Introduction to Food Process Engineering

A process may be thought of as a sequence of operations which take place in one or more pieces of equipment, giving rise to a series of physical, chemical or biological changes in the feed material and which results in a useful or desirable product. More traditional definitions of the concept of process would not include the term biological but, because of the increasing sophistication, technological advance and economic importance of, the food industry, and the rise of the biotechnology industries, it is ever more relevant to do so.

The dimensions of all physical quantities can be expressed in terms of the four basic dimensions: mass, length, time and temperature. Thus velocity has the dimensions of length per unit time and density has the dimensions of mass per unit length cubed. A system of units is required so that the magnitudes of physical quantities may be determined and compared one with another. The internationally agreed system which is used for science and engineering is the Systeme International d’Unites, usually abbreviated to SI. Table 2.1 lists the SI units for the four basic dimensions together with those for electrical current and plane angle which, although strictly are derived quantities, are usually treated as basic quantities. Also included is the unit of molar mass which somewhat illogically is the gram molecular weight or gram mole and which is usually referred to simply as a ‘mole’. However, it is often more convenient to use the kilogram molecular weight or kmol.

Thermodynamics takes its name from the Greek for ‘movement of heat’ and is the science concerned with the interchange of energy, particularly that between thermal energy and mechanical work. Thermodynamics is concerned with systems which have come to equilibrium and not with the rate at which equilibrium is achieved. Two examples can be used to illustrate this point. Chemical thermodynamics describes the extent to which a chemical reaction proceeds based upon a knowledge of the total quantity of energy involved, in contrast to chemical kinetics which attempts to describe and predict the rate at which a chemical reaction takes place. Perhaps more pertinent to the aim of this book, thermodynamics is able to specify the thermal energy changes required to bring about certain physical changes in a system: an increase or decrease in temperature; a change of state from liquid to vapour or from solid to liquid. Thus thermodynamics predicts the heat input required to raise the temperature of a food, to evaporate water from an aqueous food solution or to thaw a block of frozen food. It does not, however, have anything to say about the rate at which the transfer of thermal energy should or can take place, the latter is the province of heat transfer which is covered in Chapters 5 and 7. Thus thermodynamics is concerned with the initial and final states of a process and not with how the movement between those states is achieved. It is perhaps, cosmology and quantum mechanics apart, the most philosophical of all the sciences and indeed it underpins even those subjects.

Before a food manufacturing process can be installed and commissioned into a factory, it must be designed and the size and nature of the equipment to be used must be specified. However, before this stage an analysis of the process must be undertaken to determine, inter alia, the quantities of product and raw materials to be handled. This process analysis step consists largely of a material balance and an energy balance. Process analysis itself is preceded by a concept stage at which the broad outline of the food manufacturing process is decided upon. For example, the concept may involve, for a given product and a given rate of production, a series of operations such as mixing, evaporation, pasteurisation, filtration, drying, freezing and packaging with specified values of important quantities at each stage. In other words an answer must be found to the question, How is the product to be manufactured? When the concept has been established the very first step in the process analysis is a material balance. This allows the mass flow rate and composition of the various process streams to be determined. Thus the necessary inputs for a given production rate can be calculated or alternatively the output rate for given raw material feed rates can be determined. It is often the case in food processes that the material balance is relatively trivial, perhaps because relatively few components and few process steps are involved, although in such cases the procedure described in the next section is still extremely useful. Even where the material balance is trivial the energy balance (often reduced to a ‘heat’ balance) is usually very important; few food processes do not employ heat at some stage. In many instances (freezing, evaporation, pasteurisation) the addition or removal of heat is the substance of the manufacturing process and may involve very considerable quantities of energy. Only when the material and energy balances are complete can the detailed design or specification of process equipment begin.

Any analysis of the processing of foods must be based on a thorough understanding of the transfer of heat, of momentum and of mass. It is self-evident that a knowledge of heat transfer to and within foods (both solids and fluids) is vitally important; simply consider the following list of common processing operations, all of which involve the transfer of heat: sterilisation, evaporation, freezing, drying. The theory of heat transfer will be developed in Chapter 7. Momentum transfer will be developed into a treatment of fluid flow in Chapter 6 where the concern is the behaviour of both Newtonian and non-Newtonian fluids (amongst the latter may be numbered very many foodstuffs), especially in pipe flow. Mass transfer (Chapter 8) is very much underused in the study of food processing. Many operations, of which drying is a good example, involve both heat and mass transfer and an analysis of both is needed for a full understanding of such processes.

