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Unit Operations in Food Processing - R. L. Earle
Page 1
Unit Operations in Food Processing
Contents > Material & energy balances > Material Balances this page
CHAPTER 2
Home Contents About the book Introduction Material and energy balances Fluid-flow theory Fluid-flow applications Heat-transfer theory Heat-transfer applications Drying Evaporation Contact-equilibrium separationprocesses Mechanical separations Size reduction Mixing Appendices Index to Figures Index to Examples References Bibliography Useful links Feedback
MATERIAL AND ENERGY BALANCES (cont'd) MATERIAL BALANCES Basis and units total mass and composition concentrations Types of process situations continuous processes blending Layout The first step is to look at the three basic categories: materials in,materials out and materials stored. Then the materials in each category have to be considered whether they are to be treated as a whole, a gross mass balance, or whether various constituents should be treated separately and if so what constituents. To take a simple example, it might be to take dry solids as opposed to total material; this really means separating the two groups of constituents, nonwaterand water. More complete dissection can separate out chemical types such as minerals, or chemical elements such as carbon. The choice and the detail depend on the reasons for making the balance and on the information that is required. A major factor in industry is, of course, the value of the materials and so expensive raw materials are more likely to be considered than cheaper ones, and productsthan waste materials.
Basis and Units Having decided which constituents need consideration, the basis for the calculations has to be decided. This might be some mass of raw material entering the process in a batch system, or some mass per hour in a continuous process. It could be: some mass of a particular predominant constituent, for example mass balances in a bakery might be all related to100 kg of flour entering; or some unchanging constituent, such as in combustion calculations with air where it is helpful to relate everything to the inert nitrogen component; or carbon added in the nutrients in a fermentation system because the essential energy relationships of the growing microorganisms are related to the combined carbon in the feed; or the essentially inert non-oil constituentsof the oilseeds in an oil-extraction process. Sometimes it is unimportant what basis is chosen and in such cases a convenient quantity such as the total raw materials into one batch or passed in per hour to a continuous process are often selected. Having selected the basis, then the units can be chosen such as mass, or concentrations which can be weight or molar if reactions are important.
Totalmass and composition Material balances can be based on total mass, mass of dry solids, or mass of particular components, for example protein.
EXAMPLE 2.1. Constituent balance of milk Skim milk is prepared by the removal of some of the fat from whole milk. This skim milk is found to contain 90.5% water, 3.5% protein, 5.1% carbohydrate, 0.1% fat and 0.8% ash. If the original milk contained 4.5%http://www.nzifst.org.nz/unitoperations/matlenerg2.htm
28.04.2008 10:48:42
Unit Operations in Food Processing - R. L. Earle
Page 2
fat, calculate its composition assuming that fat only was removed to make the skim milk and that there are no losses in processing. Basis: 100 kg of skim milk. This contains, therefore, 0.1 kg of fat. Let the fat which was removed from it to make skimmilk be x kg. Total original fat = (x + 0.1 ) kg Total original mass = (100 + x) kg and as it is known that the original fat content was 4.5% so x + 0.1 100 + x whence x + 0.1 x = 0.045 = 0.045(100 + x) = 4.6 kg
So the composition of the whole milk is then fat = 4.5% , water protein carbohydrate and ash
= = = =
90.5 104.6 3.5 104.6 5.1 104.6 0.8 104.6
= = = =
86.5 % 3.3 % 4.9% 0.8%...
Page 1
Unit Operations in Food Processing
Contents > Material & energy balances > Material Balances this page
CHAPTER 2
Home Contents About the book Introduction Material and energy balances Fluid-flow theory Fluid-flow applications Heat-transfer theory Heat-transfer applications Drying Evaporation Contact-equilibrium separationprocesses Mechanical separations Size reduction Mixing Appendices Index to Figures Index to Examples References Bibliography Useful links Feedback
MATERIAL AND ENERGY BALANCES (cont'd) MATERIAL BALANCES Basis and units total mass and composition concentrations Types of process situations continuous processes blending Layout The first step is to look at the three basic categories: materials in,materials out and materials stored. Then the materials in each category have to be considered whether they are to be treated as a whole, a gross mass balance, or whether various constituents should be treated separately and if so what constituents. To take a simple example, it might be to take dry solids as opposed to total material; this really means separating the two groups of constituents, nonwaterand water. More complete dissection can separate out chemical types such as minerals, or chemical elements such as carbon. The choice and the detail depend on the reasons for making the balance and on the information that is required. A major factor in industry is, of course, the value of the materials and so expensive raw materials are more likely to be considered than cheaper ones, and productsthan waste materials.
Basis and Units Having decided which constituents need consideration, the basis for the calculations has to be decided. This might be some mass of raw material entering the process in a batch system, or some mass per hour in a continuous process. It could be: some mass of a particular predominant constituent, for example mass balances in a bakery might be all related to100 kg of flour entering; or some unchanging constituent, such as in combustion calculations with air where it is helpful to relate everything to the inert nitrogen component; or carbon added in the nutrients in a fermentation system because the essential energy relationships of the growing microorganisms are related to the combined carbon in the feed; or the essentially inert non-oil constituentsof the oilseeds in an oil-extraction process. Sometimes it is unimportant what basis is chosen and in such cases a convenient quantity such as the total raw materials into one batch or passed in per hour to a continuous process are often selected. Having selected the basis, then the units can be chosen such as mass, or concentrations which can be weight or molar if reactions are important.
Totalmass and composition Material balances can be based on total mass, mass of dry solids, or mass of particular components, for example protein.
EXAMPLE 2.1. Constituent balance of milk Skim milk is prepared by the removal of some of the fat from whole milk. This skim milk is found to contain 90.5% water, 3.5% protein, 5.1% carbohydrate, 0.1% fat and 0.8% ash. If the original milk contained 4.5%http://www.nzifst.org.nz/unitoperations/matlenerg2.htm
28.04.2008 10:48:42
Unit Operations in Food Processing - R. L. Earle
Page 2
fat, calculate its composition assuming that fat only was removed to make the skim milk and that there are no losses in processing. Basis: 100 kg of skim milk. This contains, therefore, 0.1 kg of fat. Let the fat which was removed from it to make skimmilk be x kg. Total original fat = (x + 0.1 ) kg Total original mass = (100 + x) kg and as it is known that the original fat content was 4.5% so x + 0.1 100 + x whence x + 0.1 x = 0.045 = 0.045(100 + x) = 4.6 kg
So the composition of the whole milk is then fat = 4.5% , water protein carbohydrate and ash
= = = =
90.5 104.6 3.5 104.6 5.1 104.6 0.8 104.6
= = = =
86.5 % 3.3 % 4.9% 0.8%...
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