Double-dip caustic peeling of potatoes I: Laboratory-scale development
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 Abstract 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. Resumen 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. Introduction 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. 0.6 0.4 HUXSOLL, et al- DOUBLE-DIP PEELING I NaOH ABSORBED BY U.S. # 1 POTATOES IN 77~ LYE 329 LU 0 0 O.. 6 v 0 0 "t- O (3 Z 6 v 20% NaO H / / / ~K, / NoOH j#" i i i 0.2 1981) 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 WASHED POTATOES CAUSTIC IMMERSION (C1) HOLD PERIOD (HI) CAUSTIC IMMERSION (C2) HOLD PERIOD (H2) INFRARED HEAT (IR) (Optional) PEEL FIG. 3. Flow chart for double-dip caustic peeling process. 332 AMERICAN POTATO JOURNAL (Vol. 58 C1 C2 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~ 15 - -O lo A 0~- ~Z~ q ~- s 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 zero hold period for H1, therefore, corresponds to a continuous 60 sec immersion. Curve B corresponds to an immersion in 8~ NaOH at 88~ for 30 sec for C1 and 5070 NaOH for 30 sec for C2. When caustic solutions of different concentrations in C1 and C2 were used, a minimum of 5 min was used for H1 to avoid dilution of the caustic absorbed in the first treatment by the lower concentration caustic in the second treat- ment. Curve C represents 5070 NaOH at 88~ for 30 sec for both C1 and C2. For all curves, the hold period following the second caustic treatment (H2) was 3 min at ambient temperature, and infrared heat (IR) was applied for 1 min. H2 IR 3 MIN. AMBIENT 1 MIN 3 MIN. AMBIENT 1 MIN 3 MIN. AMBIENT 1 MIN I I 0 15 30 HOLD PERIOD HI (MINUTES) FIG. 4. Peeling loss as a function of initial hold period (H1) for several combinations of lye treatment. It is evident from Figure 4 that the amount of tissue removed, as indicated by peel loss, was dependent upon the length of the hold period between the two caustic treatments. For curve A, where 8~ NaOH was used in both caustic applications, the effect of the hold time diminished after about 15 min. For curves B and C, where 5070 NaOH was used for the second treat- 1981) HUXSOLL, et al: DOUBLE-DIP PEELING I 333 ment, the peel loss continued to increase in a nearly linear manner over the entire 30 rain. period. Apparently, the peel surface in curve A was suffi- ciently modified after 15 min holding that the 8~ caustic in the second treatment readily penetrated, and additional holding resulted in little in- crease in peel removal. For curves B and C, the surface had not been suffi- ciently modified at 15 min holding so that the weaker 50 NaOH solution in the second caustic application did not immediately penetrate the underlying potato flesh; therefore, the longer holding period of 30 min after the first caustic dip did enhance lye penetration in the second dip. Peel loss, as a function of IR, is shown in Figure 5 for a double-dip caustic treatment, curve A, and a single caustic treatment, curve B. For curve A, caustic treatment C1 was 10070 NaOH at 88~ for 45 sec, H1 was 15 min at ambient temperature, C2 was 5070 NaOH at 88~ for 45 sec, and H2 was 3 min at ambient temperature. For curve B, a caustic treatment of 10070 NaOH at 93~ for 60 sec was used with a 3 min ambient temperature hold prior to application of IR. The caustic treatment and hold for curve B are typical of those used for a standard infrared peeling system. The caustic uptake was 0.066 kg NaOH/100 kg of potatoes for the double-dip treat- ment, curve A, and 0.088 kg NaOH/100 kg of potatoes for the single dip treatment, curve B. C1 H1 C2 H1 CURVE A 45 SEC, 10% NaOH, 88~ 15 MIN 45 SEC, 5% NaOH, 88~ 3 MIN (AMBIENT) (AMBIENT) CURVE B 60 SEC, 10% NaOH, 93~ 3 MIN (AMBIENT) 15 - -Z~ i,o,--'-"- ~ j~ 0 30 60 90 INFRARED IRRADIATION (SECONDS) FIG. 5. Peel loss as a function of infrared radiation (IR) for double-dip and conventional lye application. 