eChapters

10.1 Introduction

Milk is a unique substance in that it is both consumed, as fluid milk with minimal processing and it is the raw material used to manufacture a wide variety of products. From the nutrition point of view, milk is an almost ideal food and considered to be perfect food. The human body needs more than thirty distinct materials in its food supply. No single food stuff supplies all of them but milk supplies nearly all. Since milk by its nature is perishable and an ideal product for bacterial growth, so it must be carefully handled, both during production and while processing. This chapter focuses on the technological (processing) aspects of the transformation of milk into its various consumer products, including beverages, fermented products, concentrated and dried products, butter and ice cream.

The various unit operations of food process engineering applied to solve the problem of changing the compositions of solutions, mixture and solids. Often these operations must be conducted on large scale to meet the demands of a large manufacturing plant. In food processes engineering these operations are often referred to as separation / concentration / extraction / purification process. Usually these operations are directed towards separating substances into its component parts or concentrating a particular substance from another.

It is well known that lowering the temperature of food stuffs reduces microbiological and biochemical spoilage and thus extending the shelf life of the product. In addition, it is generally agreed upon that the reaction rate is reduced by half by lowering the temperature by 10°C. Though low temperature preservation process has been recognized as a preservation technique for several hundred years, the major developments in its utilization have occurred only in the last century.

7.1 Introduction

Evaporation is an unit operation which converts a liquid into a vapour, leaving behind a concentrated solution or a higher boiling liquid. In other words, the goal of evaporation is to vaporize most of the water from a solution containing a desired product. Heat is the main tool in evaporation, and the process occurs more readily at high temperature and low pressures. Removal of water from foods provides microbiological stability, reduces deteriorative chemical reactions, and reduces transportation and storage costs. Evaporation can be distinguished from some other drying methods in that the final product of evaporation is a concentrated liquid, not a solid. It also differs from distillation, since the vapours produced in the evaporator are not further divided into fractions.

6.1 Introduction

In general, the terms drying and dehydration are interpreted as the removal of moisture from a food product. Removal of moisture from a food product offers an excellent protection against most common causes of food spoilage. The drying or dehydration process achieves such protection by reducing the moisture content of the food product to levels which are adequate to limit microbial growth or other adverse chemical reactions. During drying, heat and mass transfer occur simultaneously (Figure 6.1). Heat is transferred from the bulk gas phase (i.e. hot air used for drying) to the solid phase (i.e. food product to be dried), where part of it is utilized for vaporization and the remainder is taken up by the solid phase as sensible heat. Mass (i.e. moisture of food product) is transferred in the form of vapour through various resistances, first from the interior of the solid to the surface, then through vaporization at or near the surface and finally in transport of the vapour into bulk gas phase. Dehydration is the removal of moisture close to bone dry condition.

5.1 Introduction

The medium used in various unit operations of food process engineering such as heating, cooling, drying, mixing and transport processes, is moist air, which is a mixture of two substances. One of these is dry air which itself is a mixture of number of gases and the other is water vapour which may exist in a saturated or superheated state. The properties of moist air are called psychrometric properties and the subject, which involves determination of thermodynamic properties of moist air, is known as psychrometry. Thus, psychrometry is that branch of engineering science, which deals with the study of moist air i.e. dry air mixed with water vapour or humidity. Knowledge of properties of moist air is imperative in design of systems such as air- conditioning equipment for storage of fresh produce, dryers for drying of cereal grains, and cooling towers in food processing plants. In this chapter, important thermodynamic properties used in psychrometric computations are defined. Psychrometric charts useful in determining such properties are presented. In addition, various psychrometric process used in food process engineering are discussed.

4.1 Introduction

The food preservation may be define as the state in which the food may retained over a period of time without being contaminated by pathogenic organism or chemicals and without losing its colour, texture, flavour and nutritional value. All the methods of food preservation must provide such an environment in and around the preserved food so that it produces one or more unfavourable conditions to the continued activity of the spoilage agents. The various methods used for food preservations are: