176 MISC. PUBLICATION 5 40, U. S. DEPT. OF AGRICULTURE 



and trimming cost is $60 per unprepared ton. we find (fig. 71) that the 

 cost per dry pound is 19 cents. 18 If a cheaper raw material, costing only 

 $50 per ton including trimming, is found to have a 30-percent prepara- 

 tion loss and a o 1 /^ to 1 drying ratio, the charts indicate that this ma- 

 terial also costs 19 cents per dry pound for raw material and trimming. 

 Quality of final product may be the deciding factor in this case, if 

 waste disposal is not an important problem. The operator will find 

 that similar graphs are applicable to other operating problems. 



Certain assumptions must be made and some caution exercised in the 

 use of graphs such as those shown in figures 73 and 74. For the pur- 

 pose of these graphs, it is assumed that all changes in weight during 

 preparation, either through loss of material or changes in water con- 

 tent, are included in the preparation loss. 



When wood trays are being used in the determination of prepara- 

 tion loss, errors may be caused by variation in tare weight of the trays, 

 caused by variations in wetness of the wood. This difficulty can be 

 largely overcome in test runs by wetting the trays before loading and 

 using the weight of the wet trays as the tare weight. The problem is 

 obviated by the use of metal trays. 



The changes in weight from the raw material, as purchased, to the 

 prepared material, ready for drying, include such items as dirt re- 

 moved in preliminary washing, culls graded out, the actual preparation 

 losses (peeling, coring, trimming, etc.). and leaching losses or water 

 pick-up during the peeling, cutting, washing, and blanching. Ex- 

 cessive washing after cutting and especially after blanching may re- 

 sult in losses of 10 percent or more on potatoes. 



In test runs, the drying ratio should be determined on the plant- 

 prepared material, ready to go to the dehydrator. rather than on small 

 samples. (Determination of dry ratios on samples of different raw 

 materials is. nevertheless, a valuable means of making preliminary 

 comparisons.) 



Inspection losses on the final product are frequently so small that 

 they can be neglected. If inspection losses are large enough to war- 

 rant a correction on the yield, this correction can be made by a deduc- 

 tion from the yields shown by the graphs for given values of prepara- 

 tion loss and drying ratio. It may be impossible to determine this in 

 advance of actual plant operations. If fines are removed from the 

 dry product before packaging, they may be either a total loss or may 

 have a market value at a lower price. This will require a further cor- 

 rection to the yield. The simplest procedure is to determine yield 

 after removal of the fines and credit the operations separately with 

 any return obtained from the sale of the fines. 



Failure to take account of the many factors tending to decrease the 

 final net packed weight may result in ruinously erroneous conclusions 

 regarding the probable over-all weight shrinkage from raw material 

 to final product. Solution of the problem outlined above should, how- 

 ever, be relatively easy in an operating plant, since it can be based 

 on actual operating tests. 



A greater danger lies in the assumptions that are likely to be made 

 in planning for a new plant where previous experience is not avail- 

 able on the raw material or process. The only data will be hypotheti- 



18 The preparation loss used for tbis purpose must be an overall preparation loss, deter- 

 mined in actual test runs, and must take account of the possible sources of error discussed 

 in the following paragraphs. 



