1. It occurs in innumerable instances in the food industry
MIXING
Mixing is the dispersing of components, one throughout the other.
It occurs in innumerable instances in the food industry
the most commonly encountered of all process operations.
A mixing process begins with the components, grouped together in some container, but still separate as pure components. 
Preparing a 1% iodised salt. Total of some kg to be made for sale. Potassium Iodate is added as iodizing agent
the salt is to be sold in 2 kg packets, the practical requirement might well be that each packet contains 20 g Potassium Iodate of with some specified tolerance, and adequate mixing would have to be provided to achieve this.

2. Mixing of Widely Different Quantities
The mixing of particles varying substantially in size or in density presents special problems,
gravitational forces acting in the mixer which will tend to segregate the particles into size and density ranges.
initial mixing in a mixer may then be followed by a measure of (slow gravitational) un-mixing and so the time of mixing may be quite critical.

3. Powder and Particle Mixers
Ribbon Blender
Conical Mixer
LIQUID MIXING
Most of the information that is available concerns the power requirements for the most commonly used liquid mixer - some form of paddle or propeller stirrer. 
The results have been correlated in an equation of the form
(Po) = K(Re)n(Fr)m       
               
where (Re) = (D2Nr/m),
(Po) = (P/D5N3r) the Power number(relating drag forces to inertial forces),
(Fr) = (DN2/g) the Froude number (relating inertial forces to those of gravity); D is the diameter of the propeller, N is the rotational frequency of the propeller (rev/sec), r is the density of the liquid, m is the viscosity of the liquid and P is the power consumed by the propeller.

4. Dough and pastes mixed in machines heavy and powerful
Dough and pastes mixed in machines heavy and powerful. the large power requirements - the power is dissipated in the form of heat - substantial heating of the product. require jacketing of the mixer to remove as much heat as possible with cooling water. The most common mixer for these very heavy materials - the kneader Employs two contra-rotating arms of special shape, which fold and shear the material The arms are in sigmoid shape as indicated in Fig. Kneader They rotate at differential speeds, often in the ratio of nearly 3:2. Another type of machine employs very heavy contra-rotating paddles,

6. SIZE REDUCTION
Raw materials often occur in sizes that are too large to be used. So, they must be reduced in size.
Size-reduction operation - divided into two major categories depending on state of the material
Solid - grinding and cutting,
Liquid - emulsification or atomization. 
Solids
grinding in the food industry is in the milling of grains to make flour
Cutting -break down large pieces of food into smaller pieces suitable for further processing, eg., the preparation of meat for retail sales , processed meats and processed vegetables.
Energy in grinding is defined by Kick’s Law
energy required is equal to size reduction ratio.
Rittinger’s Constant – energy is directly proportional to surface area produced. A shape factor is also considere as λ, for cube or sphere λ is 1. For other particles it is generally 1.75.

7. Hammer mills
swinging hammerheads are attached to a rotor that rotates at high speed inside a hardened casing. The principle is illustrated Figure Grinders: (a) hammer mill, (b) plate mill
The material is crushed and pulverized between the hammers and the casing
Fine enough to pass through a screen at the bottom of the casing.
Brittle and fibrous materials can be handled in hammer mills,
with fibrous material, projecting sections on the casing may be used to give a cutting action.

8. Plate mills
In plate mills the material is fed between two circular plates, one of them fixed and the other rotating. The feed comes in near the axis of rotation and is sheared and crushed as it makes its way to the edge of the plates, Fig.(b).
The plates can be mounted horizontally as in the traditional Buhr stone used for grinding corn, which has a fluted surface on the plates. The plates can be mounted vertically also. Developments of the plate mill have led to the colloid mill, which uses very fine clearances and very high speeds to produce particles of colloidal dimensions.
The range of milling equipment includes ball mills, in which the material to be ground is enclosed in a horizontal cylinder or a cone and tumbled with a large number of steel balls, natural pebbles or artificial stones, which crush and break the material.
Ball mills have limited applications in the food industry, but they are used for grinding food colouring materials.  The edge runner mill, which is basically a heavy broad wheel running round a circular trough, is used for grinding chocolate and confectionery. 
Cutters
Cutting machinery consists of rotating knives in various arrangements. 

9. EMULSIONS
Emulsions are stable suspensions of one liquid in another, the liquids being immiscible.
Dispersed phase & continuous Phase
The stability of an emulsion is controlled by 
interfacial surface forces, 
size of the disperse phase droplets, 
viscous properties of the continuous phase and 
density difference between the two phases.
very often a third component is added which is absorbed at the interface and helps to prevent the droplets from coalescing. (1-10 mm diameter. )
emulsifying agents - eg., phosphates and glycerol monostearate. Sodium stearoyl-2-lactylate (sodium stearoyl lactylate or SSL) Lecithin - rapeseed, cottonseed and sunflower

10. Shearing increases emulsification – using high pressure pump (7x 103 kPa)
Expanding through a nozzle – Homogenizer.
Centrifugal forces may also be used to obtain the shearing action.
Discs spinning at high velocities give rise to high shearing forces in liquids flowing over them.
Flow between contra-rotating discs, withpegs on the disc faces, can produce emulsions.
Designs in which small clearances are used between a stationary disc and a high speed flat or
conical rotating disc are called colloid mills.
Examples of emulsions in the food industry are - milk (fat dispersed in water),
butter (water dispersed in fat),
mayonnaise (oil in water) and
ice cream (fat in water which is then frozen).

11. Homogenizing is also used with ice cream mixes, which are dispersions of fat and air in sugar solutions, and in the manufacture of margarine.