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Historic, archived document 

Do not assume content reflects current 
scientific knowledge, policies, or practices. 

November 1948 

Eastern Regional Research Laboratory 



Pennsylvan ia 


By Paul W. Edwards, Clifford S. Redfield, Albert Eoersch, Jr. 

and Roderick K. Eskew 1 
Chemical Engineering and Development Division 







Washing I 

Grinding 2 

Mixing and drying 2 

Bagging 2 

Investment 2 

Alternate method „ 2 


Potentialities of the process H 

Alternate method q. 



Based on pilot-plant experience, methods are described for 
the use of steam tube driers in producing stock feed and 
flour and meal from ground, raw white potatoes. A plant 
processing 75 tons of potatoes daily, would produce about 
17.3 tons of feed, at a cost of about $24.40 per ton. Such a 
plant would cost approximately $.80,000. For the manufacture 
of flour and meal, an investment of about $87,500 would be 
required to make 16.6 tons daily, at a cost of about $39.00 
per ton. These estimates are based on 170 days operation and 
include all costs except that of the potatoes and sales ex- 


By Paul VI. Edwards, 1 Clifford S. Redf ield, 2 Albert lioersch, Jr. 3 

and Roderick K. Eskew 4 


The recurring surpluses of white potatoes and the large number of culls have 
given rise to a need for cheap methods of converting them to a stable form in 
which they may be conveniently stored, shipped, and used for feed or industrial 
chemicals. This circular describes such a method. 

This year part of the surplus will be used to supply the demand for about 448 
million pounds of potato flour and meal required by the Commodity Credit Cor- 
poration for export Since our normal, annual capacity for producing potato 
flour is only 25 million pounds, it was necessary to develop methods for quick- 
ly producing the much- needed flour. Bureau of Agricultural and Industrial 
Chemistry AIC-190, "Utilization of Idle Equipment in Distilleries for Produc- 
tion of White Potato Flour," describes such a method developed at the Eastern 
Regional Research Laboratory. Another simple method is described here. This 
method also uses equipment which may be idle in distilleries or other plants or 
which can be readily assembled. 


The equipment described below is that required for a factory processing 75 tons 
of potatoes in 24 hours, producing 17.31 tons of feed containing 10 percent 
moisture. This yield is based on a 2 percent loss of potatoes during processing 
A diagrammatic flow sheet of the process is shown in Eigure 1. 

Washing: The potatoes are generally conveyed from freight cars or storage bins 
into the plant by water in a flume. They pass first to the washer. A washer 
such as that ordinarily employed in potato starch factories is well suited for 
this purpose. This consists of a U-shaped trough divided into sections and 
equipped with rotating paddles, which keep the potatoes in rapid motion and 
lift them from one compartment to the next. The stones and much of the dirt 
settle in the bottom of the trough and are periodically removed by flushing. A 
trough approximately 2-1/2 feet in diameter and 25 feet long with four compart- 
ments will properly wash at least 75 tons of potatoes in 24 hours, using about 
25 gallons of water per minute. 

In Charge of Potato Products Development Section. Chemical Engineers ng and Development 
D v s I on . 

Cost Analyst. Chemical Engineering and Development Di vi sion. 
Chemical Engineer. Chemical Engineering and Development Division. 
Head Chemical Engineering and Development Di v s on. 


Grinding: To reduce the potatoes to a form suitable for drying, they are 
ground .in a hammer mill equipped with a screen having holes 1/4 inch in di- 
ameter. Ordinary blunt hammers may be used. One mill 6 inches wide by 12 
inches in diameter driven with a 7-l/2-horsepower motor, should have sufficient 
capacity. The speed of the mill should be sufficient to give a hammer tip 
speed of about 6,500 feet per minute. Much higher tip speeds may require a 
coarser screen. 

Mixing and Drying: The ground potatoes will contain on the average 21.2 per- 
cent solids 5 and 78.8 percent moisture. To prevent the material from sticking 
to the tubes of the drier, the moisture must be reduced to about 43 percent. 
This is accomplished by continuously recycling a sufficient quantity of the 
dried product and mixing .it with the moist feed. It is important that the dry 
product and the ground potatoes be mixed thoroughly before they enter the drier. 
A paddle- type mixer-conveyor 2 feet in diameter and 20 feet long and driven by 
a 3 -horsepower motor should be satisfactory. A sufficient quantity of dried 
material must be provided before the drier is put into operation. Thereafter, 
recycling 1. 1 pounds of the dried product containing 10 percent moisture per 
pound of ground potatoes containing 78.8 percent moisture will maintain a mois- 
ture content of 43 percent going to the drier.. A steam tube drier handling 
this product will evaporate about 1,4 pounds of water per square foot of tube 
heating surface per hour, when steam .is used at a pressure of 100 pounds per 
square .inch. Thus for a factory of this size, approximately 3, 600 square feet 
of tube heating surface will be required. This can be obtained by using two 
steam tube driers 6 feet in diameter by 35 feet long. 

