Potassium hydroxide for peeling potatoes

1977) MUNETA, et al: POTASSIUM HYDROXIDE FOR PEELING 83

POTASSIUM HYDROXIDE FOR PEELING POTATOES 1

P. Muneta, Wu-Wei Shen, S. Jennings, D. Everson and L. Edwards 2

Abstract

Evolutionary operations (EVOP) were used to obtain conditions for optimum peeling of potatoes with KOH. With peeling at temperatures near 93 C, 20.6% KOH with an immersion time of 1 min 40 sec resulted in good peeling. Peeling at temperatures near 68 C required a KOH concentration of 25% and immersion times near 6 rain. The higher immersion temperatures and higher KOH concentrations allowed shorter immersion times while obtaining adequate peeling of the potatoes.

Potatoes peeled with KOH and NaOH were made into French fries andd mashed potatoes. Taste panels using the triangle difference test could detect no flavor differences between NaOH and KOH peeled potatoes at the 5% significance level.

Resumen

Operaciones evolucionarias (EVOP) fueron usados para obtener las condiciones para el 6ptimo pelado de papas con KOH. E1 pelado a temperaturas cercanas a 93 C y 20.6% KOH con un tiempo de inmersi6n de 1 min 40 sec result6 en un buen pelado. E1 pelado a temperaturas cercanas a 68 C requiri6 una concentraci6n de KOH de 25% y de un tiempo de inmersi6n de cerca de 6 minutos. La temperatura mas alta de inmersi6n y las mas altas concentraciones de KOH permitieron tiempos mas cortos de inmersi6n manteni6ndose un pelado adecuado de las papas. Las papas peladas con KOH y NaOH fueron fritas a la francesca y preparados en pur6. Paneles de degustaci6n que usaron la prueba del tridmgulo diferencial no pudieron detectar diferencias en el sabor entre las papas peladas con NaOH y KOH al nivel de 5% de significaci6n.

Introduction

Lye (sodium hydroxide) is an effective and economical peeling agent for many croos including potatoes, sweet potatoes, tomatoes, red beets,

1 Published with the approval of the Director of the Idaho Agricultural Experiment Station as Research Paper No. 7652. Support for this research was provided in part by the Short Term Applied Research Program of the University of Idaho. 2 Associate Professor, graduate student, laboratory technician, Department of Bacteriology and Biochemistry; Statistician, Idaho Agricultural Experiment Station and Professor, Chemi- cal Engineering Department, University of Idaho, Moscow, Idaho 83843 Received for publication April 5, 1976.

84 AMERICAN POTATO JOURNAL (Vol. 54

turnips, peaches, apricots and pears. J.B. Thompson (13) obtained a patent in 1907 for lye peeling fruit. Stanley (12) and Olson (10) recommended NaOH for peeling peaches and apricots. Olson also tested potassium hydroxide (KOH) but NaOH was preferred because of its lower cost. NaOH peeling of potatoes was suggested by Mazzola (9). Subsequently other reports followed on lye peeling of fruits and vegetables (2, 3, 5, 6, 15, 16). The NaOH peering requires large volumes of water to remove the gelatinized outer tissue. This is undesirable for efficient waste treatment. Development of the "dry peel" process by Graham et al. (4) using NaOH results in a more concentrated peel waste which can be handled more easily.

Waste disposal and associated pollution are the major environmental problems associated with NaOH peeling plants. The settleable solids can be used for livestock feed. Most of the soluble solids can be removed by secondary waste treatment.

The sodium in the waste water is not essenfi~ for plant growth. If the waste water is applied to any but a sandy soft, the sodium content may increase sufficiently to make the soil impermeable to water alad toxic to plant growth. The time required for these effects to appear will depend on factors such as the sodium content of the waste water and the soil type.

More stringent water pollution laws will probably be enacted to limit the total salts or sodium in the waste waters entering lakes and rivers. Partial removal of the sodium will increase the costs of peering with NaOH significantly.

