Properties of foods
Properties of liquids, solids and gases Liquids, gases and some solids (for example powders and particulate materials) are termed ‘fluids’ and can flow without disintegration when a pressure is applied to them. In contrast, solids deform when pressure is applied to them. The transition from solid to fluid and back is known as a phase transition and this is important in many types of food processing (e.g. water to water vapour in evaporation and distillation and dehydration; water to ice in freezing and freeze drying or freeze concentration or crystallisation of fats). Phase transition takes place isothermally at the phase transition temperature by release or absorption of latent heat, and can be represented by a phase diagram. A second type of transition, known as glass transition, takes place without the release or absorption of latent heat and involves the transition of a food to an amorphous glass state at its glass transition temperature. The transition is dependent on the temperature of the food, time, and the moisture content of the food. When materials change to glasses, they do not become crystalline, but retain the disorder of the liquid state. However, their physical, mechanical, electrical and thermal properties change as they undergo the transition. In their glassy state, foods become very stable because compounds that are involved in chemical reactions that lead to deterioration are immobilised and take long periods of time to diffuse through the material to react together. Processes that are significantly influenced by transition to a glassy state include aroma retention, crystallisation, enzyme activity, microbial activity, non-enzymic browning, oxidation, agglomeration and caking.
Density and specific gravity
A knowledge of the density of foods is important in separation processes, and differences in density can have important effects on the operation of size reduction and mixing equipment. The density of a material is equal to its mass divided by its volume. The density of materials is not constant and changes with temperature (higher temperatures reduce the density of materials) and pressure.
The density of liquids is a straightforward measure of mass/volume at a particular temperature, but for particulate solids and powders there are two forms of density: the density of the individual pieces and the density of the bulk of material, which also includes the air spaces between the pieces. This latter measure is termed the bulk density and is ‘the mass of solids divided by the bulk volume’. The fraction of the volume that is taken up by air is termed the porosity and is calculated by:
where Va= volume of air (m3) and Vb= volume of bulk sample (m3).
The bulk density of a material depends on the solids density and the geometry, size and surface properties of the individual particles. The density of liquids can be expressed as specific gravity (SG), a dimensionless number, which is found by dividing the mass (or density) of a liquid by the mass (or density) of an equal volume of pure water at the same temperature:
SG = mass of liquid/mass water
SG = density of liquid/density water
Specific gravity is widely used instead of density in brewing and other alcoholic fermentations, where the term ‘original gravity (OG)’ is used to indicate the specific gravity of the liquor before fermentation.
Viscosity is an important characteristic of liquid foods in many areas of food processing. For example the characteristic mouth feel of food products such as tomato ketchup, cream, syrup and yoghurt is dependent on their consistency or viscosity. The viscosity of many liquids changes during heating, cooling, concentration, etc. and this has important effects on, for example, the power needed to pump these products. Viscosity may be thought of as a liquid’s internal resistance to flow. A liquid can be envisaged as having a series of layers and when it flows over a surface, the uppermost layer flows fastest and drags the next layer along at a slightly lower velocity, and so on through the layers until the one next to the surface is stationary. The force that moves the liquid is known as the shearing force or shear stress and the velocity gradient is known as the shear rate. If shear stress is plotted against shear rate, most simple liquids and gases show a linear relationship and these are termed Newtonian fluids. Examples include water, most oils, gases, and simple solutions of sugars and salts. Where the relationship is non-linear, the fluids are termed ‘non-Newtonian’. For all liquids, viscosity decreases with an increase in temperature but for most gases it increases with temperature. Many liquid foods are non-Newtonian, including emulsions and suspensions, and concentrated solutions that contain starches, pectins, gums and proteins. These liquids often display Newtonian properties at low concentrations but as the concentration of the solution is increased, the viscosity increases rapidly and there is a transition to non- Newtonian properties. Non-Newtonian fluids can be classified broadly into the following types:
Changes in viscosity of Newtonian fluid (A) and different types of non-Newtonian fluids;(B) pseudoplastic fluid; (C) dilatant fluid; (D) Bingham plastic fluid and (E) Casson plastic fluid.
