Double-dip caustic peeling of potatoes II: Studies on commercialization

1981) WEAVER, et al: DOUBLE-DIP PEELING II 355

DOUBLE-DIP CAUSTIC PEELING OF POTATOES II: STUDIES ON COMMERCIALIZATION

M.L. Weaver, C.C. Huxsoll and K.C. Ng I

Abstract

A study of "double-d ip" caustic peeling of potatoes was made at two commercial potato processing plants. The "double-d ip" system uses two caustic dips, each followed by a holding period at ambient temperature. The peel is then removed using rubber-tipped scrubbers. Four hundred and fifty- four g of pure caustic peeled about 90 to 180 kg of potatoes.

Good quality peel was attained with a 10 to 14% peel loss. The equip- ment in both plants performed with little mechanical difficulty. Quick changes in dip-time and holding-time were used to compensate for changes in the condition of raw material without causing excessive variations in caustic-use or peel-removal. Problems encountered and solutions applied to each are described in detail.

Resumen

En dos plantas comerciales de procesamiento de papa se realizaron estudios utilizando doble inmersi6n en medio cafistico. El m6todo de doble inmersi6n utiliza dos inmersiones cafisticas, cada una seguida por un periodo a temperatura ambiente. La cascara es luego removida utilizando raspadoras con puntas de goma. Aproximadamente 90 a 180 kg de papas fueron peladas usando 454 g de cat~stico puro.

Se obtuvo un pelado de buena calidad con 10 a 14% de p6rdida de pelado. E1 equipo usado en ambas plantas tuvo s61o pequefias dificultades mec~nicas. Se utiliz6 cambios r~pidos en periodos de inmersi6n y periodos intermedios para compensar los cambios en la condici6n de la materia prima sin causar variaciones excesivas en el uso de ca~stico o en la remoci6n de la cascara. Los problemas presentados durante el proceso y sus solu- ciones son descritas en detalle.

Introduction

Potatoes are commercially peeled by abraison, high-pressure steam, or sodium hydroxide (NaOH). In the mide 1960's there was an accelerated de- mand by environmentalists and government agencies, at all levels, to reduce stream pollution. With this need in mind, a new technique, "dry-caust ic" peeling (1, 2), was developed at the USDA's Western Regional Research

1Western Regional Research Center, U.S. Department of Agriculture, Berkeley, California 94710. Received for publication November 15, 1980. KEY WORDS: Caustic peeling, double dip.

356 AMERICAN POTATO JOURNAL (Vol. 58

Center to reduce the use of NaOH and to minimize peel removal. Incorpor- ated with this improved method of loosening the peel was a new method of peel removal using rubber-tipped scrubbers rather than high-volume water sprays. The technique proved quite successful and was rapidly commercial- ized in many plants both in the United States and abroad. However, recent problems of reduced supplies of natural gas and increased cost of gas and electrical power necessitated the investigation of new peeling techniques that would give the advantages of "dry-caustic" peeling without the need for high energy infra-red. "Double-dip" caustic peeling was the result of these investigations (3).

Materials and Methods

Data for this paper were collected at the commercial potato processing plants using "double-dip" peeling. Data were collected over the entire processing season (August to June) at one plant, and over two complete processing seasons at the second plant. Russet Burbank potatoes were used exclusively at one plant, while both Russet Burbank and Kennebec tubers were used at the second plant. Twelve tubers, each between 275 and 300 g, were used for all peel-loss determinations. Each trial was done in triplicate. Each tuber was marked with red dye using a special marker (4), and weighed. After going through the "double-dip" peeling system, marked tubers were again weighed to determine peel loss.

To calculate caustic used for peeling, data on the lbs of NaOH con- sumption and on lbs of tubers going into the peelers were calculated from daily plant records. These data were compared using records from the same day in the previous year, when single-dip caustic peeling was used.

Results and Discussion

Double-dip caustic peeling uses two immersions in caustic at about 93 ~ each immersion followed by a retention period at ambient temperature (Fig. 1).

Skin conditioning - - A high NaOH concentration is not needed to peel tubers, but is needed to penetrate the skin. Therefore, the first immersion in NaOH, and the prolonged hold in retention chamber I at ambient temperature, serves only to condition the skin and make it permeable to NaOH of low concentration, not to peel. If much peel was removed through the skin conditioning step, either the NaOH concentration was too high, or immersion time was too long. In all studies, using different potato cultivars and tubers at all stages of maturity, 20 to 35 sec of immersion were adequate for skin conditioning. After 40 to 45 sec of immersion, NaOH penetrated the skin of all cultivars tested and started to loosen the peel. Penetration through the skin should not be done at this stage. Changes of only a few seconds in immersion time can be quite important when measured in peel loss (Table l-a).

