4 Relationship between stress and strain force in size reduction When stress (force) is applied to a food the resulting internal strains are first absorbed, to cause deformation of the tissues.If the strain does not exceed a certain critical level named the elastic stress limit (E), the tissues return to their original shape when the stress is removed, and the stored energy is released as heat (elastic region(O–E))
5 Relationship between stress and strain force in size reduction If the strain area exceeds the elastic stress limit, the food is permanently deformed. If the stress is continued, the strain reaches a yield point(Y).Above the yield point the food begins to flow (Y–B) Finally, the breaking stress is exceeded at the breaking point (B) and thefood fractures along a line of weakness.Part of the stored energy is then released assound and heat.
6 Relationship between stress and strain force in size reduction The size of the piece is reduced, there are fewer lines of weakness available, and the breaking stress that must be exceeded increases.When no lines of weakness remain, new fissures must be created to reduce the particle size
7 Force for size reduction in food Friable or crystalline foodsCompression forceFibrous foodsImpact forceShearing force
8 Other factor Other factor which influence the energy input; Moisture content of the foodOptimum moisture-easily breakdown e.g. wet milledExceed moisture-agglomeration of particles which block the millHeat sensitivity of the foodhigh speed mill temperature increase so necessary to cool the mill
10 The size-reduction equipment Using to reduce the size of food materialsfibrous foods (as meats, fruits andvegetables) to smaller pieces or pulpsdry particulate foods to powders.
11 Size reduction of fibrous foods slicing and flaking equipmentdicing equipmentshredding equipmentpulping equipment
12 Slicing and flaking equipment used to slice the products including cheeses, pizza toppings, cooked meats, cucumber and tomato.Meats are also cut using circular rotary knives with a blade at right angles to the path of the meat.
13 The blade advances with the product on the conveyor to ensure a square cut edge regardless of the conveyor speed or cut length which can be adjusted.
15 Dicing equipmentThe products are first sliced and then cut into strips by rotating blades.The strips are fed to a second set of rotating knives which operate at right angles to the first set and cut the strips into cubes
17 Shredding equipmentis a modified hammer mill in which knives are used instead of hammers to produce a cutting action.
18 Pulping equipmentThis uses a combination of compression and shearing forces for juice extraction from fruits or vegetables, for cooking oil production and for producing pureed and pulped meats.For example; a rotary fruit crusher consists of a cylindrical metal screen fitted internally with high-speed rotating brushes or paddles
19 For bowl chopper is used to chop meat and harder fruits and vegetables into a pulp (for example for sausagemeat or mincemeat preserve).Food may be passed several times beneath the knives until the required degree of size reduction and mixing has been achieved.
22 Ball millsThese have a slowly rotating, horizontal steel cylinder which is half filled with steelballs 2.5–15 cm in diameter.At low speeds, the small balls are used.At higher speeds, the larger balls are used.They are used to produce fine powders, such as food colourants.
23 Disc mills• Single-disc mills in which food passes through an adjustable gap between a stationary casing and a grooved disc, which rotates at high speed. • Double-disc mills which have two discs that rotate in opposite directions to produce greater shearing forces.
24 Pin-and-disc millswhich have intermeshing pins fixed either to the single disc and casing or to double discs. These improve the effectiveness of milling by creating additional impact and shearing forces.
26 Hammer millsThese have a horizontal cylindrical chamber, lined with a toughened steel breaker plate.A high-speed rotor inside the chamber is fitted with swinging hammers along its length.Using for crystalline and fibrous materials including spices and sugar.
28 Roller millsUsing to mill wheat.Two or more steel rollers revolve towards each other and pull particles of food through the ‘nip’ (the space between the rollers).The size of the nip is adjustable for different foods and overload springs protect against accidental damage from metal or stones.
33 Kick’s law Rittinger’s law Bond’s law The energy required to reduce the size of solid foods is calculated using one of three equations, as follows:Kick’s lawRittinger’s lawBond’s law
34 Kick’s lawthe energy required to reduce the size of particles is proportional to the ratio of the initial size of a typical dimension to the final size of that dimensionE(J.kg-1) = the energy required per mass of feed (W/(kg/s))KK = Kick’s constant,d1 (m) = the average initial size of pieces,d2 (m) = the average size of ground particles.d1/d2 = the size reduction ratio (RR) and is used to evaluate the relative performance of different types of equipment. Coarse grinding has RRs below 8:1, whereas in fine grinding, ratios can exceed 100:1
35 Rittinger’s lawthe energy required for size reduction is proportional to the change in surface area of the pieces of foodE(J.kg-1) = the energy required per mass of feed (W/(kg/s))KR = Rittinger’s constant,d1 (m) = the average initial size of pieces,d2 (m) = the average size of ground particles.