A large number of foods comprise two or more immiscible components, which have a boundary between the phases. The phases are known as the dispersed phase (the one containing small droplets or particles) and the continuous phase (the phase in which the droplets or particles are distributed). One characteristic of these systems is the very large surface area of the dispersed phase that is in contact with the continuous phase. In order to create the increased surface area, a considerable amount of energy needs to be put into the system using for example a high-speed mixer or an homogeniser. Droplets are formed when new surfaces are created. To understand the reason for this it is necessary to know the forces acting in liquids: within the bulk of a liquid the forces acting on each individual molecule are equal in all directions and they cancel each other out. However, at the surface the net attraction is towards the bulk of the liquid and as a result, the surface molecules are ‘pulled inwards’ and are therefore in a state of tension (produced by surface tension forces). This causes liquid droplets to form into spheres because this shape has the minimum surface area for the particular volume of liquid.
Chemicals that reduce the surface tension in the surface of a liquid are termed surface active and are known as ‘surfactants’, ‘emulsifying agents’ or ‘detergents’. By reducing the surface tension, they permit new surfaces to be produced more easily when energy is put into the system (for example by homogenisers) and thus enable larger numbers of droplets to be formed. There are naturally occurring surfactants in foods, including alcohols, phospholipids and proteins and these are sometimes used to create food emulsions (for example using egg in cake batters). However, synthetic chemicals have more powerful surface activity and are used in very small amounts to create emulsions. Others are used in detergents for cleaning operations.Surface active agents contain molecules which are polar (or ‘hydrophilic’) at one end and non-polar (or ‘lipophilic’) at the other end. In emulsions, the molecules of emulsifying agents become oriented at the surfaces of droplets, with the polar end in the aqueous phase and the non-polar end in the oil phase. Detergents are surface active agents that reduce the surface tension of liquids to both promote wetting (spreading of the liquid) and to act as emulsifying agents to dissolve fats. The detergent molecules have a lipophilic region of long chain fatty acids and a hydrophilic region of either a sodium salt of carboxylic acid (soapy detergents) or the sodium salt of an alkyl or aryl sulphonate (anionic detergents). Anionic detergents are not affected by hard water, whereas soapy detergents form a scum in hard water. Non-ionic detergents, which have alcohols, esters or ethers as the hydrophilic component, produce little foam and are easily rinsed off. Enzymes may also be added to detergents to remove proteins, and other ingredients may include polyphosphates (to soften water and keep dirtin suspension), sodium sulphate or sodium silicate (to make detergent powder freeflowing) and sodium perborate (bleaching agent).
Foams are two-phase systems which have gas bubbles dispersed in a liquid or a solid, separated from each other by a thin film. In addition to food foams, foams are widely used for cleaning equipment. The main factors needed to produce a stable foam are:
• a low surface tension to allow the bubbles to contain more air and prevent them contracting
• gelation or insolubilisation of the bubble film to minimise loss of the trapped gas and to increase the rigidity of the foam and
• a low vapour pressure in the bubbles to reduce evaporation and rupturing of the film.
In food foams, the structure of the foam may be stabilised by freezing (ice cream), by gelation (setting gelatin in marshmallow), by heating (cakes, meringues) or by the addition of stabilisers such as proteins or gums.
Rheology and texture
The texture of foods has a substantial influence on consumers’ perception of ‘quality’ and during chewing (or ‘mastication’), information on the changes in texture of a food is transmitted to the brain from sensors in the mouth, from the sense of hearing and from memory, to build up an image of the textural properties of the food. This may be seen as taking place in a number of stages:
1. an initial assessment of hardness, ability to fracture and consistency during the first bite
2. a perception of chewiness, adhesiveness and gumminess during chewing, the moistness and greasiness of the food, together with an assessment of the size and geometry of individual pieces of food
3. a perception of the rate at which the food breaks down while chewing, the types of pieces formed, the release or absorption of moisture and any coating of the mouth or tongue with food.