Conversion from peeling with NaOH to a peeling agent which could be used as a fertilizer is highly desirable. Peering with KOH offers this possi- bility and it has been used for peeling fruits and vegetables (10, 1 l, 14). The higher cost of KOH compared to NaOH would be counterbalanced by the advantage of using the waste water from KOH peeling for irrigation and as a source of potash, a major plant nutrient. The many disadvantages of the disposal of waste water from NaOH peeling should also favor a change to KOH peeling.

This report discusses the conditions for peering potatoes with KOH under laboratory conditions. The results of taste panel tests to compare French fried and mashed potatoes prepared from NaOH and KOH peeled potatoes are reported.

Materials and Methods

Russet Burbank potatoes grown in south Idaho and stored for 6-12 months at 42 F (5.6 C) were used for this study. The potatoes for the taste panel tests were stored at 45 F (7.2 C) for 7 months and removed to room temperature for two weeks before use.


The KOH solutions (8-30%) were heated in stainless steel containers at temperatures from 120-200 F (48.9-93.3 C). Uniformly shaped and un- bruised potatoes weighing from 140-180 gms were placed in a heavy wire basket and immersed in the KOH solutions from 1-10 min. The potatoes were placed under cold running water and the gelatinized layer was rapidly removed with a wire handled 23/6 × 6 inch nylon brush. The potatoes were brushed vigorously until the yellow layer was removed. When pH paper was applied to the surface a neutral reaction was obtained.

The chemical reactions between KOH and the potatoes continue even after the potatoes are removed from the KOH solution. Thus the temperature and time intervals before the potatoes were scrubbed had to be carefully regulated to obtain reproducible results. Removal of most of the softer gelatinized layer quickly from all potatoes followed by a more careful scrubbing to remove the last of the soft layer gave good reproducibility. If each tuber was scrubbed completely before proceeding to the next tuber, then greater peel loss occurred from potatoes peeled last and greater variability resulted.

The potatoes for the taste panel test were peeled by immersion in 20.6% NaOH and KOH solutions at 198 F (92.2 C) for 1 min 40 sec. The French fry strips were 3/s× 3/a inches in cross section. A first stage fry at 365 F ( 185 C) for 4 min and a second stage fry at 390 F (198.9 C) for 1Vz min were used. The potatoes were cooled for 2 hours in a refrigerator before the final frying because a more desirable texture was obtained in this way.

For the mashed potatoes 3/s inch cubes were steamed for 35 min in a steamer kettle and then riced. Potatoes peeled by KOH were steamed or fried in containers different than those used for the tubers peeled with NaOH.

The triangle difference was used to determine whether differences could be detected between the NaOH and KOH peeled potato products. The panel consisted of 10 judges with 3 replications.

Two procedures used to determine optimal peeling conditions were: (1) Multiple regression equations fitted to observations taken at con-

stant temperatures of 155 (68.3), 180 (82.2),and 200 F (93.3 C) with KOH concentrations of 15, 20, 25 or 30% and immersion times of 3, 4, 6, 8 and 10 min.

(2) A simplex evolutionary operation (EVOP), a simple statistical method for optimizing processes in which many simple or compli- cated interactions affect the final product, (7, 8). Immersion time, temperature of the KOH solution and concentration were the independent variables and Rank Index or % peel loss was the dependent variable. The objective of the EVOP procedure was to find conditions that gave adequate peeling with a minimum peel loss and minimum heat ring. For each initial simplex in the EVOP

8 6 A M E R I C A N P O T A T O J O U R N A L ( V o l . 5 4

procedure the average values for the times, temperatures and concentrations were chosen by comparison with peeling conditions recommended for NaOH solutions. The stepwise differences used for the independent variables were 20 seconds for time, 5-10 F for temperature and 2% differences for KOH concentrations. If large step sizes were used in the independent variables, the simplex patterns will move rapidly to the vicinity of the optimum. Smaller step size results in much smaller changes in the points of the simplex. Once an approximation of the optimum is obtained with large step sizes, then a new simplex with smaller step sizes near the optimum can be designed. The smaller simplex should result in all or most of the points being in an acceptable range and a better definition of the optimum is obtained.

