Food Processing Technology

• Liquids, gases and some solids, such as powders and particulate materials, are referred to as fluids and can flow without disintegration when a pressure is applied to them.
• Solids deform when pressure is applied to them.
• The properties of fluids and solids that are relevant to both the design of food processes and the quality of processed food are described in this section.
• Mathematical treatments and derivations of formulae used in food engineering calculations are given in texts including Toledo (1999), Lewis (1990), Brennan et al (1990) and Earle (1983).
• The transition from solid to fluid and back is known as a phase transition and is important in many types of food processing, such as water to water vapour in evaporation and distillation, water to ice in freezing, and dehydration.
• 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.
• Examples of glass transition temperatures are given in Chapter 21 (Table 21.2).
• Moisture content is the weight of water in a food, expressed as a percentage of the food's dry weight.
• Water activity is related to the moisture content by the equation a w = P / P 0, where P (Pa) is the vapour pressure of the food, P 0 (Pa) is the vapour pressure of pure water at the same temperature.
• When materials change to glasses, they do not become crystalline, but retain the disorder of the liquid state.
• A proportion of the total water in a food is strongly bound to specific sites such as hydroxyl groups of polysaccharides, carbonyl and amino groups of proteins, and hydrogen bonding.
• When all sites are occupied by adsorbed water, the moisture content is termed the BET monolayer value.
• Typical examples of foods with a BET monolayer value include gelatin (11%), starch (11%), amorphous lactose (6%) and whole spray-dried milk (3%).
• Water activity is defined as the ratio of the vapour pressure of water in a food to the saturated vapour pressure of water at the same temperature.
• The time needed to destroy 90% of the micro-organisms is referred to as the decimal reduction time or D value.
• The slope of the TDT curve is termed the z value and is defined as the number of degrees Celsius required to bring about a ten-fold change in decimal reduction time (10.5 º C in Fig 1.14).
• Forced convection takes place when a stirrer or fan is used to agitate the fluid, this reduces the boundary film thickness to produce higher rates of heat transfer and a more rapid temperature redistribution, consequently, forced convection is more commonly used than natural convection in food processing.
• The rate of destruction is a first-order reaction; that is when food is heated to a temperature that is high enough to destroy contaminating micro-organisms, the same percentage die in a given time interval regardless of the numbers present initially.
• In freeze drying, the reduction in atmospheric pressure influences the thermal conductivity of the food.
• The death rate curve is a graphical representation of the logarithmic order of death.
• Ice has a higher thermal conductivity than water and this is important in determining the rate of freezing and thawing.
• The thermal wheel is a type of heat exchanger.
• Thermal conductivity is related to the thermal conductivity, specific heat and density of a food by a = k  c 1 : 13.
• The logarithmic order of death is described by a death rate curve.
• The factors that influence the temperature change are: the temperature of the heating medium, the thermal conductivity of the food, the specific heat of the food, and the position in the food.
• The Rotaire wheel is a type of heat exchanger.
• The physical, mechanical, electrical and thermal properties of foods change as they undergo the transition to a glassy state.
• The temperature therefore changes continuously.
• The thermal death time (TDT) curve is constructed by collating D values at different temperatures.
• When a fluid changes temperature, the resulting changes in density establish natural-convection currents, examples include natural-circulation evaporators, air movement in chest freezers, and movement of liquids inside cans during sterilisation.
• D values differ for different microbial species, and a higher D value indicates greater heat resistance.
• Reduction in moisture content causes a substantial reduction in thermal conductivity, which has important implications in unit operations which involve conduction of heat through food to remove water such as drying, frying and freeze drying.
• During processing, the temperature at a given point within a food depends on the rate of heating or cooling and the position in the food.
• The basic equation for unsteady-state heat transfer in a single direction (x) is d  d t ˆ k  c d 2  d x 2 1 : 14.
• Glassy 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.
• There are a number of definitions of 'quality' of foods, which are discussed by Cardello (1998).
• Water is a product of the condensation reaction in browning and, at higher moisture levels, browning is slowed by end product inhibition.
• Oxidation of lipids occurs at low a w values owing to the action of free radicals.