want to also learn about size reduction principles for liquids? Check out this article.
Reference: Fellow, PJ. 2009. Food processing technology, 3rd edition. Boca Raton FL: CRC Press.