334 AMERICAN POTATO JOURNAL (Vol. 58 Figure 5 illustrates the improved efficiency that was obtained when caustic was applied in a double-dip system. Even though the caustic uptake was less for the double-dip system than for the single-dip system, the peel removed in the double-dip system was greater than in the single-dip system when both caustic treatments were followed by up to 90 sec of infrared heat. In fact, the peel removed without infrared heat in the double-dip treatment corresponded very closely to the peel removed in a single-dip treatment followed by 60-90 sec of infrared heat. For the double-dip treatment, peel removal began to level off after about 60 sec of infrared irradiation, but for the single-dip treatment, peel removal continued to increase for up to 90 sec of infrared irradiation. This difference in response to IR treatment between the double-dip and single-dip system indicates that much of the absorbed caustic reacts with the potato tissue during the first hold period of the double-dip process, making it less responsive to the effects of IR treatment. However, for the single-dip treatment, the caustic apparently remains unre- acted until the IR is applied. Lye uptake for several treatments is depicted in Figure 6. Curve A is for a single caustic treatment in 10070 NaOH at 88~ for 30 and 60 sec. Curves B and C are for double-dip treatments. For both curves B and C, the C1 treat- ment was 10070 NaOH at 88~ for 30 sec, and H1 was 15 min at ambient temperature. For curve B, C2 was 5~ NaOH at 88~ for 30 sec and 60 sec. For curve C, C2 was 307o NaOH at 88~ for 60 sec and 90 sec. The results shown on this curve again illustrate the phenomena discussed in regard to Figure 1. For curve A, the caustic penetrated the suberized outer layer during the first 30 sec of immersion. For curve B, the suberized layer became penetrable after about 30 sec of the second immersion. This curve also illustrates the greater degree o f control of caustic uptake that is possible for the double-dip system compared to the single-dip system. Another interesting phenomenon is shown in curve C of Figure 6. The caustic absorbed on the potatoes was less after 60 and 90 sec in the second immersion in 307o NaOH at 88~ than it was after the initial caustic treat- ment. Apparently, some of the caustic absorbed in the initial immersion in 10o7o NaOH remained free or unreacted, after the 15 min hold period, and it was washed off or diluted by the subsequent immersion in 307o NaOH. Comparison of peel losses between ambient temperature holding and holding in 60~ heated air for H I is shown in Table 1. It is evident from this table that the holding temperature has an effect on the ultimate tissue removal. In each case, as would be expected, the higher holding temper- ature resulted in increased peel removal. A temperature of 60~ appeared to be near optimum for this hold, as higher temperatures may result in starch gelatinization in the underlying tissue and heat-ring formation in the peeled potato. It was also found that holding in heated air at temperatures 1981) HUXSOLL, et al: DOUBLE-DIP PEELING I 335 h igher than 60~ caused the sur faces to rap id ly dry out , and the absorbed caust ic became less react ive . .15 2 !or .,o .05 Z~ . . . . co 10% CURVE B -- 5% NaOH J CURVE C -- 3% NaOH l I / / I l I I 0 30 60 0 30 60 90 IMMERSION T IME (SECONDS) FIG. 6. Lye uptake for several double-dip caustic treatments. TABLE 1. - - Peel loss fo r ambient temperature and 60~ air homing (H1) and various caustic treatments 1 . H1 Treatments 15 min 15 min ambient 60~ Air C1 C2 Peel loss (07o) 10%0 NaOH 5% NaOH 11.4 13.0 88~ 88oc 45 sec 45 sec 10% NaOH 5% NaOH 10.5 12.1 88~ 88~ 45 sec 60 sec 10% NaOH 5% NaOH 6.0 10.0 88~ 88~ 30 sec 45 sec 'HI: First Holding Period C 1: First caustic application C2: Second caustic application The second holding period, H2, used for all treatments was 3 min at ambient temperature. In Tab le 2, a compar i son o f peel losses and pee l ing qua l i ty is made be- tween ho ld ing in heated air at 60~ and in a s team air mix ture at the same 336 AMERICAN POTATO JOURNAL (Vol. 58 temperature. The results indicate that using 60~ heated air was super ior to a steam-air mixture at the same temperature. This is p robab ly due to the di lut ion o f absorbed caustic that resulted f rom the condensing steam air mixture. Improved peeling always occurred when heating in 60~ air pre- ceded heating in 60~ steam-air , These results indicate that while heat is beneficial to the peel modi f icat ion dur ing the first hold ing per iod, it must be done so that the caustic concentrat ion and act ion are not decreased. TABLE 2. - - Peel loss and peel quality for various combinations of 60 ~ air and 60 ~ steam-air homing (Hlfl Sample No. Hold Treatment (HI) Peel Loss Peeling 600C 60~ Steam Air (070) Quality 1 15 min - - 9.3 Good clean peel 2 - - 15 min 5.1 Fair peel, skin remained in some areas 3 5 min 10 min 8.6 Good peel 4 10 min 5 min 11.9 Good peel 5 - - 20 min 5.5 Some skin, tight, eyes clean 6 20 min - - 11.6 Very