TABLE 1. m Rantdng levels as a function of % weight loss and heat ring depth.

Heat ring % weight loss rank A depth (mm) rank B

35% 0 >5 .0 0 30.1 - 35.0 1 4.0 - 4.9 1 2 5 . 1 - 3 0 . 0 2 3 - 3 . 9 2 2 0 . 1 - 2 5 . 0 3 2 - 2 . 9 3 15.1 - 20.0 4 1 - 1.9 4 10.1 - 15.0 5 0 - 0.9 5

Rank Index = rank A + rank B. I f the peel ing was unacceptab le because o f inadequate peeling or too m u c h peel loss (>35%), the Rank Index was rated as 0.

A Rank Index which was the sum of the peeling loss ranking and the heat ring ranking was used to evaluate the overall peeling effectiveness. Table 1 gives the ranking levels used for the % peel loss and the heat ring depth. The optimum peeling conditions resulted in potatoes with a minimum heat ring and a peel loss near 15%. With the 15% peel loss, the skin was completely removed from the eyes of the Russet Burbank potatoes. When peel losses were between 11-12%, peeling was often more difficult and the skin was not always removed from around the eye areas.

Experimental EVOP trials were begun at initial peeling temperatures near 155 F (68.3 C), 180 F (82.2 C) and 200 F (93.3 C). The 200 F temperature requires much shorter immersion times to obtain adequate peeling. However, formation of a heat ring at this temperature can result in undesirable texture and color variability in prepeeled potatoes and potato chips. Temperatures near 155 F (68.3 C) were used because the heat ring is eliminated. The 180 F (82.2 C) was chosen as an intermediate peeling temperature.


Results and Discussion

The extent of heat ring formation and peeling losses are greatly af- fected by the size and shape of the potato. Similar weights and shapes are required for reproducible results. With mixed tuber size, the smaller the tuber the greater the percenntage of peel loss. This large difference in weight loss between small and large potatoes indicates that a preliminary sorting into 2 or more size ranges would result in significant savings in peel loss with a higher yield of finished product. Lower peel loss would also help in waste disposal.

For tubers of the same weight, the greater the surface area, the greater the weight loss. Irregularly shaped tubers resulted in greater peel loss. The presence of even small cuts or bruises resulted in much greater variability. Differences in heat ring and peel loss resulted when potatoes were peeled immediately after removal from storage rather than being allowed to reach room temperature. Potatoes with little or no russetting were easier to peel than those with heavy russetting.

Table 2 lists peeling conditions, peeling losses, and heat ring for tubers considered well peeled for French frying. The peel losses were approximately 15% or slightly higher. All of the skin was removed from the eyes of the tuber and they did not require any trimming.

TABLE 2--Comparison of peel loss, heat ring and Rank Index for optimum peeling conditions.

Optimum KOH Immersion KOH Peel Heat Rank condition temp. times concn, loss ring Index number* °F (°C) min:sec (%) (%) (mm)

1 155 (68.3) 5:50 26 15.9 0 9 2 154 (67.8) 6:01 25.5 15.9 0 9 3 198 (92.2) 1:40 20.6 15.9 0-1 9 4 180 (82.2) 4:04 23.3 17.2 0-1 9 5 199 (92.8) 3:50 12 16,1 1-2 8 6 155 (68.3) 6:00 25 15.9 0 9 7 180 (82.2) 4:00 20 16 0-1 9

*Optima 1-5 were obatined by Simplex EVOP experiments. Optima 6&7 wer~ obtained from the multiple regression peeling experiments.

The good peeling at 155 F (68.3 C) required an immersion time of 6 min at a KOH concentration of 25-26%. This set of conditions was obtained by running two independent EVOP trials. One maintained a constant temperature and varied immersion time and concentration; while the other varied temperature, concentration and time. A similar peeling "optimum" was obtained from the multiple regression peeling experiments.