These various characteristics have been categorised and used to assess and monitor the changes in texture that affect the quality of foods. Rheology is the science of deformation of objects under the influence of applied forces. When a material is stressed it deforms, and the rate and type of deformation characterise its rheological properties. A large number of different methods have been used to assess the texture of food, including texture profiling by sensory methods using taste panels (e.g. Bourne, 1982), Quantitative Descriptive Analysis (QDA), described by Clark (1990), and empirical methods in which measurements of the forces needed to shear, penetrate, extrude, compress or cut a food are related to a textural characteristic. These methods are reviewed by Kilcast (1999), Rosenthal (1999), Lawless and Heyman (1998) and Brennan (1984).
Examples of these instrumental methods include the Brabender system to measure dough texture or the viscosity of starch pastes, cone or rod penetrometers to measure the yield stress of margarine and spreads or the hardness of fruits, the General Foods Texturometer which simulates mastication by compressing foods using a plunger, and the Instron Universal Testing machine, which measures stress and strain forces by compression or extension. Chemical methods include measurement of starch or pectin content, and microscopic methods include electron microscopy of emulsions or the flesh structure of meat and fish. These methods are described in detail by Sherman (1979), Prentice (1984), Bourne (1978), Brennan (1984), Kramer and Szczesniak (1973), Lewis (1990) and Bourne (1982).
The transfer of matter is an important aspect of a large number of food processing operations: it is a key factor in solvent extraction, distillation and membrane processing and it is an important factor in loss of nutrients during blanching. Mass transfer of gases and vapours is a primary factor in evaporation, dehydration , baking and roasting, frying, freeze drying, the cause of freezer burn during freezing and a cause of loss in food quality in chilled, MAP and packaged foods.
In an analogous way to heat transfer, the two factors that influence the rate of mass transfer are a driving force to move materials and a resistance to their flow. When considering dissolved solids in liquids, the driving force is a difference in the solids concentration, whereas for gases and vapours, it is a difference in partial pressure or vapour pressure. The resistance arises from the medium through which the liquid, gas or vapour moves and any interactions between the medium and the material.
An example of materials transfer is diffusion of water vapour through a boundary layer of air in operations such as dehydration, baking, etc. Packaging also creates additional boundary layers which act as barriers to movement of moisture and to heat transfer.
Barriers to mass transfer and heat flow due to packaging.
The law of conversion of mass states that ‘the mass of material entering a process equals the mass of material leaving’. This has applications in mixing,fermentation, and evaporation.
In general a mass balance for a process takes the following form:
mass of raw materials in= mass of products and wastes out+ mass of stored materials+losses
Many mass balances are analysed under steady-state conditions where the mass of stored materials and losses are equal to zero.
Many types of liquid food are transported through pipes during processing, and powders and small-particulate foods are more easily handled as fluids (by fluidisation). Gases obey the same laws as liquids and, for the purposes of calculations, gases are treated as compressible fluids. The study of fluids is therefore of great importance in food processing. It is divided into fluid statics (stationary fluids) and fluid dynamics (moving fluids). A property of static liquids is the pressure that they exert on the containing vessel. The pressure is related to the density of the liquid and the depth or the mass of liquid in the vessel. Liquids at the base of a vessel are at a higher pressure than at the surface, owing to the weight of liquid above (the hydrostatic head). This is important in the design of holding tanks and processing vessel, to ensure that the vessel is constructed using materials of adequate strength. A large hydrostatic head also affects the boiling point of liquids, which is important in the design of some types of evaporation equipment. When a fluid flows through pipes or processing equipment, there is a loss of energy and a drop in pressure which are due to frictional resistance to flow. These friction losses and changes in the potential energy, kinetic energy and pressure energy are described in detail in the food engineering texts referenced at the start of this chapter and by Loncin and Merson (1979). The loss of pressure in pipes is determined by a number of factors including the density and viscosity of the fluid, the length and diameter of the pipe and the number of bends, valves, etc., in the pipeline. To overcome this loss in energy, it is necessary to apply power using pumps to transport the fluid. The amount of power required is determined by the viscosity of the fluid, the size of the pipework, the number of bends and fittings, and the height and distance that the fluid is to be moved.