1981) WEAVER, e t al." DOUBLE-DIP PEELING II 357

Dip I

Retention

20-35 sec

4-12~ NaOH

93 C

ambient temp

6-15 mln

Skin

conditioning

Dtp I I 30-75 sec

4-109 NaOH

93 C

Retention II ambient temp

30-180 see

Rubber-tl pped scrubber

1 Brush washer

Peel ing

FIG. 1. Double-dip caustic peeling.

The extremely fast time of immersion caused some problems when using standard commercial lye-dippers which are designed for a single immersion of about 1.5 to 4 min. The much slower movement of the paddles in standard single-dip peelers gives sufficient time for NaOH to drain off the paddles and off the tubers being removed. The paddles move so fast in "double-dip" peeling that they actually throw NaOH solution out of the dipper, almost like a pump. Also, the short immersion does not allow sufficient time for softening of the peel, and much NaOH solution adheres to the tuber surface


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and is carried out into the first retention chamber. Thus, the more potatoes peeled, the more the carry out of NaOH. The quantity of NaOH removed amounted to more than one-half of the total caustic use in some trials (Table 2). Thus, an efficient recovery system to reclaim caustic drained from the first retention chamber is essential for an efficient operation. When "single-dip" caustic peeling was used just prior to changing over to "double- dip" peeling, recovery of excess caustic, if there was any, was not done and caustic use was extremely high (Table 2).

TABLE 2. - - Quantity o f NaOH carried out o f dipper L both with and without a load o f tubers, and the effect o f NaOH recovery on

peeling efficiency.

NaOH NaOH removed Total NaOH removed from retention NaOH

used from dipper I chamber I with actually used Potatoes Peel with no load a load to peel I peeled/454 kg loss

Trial (kg of 1000/o)(kg of 1000/o) (kg of 1000/o) (kg of 1000/o) of 1000/o (0/0) NaOH (kg)

1 1669 199 659 1010 (600/0) 126 10.6 2 1828 186 1312 516 (400/0) 190 10.1 3 2087 209 694 1392 (660/0) 128 11.1 4 3461 - - - - 3461 (1000/0) 44 21.0

5 3200 -- -- 3200 (1000/0) 54 18.3

1Percentage value in parenthesis is the fraction of the total NaOH used that could actually be attributed to peel removal. Kennebec tubers used in all trials.

With the extremely short immersion time, some standard caustic dippers did not have an adequate angle of fall to rapidly remove tubers from the dipper. One peeler had a fall angle of about 25 ~ and at least one-third of the tubers in each chamber were still on the paddle by the time it reached a posi- tion horizontal to the exit. The tubers left on the paddle were then carried back into the dipper for another immersion. Some tubers, as measured by depth of heat ring, were returned to the dipper several times. By raising the dipper and changing the slope of the exit, so that the paddle and the exit slide made a straight angle of about 45 ~ carry back of the tubers was elim- inated.

The retention chamber can be quite deep, 24 to 48 inches in tested trials, as long as the flow of tubers is on a first-in, first-out basis. The chamber can be covered, or open. If covered, the chamber should not be attached to the caustic dippers in such a way as to make it a totally enclosed unit unless the dippers are properly vented. In one instance, with a totally enclosed non- vented unit, a deep heat ring in the peeled tubers was traced to a failure of the temperature control unit on the second dipper which allowed the NaOH solution to boil. Vapor from the boiling caustic rose from the dipper, drifted


back into the discharge end of the first retention belt for a distance of 10o12 ft raising the temperature in this area to about 77~ The extra exposure to heat above the starch gelation temperature caused additional cooking of the tissue at the tuber surface, and, as a result, peel loss exceeded the 10-14070

desired and all of the heat ring was not removed. Ordinarily, when well- vented, the covered retention chamber stayed at about 37 to 45~

The belt speed in retention chamber I should be capable of providing a holding period of about 6 to 15 min to give the needed flexibility to peel all types of potatoes, at all stages of maturity. Changing retention belt speeds can also be used to decrease, or increase, peel loss (Table l-b).

Peeling - - The second (peeling) dip was also kept at a temperature just below the boiling point for optimum operation. Since the skin had al- ready been prepared, by the skin-conditioning steps, to promote more rapid penetration of the NaOH solution, it was usually possible to use NaOH concentrations in the second immersion equal to, or less than, that used in the first immersion (Table 1-c). Immersion time and NaOH concentration needed in dipper II depended on the degree of peeling required for the product being produced. It is desirable to keep the immersion time under 60 sec in dipper II. This was done by adjusting the NaOH concentration. When total immersion time at 93~ in dippers 1 and 2 exceeded 115 sec, a heat ring was produced that was deeper than could be removed by a 10 to 1407o peel loss.