This chapter develops the concept of momentum transfer into a treatment of the flow of fluids over surfaces and in pipes and ducts. A later chapter will deal with the behaviour of food particles in a fluid stream. Many foods are of course liquid and the study of fluid flow, or fluid mechanics, is necessary to understand how fluids are transported, how they can be pumped, mixed and so on. The high viscosity of many liquid foods means that laminar flow is particularly important. However, very many foodstuffs are non-Newtonian and later sections of this chapter cover a wide variety of rheological models; these are treated as mathematical descriptions of physical behaviour with the objective of enabling the reader to apply models to experimental data in order to determine whether or not they can be used predictively.

Very many food processing operations involve the transfer of heat: cooking, roasting, drying, evaporation, sterilisation (either of bulk liquids or of packaged foods), chilling and freezing are utilised to preserve food or to prepare it directly for eating. Thus the student of food engineering needs a thorough understanding of the mechanisms of the transfer of heat together with a knowledge of heat exchange equipment. Many applications of heat transfer to food processing require a knowledge of unsteady-state theory; this is particularly true of freezing, for example. However, it is undoubtedly easier to grasp the principles of heat transfer by studying steady-state processes first and, although steady state is simply a special case of the general unsteady-state theory, the approach adopted here is to study the former first. More complex problems will be introduced only when steady-state heat exchange has been covered in detail.

Mass transfer is concerned with the movement of material in fluid systems, that is both gases or liquids, under the influence of a concentration gradient. As we saw in Chapter 5, this is analogous to the movement of heat under the influence of a temperature gradient. For example, in drying operations water is removed in vapour form from either a liquid or a solid food into a warm gas stream (usually air). Thus the mass transfer of water occurs because there is a high concentration of water in the food and a lower concentration of water in the air. Most examples of mass transfer in food processes involve the transfer of a given component from one phase across an interface to a second phase. Some examples are listed in Table 8.1

Psychrometry is concerned with the behaviour of humid air and the prediction of its properties. More strictly it covers the behaviour of any vapour (not just water vapour) when mixed with a gas (not just air). However, because the air/water system is of huge importance, not least to food processing, and is the most commonly encountered gas/vapour mixture, the terminology employed often appears to be specific to air and water. Prediction of the properties of moist air, for example, humidity, maximum possible humidity, temperature, density and so on, is especially important in drying operations. The psychrometric chart is a simple graphical method of presenting this information, and the principles behind the chart are explained in this chapter. The outcome of a drying process can often be followed more easily by monitoring the condition of the air with the aid of a psychrometric chart than by directly measuring the moisture content of the substance being dried. Of equal interest is the establishment of the correct atmospheric conditions for food storage.

Chapter 7 dealt only with steady-state heat transfer where conditions are constant with time. This is sufficient for an understanding of heat exchange in a continuous process, for example a heat exchanger where the flow rates of the two fluids and their inlet temperatures remain constant. However, an analysis of unsteady-state heat transfer is required for a proper understanding of both freezing and sterilisation which are batch processes and where heating or cooling is stopped when a pre-determined temperature is reached. At steady state the temperature profile in a body through which heat is being transferred remains constant with time and the rate of heat transfer is also constant. Consider as an example the slab shown in Fig. 7.1. The temperature at each surface is constant with time and therefore the temperature gradient (equal in this case to the temperature difference divided by the slab thickness) remains constant. However, if one surface of the slab is suddenly increased in temperature the temperature profile will change immediately and adjust over a period until equilibrium is re-established. Figure 10.1 shows the temperature profile in a food block at successive times t ₁, t ₂ and t ₃ after the surface temperature is changed from T ₀ to T ₁ at time \(t = 0\). Thus the temperature at any given point changes with time and it becomes important to be able to predict this change and to determine how long it is required for thermal equilibrium to be regained.