Bagging: That portion of the dried material not required for recycling is 
drawn off and bagged. Ordinary paper shipping bags, each holding approximately 
100 pounds, may be used. The product '.is a dark granular material suitable for 
feed. It has a bulk density of about 45 pounds per cubic foot 

Investment: A factory costing approximately $80,000 (Table I) could produce 
about 17.3 tons of product per 24 hour day, at a cost of about $24.40 per ton 
(Table II) . This figure includes all costs except that of the potatoes and 

sales expense for the product. 

Alternate Method: One might logically inquire why the ground potatoes could 
not be pressed to reduce drying costs. Although such a step is entirely feasi 
ble (if about 0.8 percent lime is added), it is not generally desirable because 
the press effluent may constitute a stream contamination problem. Furthermore, 
up to 20 percent of the total solids in the potato may be lost in this effluent* 
a loss which would more than offset any economy in drying. Although some of 
the .insoluble solids in the effluent might be recovered by settling, cost 
calculations indicate that it is cheaper not to press. 

5 morri son Feeds and Feeding. 20th Edition. P. 970 The Morrison Publishing Company Ithaca 
New York 1939 . 





The same basic method described for the production of feed can be adapted to 
the production of potato flour and meal. This product will be slightly more 
cream colored than the highest quality potato flour made from cooked, peeled 
potatoes. It compares favorably with flour now being produced commercially 
with drum driers from cooked unpeeled potatoes. 

The various steps are the same as those described for the production of feed 
with the following exceptions: 

Because the product is to be used for food, the washed potatoes would have to 
be sorted to remove spoiled parts. Sulfur dioxide should be added to the 
ground potatoes to maintain a good color, and the moisture content must be re- 
duced to not more than 9 percent. The dried product must also be ground and 
screened to produce flour or meal. Figure 2 is a diagrammatic flow sheet of 
the process. 

The sulfur dioxide required to maintain the color is not more than 0.2 percent 
of the wet weight of the potatoes. It is believed that in most cases less than 
0.1 percent would be sufficient. The sulfur dioxide treatment can be carried 
on effectively in a wooden tank 4 feet In diameter and 5 feet high, equipped 
with a bronze turbine-type agitator. The sulfur dioxide can be fed continu- 
ously through a flowmeter at a rate commensurate with the rate of ground pota 
toes entering the tank. Since the average retention time is about half an 
hour, the tank also serves as a reservoir to equalize discrepancies in oper- 
ating rates. The ground potatoes are delivered from the tank to the mixer by a 
positive delivery pump. 

The moisture content must be reduced in the drier to not more than 9 percent. 
A drier handling this product will evaporate about 1.3 pounds of water per 
square foot of tube heating surface per hour when steam is used at a pressure 
of 100 pounds per square inch. Two driers 6 feet in diameter. 35 feet long and 
each having an evaporative area of 1,800 square feet will handle 75 tons of 
potatoes .in 24 hours, making 16.6 tons of product. The lower yield of flour, 
as compared with that of feed, results from a higher loss of solids (5 percent) 
in the preparation of the food product and from its lower moisture content. 

When mixed with 0.2 percent sulfur dioxide, the ground potatoes have a pH of 
about 4.6- Some of the sulfur dioxide driven off in the drier will be dis 
solved in the water which condenses in the drier stack. This will result in 
corrosion of the stack if .it is not constructed of metal resistant to sulfur 
dioxide corrosion. Manufacturers who propose to use this process beyond the 
present emergency demand for flour should replace the stack, when required, 
with one constructed of such metal. 

To produce flour and meal, the dried product must be ground in a hammer mill. 
One mill 6 inches wide by 12 inches in diameter equipped with blunt hammers and 
driven with a 7- l/2 -horsepower motor should have sufficient capacity. The size 
of the holes in the hammer mill screen and the speed of the mill will be 
governed in part by the relative proportions of meal and flour desired. The 
choice may also be influenced by the moisture content and friability of the 
product . 


