1981) HUXSOLL, et al: DOUBLE-DIP PEELING I 327
DOUBLE-DIP CAUSTIC PEELING OF POTATOES I: LABORATORY-SCALE DEVELOPMENT
C.C. Huxsoll, M.L. Weaver and K.C. Ng 1
A modified caustic peeling process for potatoes was developed and evaluated. The process, called the double-dip process, comprises two separate caustic applications with an intervening holding period. Results on Russet Burbank potatoes showed that tissue removal was enhanced for the double-dip process when compared to the standard single dip process and that the double-dip process can effect reduction in caustic usage without re- quiring additional heat input.
Un m~todo modificado de pelado de papas en medio cafistico fue desarrollado y evaluado. E1 m6todo, llamado de proceso de doble inmer- si6n, comprende dos aplicaciones cafisticas con un periodo intermedio. Los resultados con papas Russet Burbank demostraron que la remoci6n del te- jido fue acelerado por el proceso a doble inmersi6n al compararlo con el m&odo standard de inmersi6n simple y que puede permitir reducir la can- tidad de medio ca0stico a utilizarse sin requerir incorpocaci6n adicional de temperatura en el proceso.
Various methods have been developed for peeling processing potatoes, and these have been described and analyzed in some detail (2, 5). The methods that are used commercially may be categorized as: Abrasion, Pressure- Steam, Conventional Caustic, and Dry Caustic. The method chosen may be determined by a number of factors, but the dominant factors are: the quali- ty of peeling required, the efficiency of the system, and the degree to which waste or pollution must be controlled.
The quality of peeling required is dependent upon the product being made. While a grading system has been described (7) for peeled potatoes which relates the numbers and kinds of defects that remain to the kinds of product that can be made, the actual peeling quality required is determined rather subjectively. Because of the wide range of product quality that is acceptable in the marketplace, the quality of peeling that is considered acceptable for a
*Western Regional Research Center, Science and Education Administration, U.S. Department of Agriculture, Berkeley, California 94710. Received for publication November 15, 1980. KEY WORDS: Caustic peeling, double-dip.
328 AMERICAN POTATO JOURNAL (Vo1. 58
given product type may also vary widely. For most product types, in- cluding the high production items such as French fries and dehydrated piece-form products, conventional-caustic and dry-caustic peeling systems have been used.
To improve the efficiency of caustic peeling, we have examined the ef- fects of different procedures of lye application. Earlier work (1, 3), which improved the efficiency of caustic peeling systems by making small amounts of absorbed caustic more effective, introduced heat, such as high pressure steam or infrared radiation. With recent increases in energy costs, and perhaps limited availability of energy, substitution of heat for caustic may not improve overall efficiency. Our objectives in the work described here was to improve the efficiency of caustic peeling by modifying the caustic ap- plication procedures so that a given amount of absorbed caustic would be more effective for peeling with a given heat input or without requiring addi- tional heat input.
In the initial development of the dry caustic peeling process (3), we obtained the results depicted in Figures 1 and 2. Figure 1 depicts caustic uptake as a function of caustic concentration and immersion time in the caustic solu- tion. The curves indicate a lag period followed by a rapid, almost linear, rate of caustic uptake. Figure 2 depicts peel loss as a function of hold time following lye application for holding periods of up to 15 min.
From the information contained in these two figures, we reasoned that caustic first modifies the suberized layer of the potato, making it more penetrable for additional caustic diffusion, and subsequently attacks the underlying potato flesh, resulting in rapid caustic uptake. It was also evi- dent that absorbed caustic continued to attack the potato tissue even though the potatoes were held at ambient temperature.
Based on these results, it was concluded that a caustic peeling process would be most efficient in the use of caustic when caustic applications are staged, i.e., one to modify the outer suberized layer and one to attack the underlying tissue, with holding periods following each lye application to maximize the effects of absorbed caustic. Such a process is diagrammed in Figure 3. Incoming washed potatoes are given a caustic treatment (C1). They are then subjected to a holding step (H1). A second caustic application (C2) and a second holding step (H2) follow. An optimal heat treatment, such as infrared heat (IR), may be applied prior to peeling with standard peel removal devices. This process has been disclosed in U.S. Patents (4, 6) and is called the "double-dip" caustic peeling method.
Materials and Methods
Representative tubers were selected from lots of Russet Burbank potatoes so that all potatoes for a given test conformed closely to one another in size, Shape, and skin condition.