It was possible to peel at temperatures lower than 93~ (Table l-d). When Kennebecs were immersed one time only for 4 min at 71~ in caustic of 15070, peel loss was 18070, and 454 g of NaOH peeled 60 kg of potatoes (Table 3). However, when "double-dip" peeling was used at the optimum temperature and immersion time, peel loss was cut nearly in half, and 454 g of NaOH peeled about 4 times as many potatoes.

The NaOH solution should be continuously cleaned, especially in dipper II, where peels are loosened and the rubbing together of tubers causes pieces to fall off. As sludge builds in the dipper some of the NaOH is being used to break down the organic matter, and is not then available to act on newly in- troduced tubers. Screens, or settling tanks, can be used for sludge removal. Effectiveness of a clean dip solution over a dirty solution was demonstrated at one of the processing plants where NaOH concentration had to be continuously increased from 8 to 15070, in both dippers, over a 2 wk period just to keep peel loss at about 14~ When some of the same Russet Burbank tubers were taken to our laboratory and immersed for 35 sec in clean 13.5070 NaOH, held 14 min at ambient temperature, immersed a second time in clean 13.5070 NaOH for 50 sec, and held 3 min at ambient temperature, they had a peel loss of 20%. When other tubers from this lot were treated for similar times in clean caustic using only 10070 NaOH in each dipper, the peel loss was cut 11.1 07o and 454 g of pure NaOH peeled 137 kg of potatoes (Table 3). When tubers of the same lot were given a single dip in 15070 NaOH for 3.5 min,

1981) WEAVER, et ai: DOUBLE-DIP PEELING II 361

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peel loss was 20% and 454 g of NaOH peeled less than 45 kg of potatoes. Heat ring was also quite deep. Most adjustment for control of peeling was obtained by regulating the time of immersion and NaOH concentration in dipper II. It was here that most of the real savings of NaOH and raw material were realized.

The belt in retention chamber II should be adjustable to provide a holding period of 30 sec to 3 rain. When peeling newly-harvested tubers, it was often not desirable to hold them, after the peeling dip, for an extended time at ambient temperatures to let NaOH penetrate the skin. However, when peeling thick-skinned tubers late in the storage season, it was usually necessary to allow more time for the NaOH to work on the tuber tissue.

T i m e o f y e a r - - N a O H C o n c e n t r a t i o n ~ Pee l i n g L o s s

Early harvested tubers have a non-uniform skin, especially where grow- ers do not set the skin by beating vines, cutting of f water, and leaving tubers in the ground for 2 to 3 weeks before harvesting. Now, there is a tendency to forget vine beating and delayed harvesting in favor of getting into produc- tion early. Tubers harvested in this manner are very immature with patches of little, or no skin cover. Penetration of NaOH is very rapid in areas of poor skin cover, and the deeper penetration of NaOH results in isolated areas of deep tissue removal ("cra ter ing") . "Cra ter ing" usually occurs only during the first few weeks of the season. While it is virtually impossible to eliminate without full skin set prior to digging, it was reduced by using "double-dip" peeling. Most success was obtained by keeping the concentration of NaOH in the first immersion to about 3 to 4~ reducing retention time in chamber #1 to 6 to 8 min, increasing NaOH concentration in dipper II to about 5 to 6~ and reducing hold time in retention chamber II to the mini- mum, about 30 sec. NaOH then had less time to penetrate skinless areas and to react with tissue just under the skin. As a result "cra ter ing" was reduced, and a more uniform peel depth was obtained over the entire tuber surface and peel loss was minimized. Russet Burbank, Kennebec, and White Rose potatoes were peeled using about the same NaOH concentration and immersion time when first harvested. When peeling Russets, the NaOH concentration had to be slowly increased from time of harvest until it reached 7 to 8% in both dippers I and II in December. When peeling Kennebec, or White Rose tubers, NaOH concentration was also increased as the season progressed, but, by December, only about a 5.0 to 5.5070 concentration was needed in each dipper. After the first of the year, NaOH was slowly raised to 10 to 12070 in dipper I and 7 to 10070 in dipper II at one processing plant, but did not have to be raised over 8~ at the second plant, when peeling Russets. It was not necessary to raise the NaOH concentration to more than 6~ in either dipper when peeling Kennebec or White Rose tubers through the rest of the season. Kennebec and Russet Burbank potatoes were peeled during the same day, with the same peeler, even late in the storage season. It


was possible to overcome the variations in peel characteristics of the two cultivars by changing only the retention belt speeds and immersion times in NaOH, not by changing NaOH concentration.