The ground product must be screened. A vibrating screen driven by a 3-horse- 
power motor and equipped with 30-mesh and 70-mesh screens each 6 feet long and 
3 feet wide, should be satisfactory. The ground product is fed to the 30-mesh 
screen, which is placed over the 70-mesh screen. Material passing through the 
70-mesh screen is flour; that held on it is meal. The portion that does not 
pass through the 30-mesh screen is returned to the hammer mill. 

The Commodity Credit Corporation specifications for flour may be obtained from 
Claude S. Morris, Potato Division, Fruit and Vegetable Branch, Production and 
Marketing Administration, U. S. Department of Agriculture, Washington 25, D„ C. 

Potentialities of the Process: For an investment of approximately $87,500 
(Table I) , 16.6 tons per day of potato flour or meal containing 9 percent mois- 
ture can be produced, at a cost of about $39.00 per ton (Table II). This fig- 
ure includes all costs except that of the potatoes and sales expense for the 
product. This is considerably below the cost of producing potato flour by con- 
ventional means. Thus the potentialities of this method may well go beyond the 
present emergency. There should be an expandible domestic market for a potato 
flour which deviates only slightly in color from the best grade and costs only 
about half as much to make. The baking properties of this flour have not been 
determined. They may be different from that of flour made from cooked potatoes. 
However, its utility .in other fields, such as dehydrated soups, .is unquestioned. 
The shelf life of flour made by the method described here has not been deter- 
mined, It may be shorter than that made from cooked potatoes since the flour 
has not been exposed to the higher temperatures incident to cooking. 

Should a still lighter colored flour be required for domestic use, it can be 
made by drying in two stages (two driers in series) using steam at 100 pounds 
per square inch in the first drier and at 50 pounds per square inch in the 
second, Obviously, peeling the potatoes would also improve the color but at a 
significant increase in cost. 

Alternate Method: In general, water can be expressed more cheaply than it can 
be evaporated. Consideration was therefore given to expressing sufficient 
water from the ground potatoes, after treating them with sulfur dioxide, to 
prevent sticking in the drier without recycling. It was found that the criti- 
cal moisture content of the material to the drier was about 56 percent, instead 
of 43 percent, the moisture content of unpressed potatoes. However, it is not 
economically feasible to reach 56 percent moisture in continuous- type rotary 
presses. Batch-type cider presses capable of developing 200 pounds pressure 
per square .inch must be used, and the labor costs for this are high. Moreover, 
about 15 percent or more of the potato solids are lost, including valuable pro- 
teins, starch and sugar. Disposal of the press effluents may also constitute a 
stream -pollution problem. Thus although the color of flour from pressed ground 
potatoes which have not been recycled during drying is slightly superior to 
that of the unpressed, recycled product, in our opinion the additional cost of 
pressing is not justified. It is estimated that pressing instead of recycling 
would cost about an additional $2 per ton of product. 


78.8 % WATER 





To have definite figures on all items entering into the cost calculations, a 
specific area for the plant was chosen. Long Island, New York, was selected 
because of its central location on the eastern seaboard and because of the 
large quantities of potatoes grown in that vicinity. 

The following assumptions are made. Both feed and flour plants would operate 
170 days yearly, and each would process daily 75 tons of potatoes which contain 
21.2 percent solids 5 and 78.8 percent moisture. Based on a 2-percent loss of 
potatoes during processing, the feed plant would produce daily 17.31 tons of 
product having 10 percent moisture. Based on 5-percent loss of potatoes during 
processing, the flour factory would produce daily 16.60 tons of product having 
9 percent moisture. 

It is further assumed that no one would go into the production of potato flour 
or feed without already possessing some of the facilities. The estimates 
therefore, are based on the assumption that an already established business 
will provide such items as storage, office facilities and watchmen. Since the 
manager would also operate the new enterprise, a charge of $10.00 per day 
throughout a 300-day year is made to cover his services to the process. One 
operator on each of the two shifts, 4:00 to 12:00 and 12:00 to 8:00, is given 
50 cents per hour additional to act as foreman, making a total daily charge of 
$8 00 Thus, the total for supervision is $25.65 per day. One-quarter of the 
time of a secretary- stenographer-clerk of the already going business is charged 
to the new process. This represents a charge of $3.09 per operating day. For 
the operating period of 28 weeks, one-quarter of the time of two watchmen at 
$25.00 per week each is charged, $2.06 per operating day. 

Owing to the nature of the operation, with its long idle period, the investment 
is amortized at 8 years except that for the building for which 10 years is 
allowed. For the same reason, maintenance, repairs, and renewals are charged 
at a relatively high rate. 

No provision is made for vacations, as the plant operates only 170 days a year. 