When EVOP was run near 200 F (93.3 C), the optimum conditions changed depending on the concentration of KOH. At 12% KOH an immer-

88 AMERICAN POTATO JOURNAL (Vol. 54

sion time of 3 min 50 sec was required. At 20.6% KOH an immersion time of 1 min 40 sec was required for equivalent peeling losses. The increased immersion time resulted in greater heat ring formation at 12% KOH than with the 20.6% KOH.

When EVOP was run near 180 F (82.2 C), an optimum at 180 F (82.2 C), 23.3% KOH and a 4 min 4 sec immersion time was obtained. At a constant temperature of 180 F (82.2 C) in the multiple regression experiments an optimum was obtained with 20% KOH and a 4 min immersion time. The heat ring was beginning to form at the 180 F (82.2 C) temperature.

The simplex EVOP method offered a quick method to obtain good peeling results while minimizing the number of peeling experiments required. Even though all of the experimental points in the initial simplex may give poor peeling, good peeling was obtained by the time 10 to i2 peeling conditions were tested.

A stepwise multiple regression was run on the peeling at constant temperature and varying KOH concentration and immersion times. Thirty prediction equations were obtained and the best prediction equation with the least standard error of estimate was the following.

R.I. = 120.57 - 2.309t - 0.976T - 16.13 × 10-2C + 8.84 x 10-~t ~ + 2.29 x 10-3T ~ R 2 (Coefficient of variation)=0.962, significant at 1% level. Sy.x=0.4994 R.I .=Rank Index T= temperature (°F) t=time (min) C=concentration (weight %)

The simplest linear equation given below had a lower R 2 value of.9160 and a much higher Sy.x of 0.8419.

R.I.=41.859 - 1.043t - .1478T - 0.1197 C

This equation is easier to manipulate but not as accurate as the previ- ous equation.

Temperature and time have the greatest effect on the Rank Index when the temperatures, concentrations and times used in the peeling process are substituted in the Rank Index equation. The KOH concentration was much less important. The temperature effect between 155 F (68.3 C) and 180 F (82.2 C) was much greater than between 180 F (82.2 C) and 200 F (93.3 C) because the heat ring appeared at 180°F (82.2 C) and was greater at 200°F (93.3 C).

If a heat ring is undesirable, it can easily be eliminated by peeling at temperatures near 155 F (68.3 C). If a heat ring is not important, then high temperature peeling is desirable because shorter immersion times can be used. Preliminary evidence also indicated less KOH was consumed per unit weight of potatoes peeled at the higher temperatures.

1977) MUNETA, e t al: POTASSIUM HYDROXIDE FOR PEELING 89

Elimination of concern over the heat ring can simplify the prediction equation by changing the equation to measure percent peel loss only. The best and simplest equation was the following:

% Peel loss=-0.592+0.0268t+2.72x 10-aT+7.74x 10-3C Rz=0.962; Sy.x=0.0122

The equation predicted % peel loss with a low standard error of estimate. Substitution of the temperature, concentration and time values used in the peeling process into the % peel loss equation shows that the temperature has the greatest effect on % peel loss. The effects of concentration and time on the % peel loss equation are similar in value but considerably lower than the temperature effects. The simple equation can be used to calculate peel losses from any combination of time, temperature, and concentration. The calculated peeling loss can be tested and further tests can be conducted to obtain the actual optimum conditions.

A comparison of the best equation for Rank Index and % peel loss showed that squared terms for immersion time and temperature were included in the Rank Index. The % peel loss included only the linear terms of time, temperature and concentration.

Peel losses were approximately 1.0-2.5% higher with NaOH than with KOH at the same peeling conditions. Thus, to obtain equal peeling losses at the same temperature and immersion time, lower NaOH concentrations would be required than with KOH.

Taste panel tests were run to determine whether differences could be detected between potatoes peeled by NaOH and KOH and made into mashed or French fried potatoes. The triangle difference test indicated the judges could not detect flavor differences at the 5% significance level. No visual differences were noted between potatoes peeled with NaOH and KOH and cooked into mashed or French fried potatoes.

