ANÁLISIS DE ALTERNATIVAS

Heat Transfer Operations Mixing Operations Separation

Microbial Transformation From farm to Factory Storage of Raw Milk Centrifugal Operations Thermal Processing Systems Homogenization

Membrane Technology Bibliography

INTRODUCTION

Milk is a highly perishable biological fluid. The com- position of milk and the factors that contribute to variability in the composition have been discussed in Chapter 2. Milk from many farms are collected in tankers two to three times a week and delivered to a processing facility. At this facility (also known as a dairy plant or factory) the milk is stored and pro- cessed further to make the appropriate products for which the dairy plant is designed for.

The safety of products is of major concern in dairy processing. Hence the regulations for the production and storage of milk at the farm, for the transporta- tion from the farm to the factory, and for the holding and processing required on the factory premises have been promulgated, and these have been discussed in Chapter 3. Regulations also apply for standard- ized food products that have to meet compositional requirements as well as the use of approved ingre- dients and processes. These aspects have been dis- cussed in Chapter 4. In addition, manufacturers of

products may have internal standards for insuring the quality of the products important to the consumer. Such attributes may include taste, texture, odor, fla- vor, mouthfeel, color, and keeping quality. These as- pects are covered in detail in Chapters 1, 9, 14, and 15. The processing steps may involve one or more op- erations in combination, and the most common op- erations involve pumping or transfer of fluids, heat transfer (cooling and heating), mixing of ingredients, separation (fat standardization), and microbial trans- formation of milk (acid gel formation). These aspects are discussed in the next section of this chapter.

OVERVIEW OF PROCESSING

EQUIPMENT IN A DAIRY PLANT

Fluid Transfer Operations

Fluid transfer processes involve transferring milk from the receiving tankers to storage silos and then for further transfer to appropriate unit operations. These transfers are achieved by means of pumps. There are two main categories of these transfer agents used in the dairy industry called centrifugal and positive dis- placement pumps. Within each category there are dif- ferent types of pumps.

The selection of the right type of pump for use in an operation is dependent upon a number of fac- tors including flow rate, product to be handled by the pump, viscosity, density, temperature, and pressure in the system. Pumps should be installed as close to the tanks from which process liquids are being transferred with as few valves and bends in the line as feasible. Any devices to restrict flow should be placed at the exit or discharge side of the pump. Cav- itation is a problem in pumping caused by too low a 73

Edited by Ramesh C. Chandan Copyright © 2006 by Blackwell Publishing

pressure at the inlet end of a pump relative to the vapor pressure of the fluid being transferred. As cavitation progresses, pumping efficiencies decrease and even- tually the pump ceases to transfer the fluid. The ap- propriate size of the pump required for the transfer de- pends upon flow rate and head, required motor power, and the net positive suction head. Engineers using charts and formulas easily calculate these parameters.

Centrifugal Pumps

A motor drives an impeller that has vanes (Fig. 5.1). The motion is circular and the liquid being pumped enters to the center of the impeller that imparts a circular motion to the liquid. The liquid exits the pump at a higher pressure than the pressure at the inlet. Centrifugal pumps are useful for transferring liquids that are not very viscous. Because of the lower costs (when compared with positive displace- ment pumps) of these pumps, they are widely used in most applications in a dairy factory. These pumps are not suitable for high-viscosity liquids or those items requiring care in handling, for example fluids where structures should not be disturbed or ingre- dients whose identity is critical to product appeal. Flow control is achievable by three different means. The first is by throttling. This procedure is expensive but offers the greatest flexibility. The second means of achieving flow control is by changing the impeller diameter. This method is the most economical but the least flexible. A third means is to install an elec- tronic speed controller, which is both economical and flexible.

Positive Displacement Pumps

These pumps work on the principle of positive dis- placement in which in each rotation or reciprocat- ing movement a finite amount of fluid is pumped re- gardless of the manometric head. The main types of positive displacement pumps have been called rotary and reciprocating pumps. These pumps are useful for higher viscosity fluids and at lower viscosities may exhibit some slip as the pressure increases. The net result is a reduction in volumetric flow on each stroke. Throttling by flow control valves at the discharge end of the pump should be avoided and these pumps have to be fitted with a pressure relief valve. Flow control in positive displacement pumps is achieved by con- trolling the speed of the motor or by adjusting the volume of reciprocating pumps. Positive displace- ment pumps must be placed as close to the feed tank as possible, and the diameters of the pipes should be large relative to those of centrifugal pumps. If pipe diameters are too small the pressure drop may be high enough to cause cavitation in the pump.

Positive lobe pumps generally have two rotors and on each rotor there are three lobes (Fig. 5.2). A vac- uum is created when the lobes move causing the pro- cess fluid to be inspired into the cavities of the lobes. The process fluid is then moved along the outer walls of the pump toward the discharge end. The rotors are driven independently by a reducing gear motor. And the lobes do not touch each other or the walls of the pump casing. These pumps are used when the viscosity of the process fluid exceeds 300 cP, as is the choice for transferring cream and cultured products.

Figure 5.1. Centrifugal pump: (1) delivery

line, (2) shaft seal, (3) suction line, (4) impeller, (5) pump casing, (6) back plate, (7) motor shaft, (8) motor, (9) stainless steel shroud and sound insulation. Reproduced with permission from Tetra Pak.

Figure 5.2. Lobe rotor principle. Reproduced with

permission from Tetra Pak.

Eccentric screw, piston, and diaphragm pumps are also positive displacement pumps used for special- ized purposes in dairy plants.