Solids separation processes Foods Tables(Paul and Southgate, 1978). A simple two-component model can be used (n= 2; water and solids) or a multicomponent system. The density of the major ce nts are given as(kg m -)(Peleg, 1983) 1000 salt 2160 900950 citric acid 154 protein 1400 cellulose 1270-1610 sucrose 1500 glucose It is noteworthy that all solid components except fat are substantially more dense than water. However the differences between protein and the various types of carbohydrates are less marked, although minerals are much higher. In comparison air has a density of 1.27 kg m This equation is not applicable where there is a substantial volume fraction of air in the particle. Any deviation between the experimentally determined value and the value calculated from the above equation may mean that there is substantial air within the solid. An estimate of the volume fraction of air( va) can be made from p=VaPa +vsPs= vaPa+(l-va)Ps where Pa= density of air; Ps =density of solid(estimated using eq (9.2))and p=true lid density, measured experimentally. This volume fraction (Va) of air is sometimes known as the intenal porosity Many other foods contain substantial amounts of air, for example mechanically worked doughs. One solution to determine the unaerated density is to measure the dough density at different pressures and extrapolate back to zero pressure(absolute) to obtain the unaerated density. This methodology could then be used to determine the extent of aeration during the mixing process Note that from the compositional data, the calculated particle density of an apple is about 1064 kg m, Most apples float in water, indicating a density less than 1000 kg m-3 Mohsenin(1986)quotes a value of 846 kg m-3, suggesting an air content of about 20%. One important objective of blanching is to remove as much air as possible from fruit and vegetables prior to heat-treatment in sealed containers, to prevent exces- sive pressure development during their thermal processing. Data on the amount of air in fruits and vegetables are scarce in the food literature. There is evidence that this air is quickly displaced by water during soaking Data on particle densities are provided by Lewis(1990), Mohsenin(1986), and Hayes (916 kg m3 at 0 C). However, not all the water is likely to be frozen, even at-300 ted (1987). Note that if the food is frozen, the density of ice should be substi The particle density of dehydrated powders is considerably affected by the conditions of spray drying. Increasing the solids content of the feed to the drier will result in higher particle densities and bulk densities. High particle densities will enhance sinkability and ing and separation techniques, e.g. flotation, sedimentation and air classification ]l clean reconstitution properties. Differences in particle densities are exploited for seveSolids separation processes 25 1 Foods Tables (Paul and Southgate, 1978). A simple two-component model can be used (n = 2; water and solids) or a multicomponent system. The density of the major components are given as (kg m-3) (Peleg, 1983): water 1000 salt 2160 fat 900-950 citric acid 1540 protein 1400 cellulose 1270-1 6 10 sucrose 1590 starch 1500 glucose 1560 It is noteworthy that all solid components except fat are substantially more dense than water. However the differences between protein and the various types of carbohydrates are less marked, although minerals are much higher. In comparison air has a density of 1.27 kg m-3. This equation is not applicable where there is a substantial volume fraction of air in the particle. Any deviation between the experimentally determined value and the value calculated from the above equation may mean that there is substantial air within the solid. An estimate of the volume fraction of air (V,) can be made from P= &pa + V,P~ =VaPa +(l-va)Ps (9.4) where pa = density of air; ps = density of solid (estimated using eq. (9.2)) and p = true solid density, measured experimentally. This volume fraction (V,) of air is sometimes known as the internal porosity. Many other foods contain substantial amounts of air, for example mechanically worked doughs. One solution to determine the unaerated density is to measure the dough density at different pressures and extrapolate back to zero pressure (absolute) to obtain the unaerated density. This methodology could then be used to determine the extent of aeration during the mixing process. Note that from the compositional data, the calculated particle density of an apple is about 1064 kg m-3. Most apples float in water, indicating a density less than 1000 kg m-3, Mohsenin (1986) quotes a value of 846 kg m-3, suggesting an air content of about 20%. One important objective of blanching is to remove as much air as possible from fruit and vegetables prior to heat-treatment in sealed containers, to prevent exces￾sive pressure development during their thermal processing. Data on the amount of air in fruits and vegetables are scarce in the food literature. There is evidence that this air is quickly displaced by water during soaking. Data on particle densities are provided by Lewis (1990), Mohsenin (1986), and Hayes (1987). Note that if the food is frozen, the density of ice should be substituted (916 kg m-3 at 0°C). However, not all the water is likely to be frozen, even at -30°C. The particle density of dehydrated powders is considerably affected by the conditions of spray drying. Increasing the solids content of the feed to the drier will result in higher particle densities and bulk densities. High particle densities will enhance sinkability and reconstitution properties. Differences in particle densities are exploited for several clean￾ing and separation techniques, e.g. flotation, sedimentation and air classification