No general cost estimate of this type will exactly fit the conditions of any 
prospective manufacturer. With an understanding of the assumptions upon which 
the estimate is based, however, a manufacturer should be able to make a reason- 
able estimate of his own costs. 

It should be emphasized that the capital costs given here and the cost per ton 
for producing the products are based on the use of a new building and the pur- 
chase and installation of new equipment. When idle equipment, such as steam 
tube driers, is available, the capital costs will be proportionately less, and 
hence the overall costs will be somewhat less than those shown. 


Feed Flour and Meal 

895.00 895.00 

Building, galvanized iron on structural steel, 

50 x 60 ft. and '25 ft. high $15,029.60 $15,029.60 

Boiler, 290 h. p., housed and ready to run 15,933.50 16,010.50 

Equipment : 

Flume, wood, 12 in. x 18 ft. and 9 in. high 85.20 85. '20 
Conveyor, belt, 3- ft. centers, 12 in. wide, 

with cleats and side rails 498.60 498.60 
Washer, 25 ft., x 30 in. x 30 in., with 4 

compartments 750.83 750.83 
Conveyor, belt, 14 in. x 25 ft. with cleats and 

side rails 591-30 591.30 
Hammer mill, 6 in. x 12 in., dia., with l/4-in.- 

screen, with 7 -1/2-h.p. motor . 668.19 668 .19 
2 Driers, each 6 ft. dia. x 35 ft., 1800 sq. ft. 

of heating surface 20,020.00 20,020.00 
Cyclone separator, rated capacity 5300 cu.ft. 

per min. 

Blower to cyclone, standard E, single width, 

approx. 5600 cu.ft. per min. with 5- h. p. motor 434.47 434.47 

Conveyor, screw tube, 6 ft long, with l/2-h. p. motor 244.48 244.48 

Bagging bin, wood, 3 ft. dia. x 5 ft. deep 115.00 115.00 

Bagging head, sheet metal breeches 20.00 '20.00 

Pump, centrifugal, 100 gal. per min., 50-foot head 277.00 277.00 

Bin under cyclone, steel tank, 4 ft. x 6 ft. deep 354.77 354.77 
Mixer- conveyor, paddle type, 2 ft. dia. x 20 ft. 

long, with 3-h.p. motor 1,643.23 1,643.23 

Bin for potatoes, wood, 25 x 15 ft. and 8 ft deep 482.37 482.37 

Inspection belt and feeder, belt 18 in. x 12 ft. 590.50 

Screen, vibrating, 3 ft. x 6 ft., with 3-h.p. motor 1,321.94 
Hammer mill for dried product, 6 in. x 12 in. 

dia. , with 7 -1/2-h.p. motor cyc * ^ 
Blower, standard A, single width, approximately 

1700 cu.ft. per min., with 2-h.p. motor 255.30 
Cvclone separator, rated capacity 1635 cu.ft. per 

. > 405.00 

Tank for sulfur dioxide treatment, wood, 4 ft. dia 

x 5 ft high 127 

Agitator, turbine type, bronze, with 5-h. p. motor 1,043.02 
Pump, positive delivery, to handle 6250 lb. per 

nr.:, with 1 -1/2-h.p. motor 308.85 

Freight for equipment 531.96 ^ 607.00 

E-ection of equipment 6,782.51 8,109.12 

piping and duct work . 834 75 851.90 

648.31 665.17 
819.58 819.58 
394. 78 394. 78 

Erection of piping and duct work 
Heating, installed 
Lighting, installed 

Contingencies 4,000.00 4,371.59 

Engineering fees 8, 006. 16 8,74o. 18 

Total |80,061.59 |87,431.91 


Based on a 24-hour Day 


Flour and Meal 


Sulfur dioxide - $ 18.00 

Labor $. 90.86 179. 66 

PRIME COST | 90.86 


Indirect materials 

Bags 34. 62 132. 80 

Indirect labor 

Supervision '25.65 25.65 

Factory clerk 3.09 3.09 

Indirect expense 

Insurance, public liability and fire 2.35 2.57 

Workmens Compensation 1.34 '2.45 

Unemployment Insurance 3.59 6.25 

Social Security 1.18 2.. 08 

Taxes 9.42 10. '29 

Interest on investment . 11.77 12.86 

Depreciation 58.99 60.72 

Maintenance, repairs and renewals 28. '26 30.86 

Power ' 13.19 18.67 

•Steam 133. 02 135. 14 

Freight and cartage into plant - 1. 95 

Miscellaneous factory expenses 5. 00 5.00 

Total factory overhead 331.47 450.38 

FACTORY COST 422.33 648.04 


per ton 

24. 40