Freezing has long been used to preserve high-value food products such as meat; fish; particular foods where the quality of the frozen product is significantly better than the alternative, such as peas; and increasingly for other convenience foods ranging from chipped potatoes to complete ready meals. Lowering the temperature of foodstuffs reduces microbiological and biochemical spoilage by decreasing microbial growth rates and by removing liquid water which then becomes unavailable to support microbial growth. Freezing refers to the storage of food at temperatures between −18 and −30°C. In general lower storage temperatures give a longer shelf life. For example soft fruits may be stored for between 3 and 6 months at −12°C but for 24 months and beyond at −24°C. Most meat has a shelf life of 6–9 months at −12°C and this increases to between 15 and 24 months at temperatures down to −24°C. In contrast, chilling is defined by a storage temperature range between −1 and 8°C and is used for meat, fish, dairy products and chilled recipe dishes prior to consumption. It has little or no effect on the nutritional content or organoleptic properties of food. Similarly, the freezing process itself has little or no effect on the nutritional value of frozen foods. Conversely the quality of the initial raw material cannot be improved by freezing and only high-quality raw materials should be selected for freezing. Thus the quality and nutrient content of any food at the point of consumption are dependent upon the quality of the original raw material, the length of storage and the storage conditions and the extent and nature of the freezing process.

Evaporation is a food preservation technique in which dilute liquid foods and solutions are concentrated by the evaporation of water, with the aim of increasing microbiological stability and shelf life. A second major reason for the concentration of liquids is the reduction in transport and storage costs which can be achieved by reducing the product bulk volume. In this way concentrated liquids can be transported at relatively low cost and water added later, closer to the point of sale. In addition, evaporation can be used to increase the concentration of solutions prior to the removal of the remaining water by drying, particularly by spray drying. This is an attractive option because high-efficiency evaporation is significantly less costly than drying and other methods of removing water as the data in Table 12.1 show.

Any sample of solids whether natural (such as the grains of sand on a beach) or manufactured (such as particles of spray-dried milk) will contain a distribution of particle sizes. The size of particulate material is an important product characteristic, both because of its effect on product appearance, dissolution, flowability and because particle size determines the behaviour of powders in process equipment. It is necessary therefore to be able to measure the particle size distribution and to present the data in an intelligible way.

The mixing and/or agitation of liquids, solids and (to a lesser extent) gases is one of the commonest of all operations in the food processing industries. Of the possible combinations of these states, those of principal interest are liquid–liquid mixtures, solid–solid mixtures and liquid–solid mixtures or pastes. However, it is important at this early stage to define exactly what is meant by the terms ‘agitation’ and ‘mixing’ and it is perhaps easiest to do this by considering liquid–liquid systems. The agitation of a liquid may be defined as the establishment of a particular flow pattern within the liquid, usually a circulatory motion within a container. On the other hand mixing implies the random distribution, throughout a system, of two or more initially separate ingredients.

Distillation is the process in which the separation of components in a mixture is achieved due to differences in volatility, that is differences in vapour pressure, of the components in the mixture to be separated. The simplest kind of batch distillation process results in only a limited increase in the concentration of the more volatile component (MVC) in the distillate; a greater degree of separation can be obtained by using a fractionating column. In the food industry, distillation is used in the purification of raw alcohol and the production of beverage alcohol from wheat; the separation of flavour components, for example from the condensed vapour resulting from evaporation of fruit juices; as an important step in the production of bioethanol from sugar beet; and in the production of food grade white oils which are used as binders or coating materials. Other applications of distillation are the recovery, concentration and fractionation of aromas; the recovery of solvents from miscella; and the concentration and recovery of solvents used in extraction process such as the recovery of isopropanol used in the extraction of pectin from fruit peel.

The growing demand for safer food of ever higher quality has led to the investigation of a range of techniques which may together be labelled as minimum processing technologies. The principles of some of these techniques are outlined in this chapter. A number of the methods employ dielectrical heating either by applying an electrical current directly to the food as in ohmic heating or by exposing the food to electromagnetic radiation such as microwaves and radio frequencies. Of these, microwave heating is by far the most successful and widely used and was treated separately in Chapter 7. Other, non-thermal, methods such as the use of very high pressures, ionising radiation or high-energy sound waves are either being used increasingly in commercial production or under active development. It is probable that the non-thermal methods of food preservation are best used in combination with conventional convective heating so as to reduce the need to expose food to high temperatures for long periods. Therefore they offer the potential of delivering processed food with better retention of nutrients and with improved sensory properties.