The effectiveness of KOH for peeling and the taste panel test indicate that KOH should be seriously considered as a substitute for NaOH for peeling potatoes. Other advantages of using KOH are:

1) The wastes from the primary and secondary waste treatment could be irrigated onto farm land because K is a major plant nutrient.

2) The increase of K in the soil could be decreased considerably by the choice of crops requiring large amounts of K.

3) The settleable solids should be available for feeding livestock. Settleable solids from NaOH peeling operations are being fed to livestock now.

4) Solid waste not fed to livestock could be incorporated into the soil if necessary.

5) No change is equipment would be required if regular NaOH peeling or dry peeling is currently being used.

90 AMERICAN POTATO JOURNAL (Vol . 54

6) High concentration of KOH (45%) crystallize at -22 F ( -30 C) while NaOH (45-50%) will crystallize at much higher temperatures near 45-50 F (7.2-10 C). The KOH would offer special advantages where cold weather storage is necessary.

7) Residual potassium left on the potato would be more desirable than sodium for people suffering from hypertension and who are on sodium restricted diets.

8) A patent issued to Cole (1) reported that potassium phosphate in the precook water produced potato flakes superior in both flavor and texture.

The higher cost of KOH compared to NaOH is the major disadvan- tage. The higher cost should be considered in relation to costs of pollution control now and in the future, and the public relations value of elimination of pollution. The use of KOH for peeling and the careful use of the waste for livestock feed and irrigation can eliminate the pollution of lakes and rivers. The KOH peeling should also be considered for peeling other fruits and vegetables.

Literature Cited

1. Cole. M.S. 1965. Process for dehydrating potatoes. U.S. Patent 3,219,464.

2. Dunlap. R.L. 1944. Lye peeling pays off. Food Ind. 16:969-971. 3. Eidt, C.E. and M. MacArthur. 1944. The peeling of fruits and vegetables for processing.

Foods in Canada. 4 (7):31-35. 4. Graham, R.P., C.C. Huxsoll, M.R. Hart, M.L. Weaver, and A.I. Morgan, Jr. 1969. Dry

causttic peeling of potatoes. Food Technol 23 (2): 195-201. 5. Harrington, W.D., P.C. Mayer, R.L. Olson, W.R. Mullins, and R.L. Potter, Jr. 1956.

Observation on pre-peeling potatoes. Food Technol 10:347-351. 6. Harrington. W.D., and R. Shaw. 1967. Peeling potatoes for processing, In Potato Proces-

sing, ed. W.F. Talburt and O. Smith. The Avi Publ. Co., Westport, Conn., p. 247-259. 7. Lowe, C.W. 1964. Some techniques of evolutionary operations. Trans. Instn. Chem.

Engrs. 42:T334-T344. 8. Lowe, C. W. 1967. A report on a simplex evolutionary operation for multiple responses.

Trans. Instn. Chem. Engr. 45:T3-T8. 9. Mazzola, L.C. 1943. New caustic peeling method reduces waste, saves labor. Food Ind

15:53-54, 104. 10. Olson, I.T. 1947. Wetting agents speed chemical peeling. Food Ind 13 (4):51-52. 11. Powers, M.J. 1975. KOH plus pressure saves up to 25% apple waste. Food Eng (NY) 47

(4):74-75. 12. Stanley, L. 1926. Canning fruits and vegetables in the home. US Dep Agric Farmers Bull,

No 1471. 13. Thompson, J.B. 1907, Process of skinning fruits. U.S. Patent 834, 311. 14. Van Blaricom, L.D. 1969. Caustic lye peeling of tomatoes and peaches with sodium and

potassium hydroxide. SC Agric Exp Stn Circ 159, llpp. 15. Wager, H.G., R.G. Tompkins, S.T.P. Brightwell, R.J.L. Allen, and L.W. Mapson. 1945.

The drying of potatoes. Food Manuf 20 (8):287-293. 16. Woodroof. J.G., S.R. Cecil. E. Shelor, and I.A. Cecil. 1948. Peeling with Lye. Food Ind 20

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Potassium hydroxide for peeling potatoes