If maximum advantage of the system is to be realized, it is imperative that a means be provided to quickly and accurately change the speeds of caustic dippers and retention belts. The best control was obtained using hy- draulic motors.

From data obtained in both processing plants, it appears that the varia- bility needed for a "double-dip" peeling system for potatoes is: 15 to 60 sec in dipper I; 6 to 15 min in retention chamber I; 30 to 75 sec in dipper II; and, 30 to 180 sec in retention Chamber II. This system allows for quick changes in peel removal to satisfy the different quality control standards for different types of processed products and for raw material of different quality.

One of the most difficult tasks in this study was to teach peeling operators, especially experienced personnel, how to run the"double-dip" system. These operators had great difficulty adjusting to the idea of peeling with very low NaOH concentrations and short immersion times, even late in the season. Initially, as adjustments were being made and quality control indicated the product was not meeting grade standards, and before the cause of the defects was determined, the operators' first response was to add more caustic. Also, when more caustic was added to peel poor quality tubers, the tendency was to keep peeling at that same high NaOH level even after switching to good tubers. Such carelessness caused excess peel removal and greatly increased NaOH use.

In these processing plant tests, wide ranges of NaOH usage and peeling losses were recorded. In one of the plants, which was not equipped with a clean-up system for the NaOH dippers, or a system to recover NaOH carried from dipper I into retention chamber I, the carry-out of NaOH was experi- mentally determined. This amount, added to the quantity of NaOH dumped during clean up of dippers I and II, was subtracted from the total amount of NaOH used. Four hundred and fifty-four g of pure NaOH peeled 91 kg of potatoes in January and 77 kg of potatoes in February. Peel loss during this period ranged from 12 to 14%. The average daily quantity of potatoes peeled for this period was in excess of 454,000 kg.

The second plant did not have a NaOH clean-up system for the dippers, but did have a recovery system to collect NaOH removed from dipper I. About 68,000 kg Kennebec and Russet Burbank potatoes were peeled on a daily basis. During February and March, 454 g of NaOH peeled from 125 to 227 kg of potatoes (Table 2). The overall average usage during the year being about 113 to 136 kg of potatoes peeled using 454 g of NaOH. Based on laboratory and processing plant studies, the "double-dip" system should handle more than 91 kg of potatoes with each 454 g of pure NaOH, with peel loss between 10 to 14070. This should be possible if there is: a) automated control of NaOH concentration; b) quick and accurate changing of immer-


sion and retention times; c) good clean-up of NaOH; d) recovery of NaOH carried out of dipper I; and e) a conscientious operator paying strict attention to the quality of different lots of tubers as well as to the type of product being made.

Loosened peel was removed using one of the conventional rubber-tipped scrubbers and brush washers to reduce water use. Very stiff bristles on brush washers increased peel loss by about 1.0 to 2.5%. However, stiff bristles assisted in removing such external defects as rhizoctonia or scab.

"Double-dip" peeling also offers another advantage to the processor. Short immersions in hot NaOH solution, and long retention times at some- what elevated temperatures, raised the core temperatures of stored tubers from about 7~ to about 15 to 22~ as they came from storage and went through the peeling system. The warmer tissue temperature reduced shatter- ing of the tubers during cutting. The effect on improved quality was quite apparent in the finished product.

Double-dip peeling was also found to be well suited for peeling other fruit and vegetable crops, i.e., sweet potatoes, beets, carrots, apples, etc. Papers on these subjects will be submitted at a later date.

Literxtare Cited

1. Graham, R. P., C. C. Huxsoll, M. R. Hart, M. L. Weaver, and A. I. Morgan, Jr. 1967. Dry caustic peering of potatoes. Part 1. Laboratory experiments on single potatoes. Part 2. Design and operation of continuous pilot plant. Food Technol 23: 61-67.

2. GrAham, R.P., C.C. Huxsoll, M.R. Hart, M.L. Weaver, and A.L Morgan, Jr. 1969. Prevents potato peel pollution. Food Engineering 41: 91-93, 1969.

3. Huxsoll, C. C., M. L. Weaver, and R. P. Graham. Process for peeling fruits and vegetables. U.S. Patent #3,950,556, April 13, 1976.

4. Weaver, M. L., K. C. Ng, and C. C. Huxsoll. 1981. Sampling potato tubers to determine peel loss. Am Potato J "In press."

Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.

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Double-dip caustic peeling of potatoes II: Studies on commercialization