36 Bond’s law E(J.kg-1) = the energy required per mass of feed (W/(kg/s)) W (J kg-1) = the Bond Work Index (40,000–80,000 J kg-1for hard foods Such as sugar or grain)d1 (m) = diameter of sieve aperture that allows 80% ofthe mass of the feed to passd2 (m) = diameter of sieve aperture that allows 80% ofthe mass of the ground material to pass.
37 Bond’s law is intermediate between these two. Kick’s law gives reasonably good results for coarse grinding in which there is a relatively small increase in surface area per unit mass.Rittinger’s law gives better results with fine grinding where there is a much larger increase in surface areaBond’s law is intermediate between these two.However,equations Rittinger’s law and Bond’s law were developed from studies of hard materials (coal and limestone) and deviation from predicted results is likely with many foods.
38 EXAMPLE 1Food is milled from 6 mm to mm using a 10 hp motor. Would this motor be adequate to reduce the size of the particles to mm? Assume Rittinger’s equation and that 1 hp W.Given1. d1 = 6 mm. = 6 x 10-3 m. ,d2 = mm. = x10-3 m.E1 = 10 hp. x (745.7 W/hp.)E2 = ? When d2 = mm. = x10-3 m.Assume rate of throughout no change
39 From Rittinger’s equation Therefore,To produce particles of mm.Therefore the motor is unsuitable and an increase in power of 50% is required
40 EXAMPLE 2Sugar is ground from crystals of which it is acceptable that 80% pass a 500 m sieve (US Standard Sieve No.35), down to a size in which it is acceptable that 80% passes a 88 m (No.170) sieve, and a 5-horsepower motor is found just sufficient for the required throughput. If the requirements are changed such that the grinding is only down to 80% through a 125 m (No.120) sieve but the throughput is to be increased by 50% would the existing motor have sufficient power to operate the grinder? Assume Bond's equation.Given :1st condition E1 = 5 hp. , rate of throughout = M kg./s.d1 =500 m. = 500x10-6 m. , d2=88 m. = 88x10-6 m2nd condition E2 = ? , rate of throughout = 1.5M kg./s.d1 =500 m. = 500x10-6 m. , d2=125 m. = 125x10-6 m
41 Assume Bond's equation. 1st condition 2nd condition (E2/1.5M) = W1(5 hp./M) WSo, E2 = 5.42 hp.So the motor would be expected to have insufficient power to operate the grinder equal hp.
43 SievingSieving is a mechanical sizeSeparation process.Widely used in the foodIndustry for* separating fine fromlarger particle* removing large solidParticle from liquid stream
44 Sieve AnalysisInvolves :- Passing the material being sized through openings of a particular standard size in a screen.- The particle-size distribution is then reported as the weight percentage retained on each of a series of standard sieves of decreasing size and the percentage passed of the finest size.
45 Sieving is a gravity-driven process. usually a stack of sieves are used when fraction of various sizes areto be produce from a mixture of particle size
46 The shaker may be in the form of an eccentric drive which a screens a gyratory or oscillating motion orvibrator which gives the screens small-amplitude,high frequency, up and down motion
47 When the sieve are inclined, the particles retained on the screen fall off at the lower end and are collected by a conveyor. Screening and particle size separation can thus be carried out automatically
48 Standard sieve sizeSieves may be designated by the opening size,US sieve mesh or Tyler sieve meshThe US-sieve mesh designation is the metricationThe Tyler mesh designation refer to the number ofopening per inch.The two mesh designations have equivalent openingsize although the sieve number designations are notexactly the same.
50 Method for sieve analysis. The percentage frequency curve graph Method for sieve analysis * The percentage frequency curve graph *The cumulative percentage curve graph or The probability curve graph * Calculate method
51 The percentage frequency curve graph Figure 2 : Schematic of relative percentage frequencydistribution curve.
52 The probability curve graph Plot opening sieve diameter against probability percentageThe diameter at 0.5 or 50% probability is particle size
53 The cumulative percentage curve graph Figure 1 : Schematic of cumulative percentagefrequency distribution curve.
54 The probability curve graph Figure 3 : Schematic of the probability curve
57 The mass fraction of a sample of milled corn retained on each of a series of sieves. Calculate a mean particle diameter which should be specified for this mixture.U.S.MicronWt.X (%)% accumulateSieveSizegrams63,3601.61.6282,3803.23.244.85121,6807.97.9912.84161,19119.419.6232.46208411818.2050.66305941515.1765.824042011.611.7377.55502978.0985.64702126.66.6792.321001503.43.4495.7514010398.99200730.90.9199.90270530.10.10100.00Pan370.00Sum.98.9