HUXSOLL, et al- DOUBLE-DIP PEELING I
NaOH ABSORBED BY U.S. # 1 POTATOES IN 77~ LYE
"t- O (3 Z 6 v
20% NaO H / / /
~K, / NoOH j#" i i i
I I I I I I 0 40 120 200 280
DIP TIME (Sec.) FIG. 1. Lye uptake in potatoes as a function of immersion time for several levels of lye concen- tration.
Caustic solutions were made to given concentrations using reagent grade sodium hydroxide pellets (minimum assay, 95~ NaOH). About 3.2 kg of caustic solution were placed in a stainless steel pot which was heated by an electric hot plate. Calibrated dial thermometers were used to measure the solution temperature.
Selected potatoes, individually impaled on stainless steel spits, were im- mersed in the caustic solution at the desired temperature. During immer- sion, the spit was rotated to thoroughly stir the solution. The temperature of the solution was controlled manually. After immersion for a specified time, the impaled potato was removed from the solution.
Ambient temperature holding was accomplished by merely exposing the potato, on the spit, to the ambient conditions of the laboratory, about 21-14~ Holding in a steam-air atmosphere was accomplished by placing the potatoes in a steam blancher equipped with air injection to control tem- perature. Holding in warm air was accomplished by placing the potatoes in a small, temperature controlled, laboratory bin-dryer with an air velocity of about 0.5 m/sec.
Following a given series of immersions and holding periods, the po- tatoes were either peeled directly or subjected to infrared heat. When in-
330 AMERICAN POTATO JOURNAL (VoI. 58
EFFECT OF HOLDING TIME AT AMBIENT TEMPERATURE
Lye d ip 20%-77~
24 t 90 SEC. DIP .33Kg NaOH/IOOKgPOTATOES)
20 60 SEC. DIP (0.25Kg NaOH/IOOKgPOTATOES)
0 16 --' 45 SEC. DIP ('~ (0 .23Kg NaO H/IOOKg POTATOES) Z 9 ~ 12 M.I LLI a.
8 I l I I I I i I I
0 2 6 10 14 18
HOLDING TIME (Min.) FIG. 2. Peeling loss for potatoes as a function of holding time for several levels of lye uptake.
frared heat was used, the impaled tuber was rotated about l0 times per minute under a gas-fired infrared burner with the nearest surface about 5 cm from the 870~ radiating mantle.
Peeling was accomplished by hand brushing under a small flow of wa- ter or by use of a small mechanical, 3-roll, dry peeler (3).
The peeled potatoes were weighed to determine the amount of material removed, and a visual inspection determined "heat ring" formation and overall peeling quality.
Lye uptake was measured by a previously described method (3) in which caustic treated potatoes were placed in a known volume of standard dilute acid. After the caustic in the potatoes had time to react with the acid - - about 15 min - - an aliquot of the acid solution was taken. Back titration with sodium hydroxide then determined the quantity of caustic absorbed by a given weight of potatoes. While all tests reported here were conducted on Russet Burbank potatoes, it is well-known that potatoes, even of the same variety, vary widely in their peeling characteristics due to variations in culture and storage. Therefore, tests were conducted on different lots of potatoes throughout the storage season so that the system could be eval- uated for a range of potato skin conditions. While many lots of potatoes were tested, the data for each direct comparison reported here came from raw material of the same lot.
1981) HUXSOLL, et al: DOUBLE-DIP PEELING I 331
CAUSTIC IMMERSION (C1)
HOLD PERIOD (HI)
CAUSTIC IMMERSION (C2)
HOLD PERIOD (H2)
INFRARED HEAT (IR)
PEEL FIG. 3. Flow chart for double-dip caustic peeling process.
332 AMERICAN POTATO JOURNAL (Vol. 58
CURVE A 30 SEC, 8% NaOH, 88~ 30 SEC, 8% NaOH, 88~
CURVE B 30 SEC 8% NaOH, 88~ 30 SEC, 5% NaOH, 88~ CURVE C 30 SEC 5% NaOH, 88~ 30 $EC, 5% NaOH, 88~
lo A 0~- ~Z~
Resu l ts and D iscuss ion
Figure 4 shows a typical effect of the hold period (H1) at ambient temperature between two caustic applications on the peel loss for several combinations of caustic treatments. Curve A represents immersions in 8~ NaOH at 88~ for 30 sec in both the first immersion (C1) and the second immersion (C2). A zer