good to overpeeled 7 10 rain 10 rain 8 Good peel 8 5 rain 15 min 8.1 Fair to good peel 9 15 min 5 min 11.1 Very clean ped 'For all samples: C1 - 10070 NaOH at 88~ for 30 sec C2 - 5070 NaOH at 88~ for 30 sec H2 - 3 min ambient temperature IR- 1 min Conclusions Our results show that the double-d ip caustic peeling system can effec- tively increase peeling eff iciency without the use o f addi t ional energy input. The caustic eff ic iency o f the double-d ip system without infrared heat input is about equivalent to the caustic eff iciency o f a single-dip system combined with 60-90 sec o f infrared heat. Whi le addi t ional energy input in the form of infrared i r radiat ion further improved caustic eff ic iency for the double-d ip system, the improvement would probab ly not just i fy the added cost o f in- f rared heat in most pract ical s ituations. Likewise, while caustic eff iciency was enhanced by added heat dur ing the hold per iod between caustic appl ica- 1981) HUXSOLL, et ai: DOUBLE-DIP PEELING I 337 tions, the amount of imprqvement would probably not justify the cost of the added heat input, unless the heat could be readily obtained as waste heat from another operation. While the suberized peel of the potato may account for only about 5~ or less of the total potato weight, normal peel losses usually amount to 10% or more. This means that much of the material removed in peeling is tissue underlying the suberized peel, and this material can be softened with very low caustic concentrations. When single dip caustic treatments are used, the caustic concentration must be sufficient to attack the suberized peel, and the same solution is used to attack the underlying flesh. As a result, the caustic concentration is higher than is needed to release the tissue, causing a higher than necessary caustic use. On the other hand, when the double-dip system is used, the initial application of caustic must be sufficient to attack the outer suberized tissue only to the extent that it becomes penetrable to the lower concentration caustic in the second application. The second caustic concentration is just sufficient to cause the underlying tissue to separate. Thus, a substantial reduction in caustic usage is effected. Because of the highly idealized conditions used for these laboratory tests, it would seldom be possible to attain the same results on caustic usage in pro- duction operations. Therefore, we emphasize again that it is the relative values, rather than the absolute values, that should be noted. Also, much of our discussion here deals with caustic efficiency in terms of tissue removed versus caustic uptake. Obviously, good peeling is not the result of high tissue loss for a given amount of caustic consumed. Good quality peeling occurs when the peel and minor defects are removed without removing excessive amounts of other tissue and without the development of other undesirable characteristics, such as heat ring. While it is difficult to make meaningful comparisons of the subjective factors that determine peeling quality with the small-sized samples used here, we observed that at comparable peel losses, the double-dip system resulted in defect and peel removal that was as good as, or better than, that obtained in the single-dip system, and the heat ring formation was substantially less for the double-dip system. In summary, we have developed in the laboratory, a system for caustic peeling potatoes that effectively peels potatoes using minimal amounts of caustic. An analysis of this system under commercial operating conditions is the subject of the second part of this communication. Literature Cited 1. Adams, H.W., F.D. Hickey, and M.J. Willard, Jr. 1960. Lye-pressure steam peeling of po- tatoes. Food Technol 14:1-3. 2. Feustel, I.C., C.E. Hendel, and M.E. Juilly. 1964. Potatoes, in Food Dehydration, Vol. II - Products and Technology. M.J. Copley and W.B. Van Arsdel, eds. The AVI Publishing Company, Inc., Westport, Connecticut. 338 AMERICAN POTATO JOURNAL (Vol . 58 3. Graham, R.P., C.C. Huxsoll, M.R. Hart, M.L. Weaver, and A.I. Morgan, Jr. 1969. Dry Caustic Peeling of Potatoes. Food Technol 23:61. 4. Huxsoll, C.C., M.L. Weaver, and R.P. Graham. 1973. Process for peeling potatoes. U.S. Patent 3,759,160. 5. Huxsoll, C.C. and T.J. Smith. 1975. Peeling potatoes for processing, in Potato Processing, 3rd Ed. W.F. Talburt and O. Smith, eds. The AVI Publishing Company. Inc., Westport, Connecticut. 6. Huxsoll, C.C., M.L. Weaver, and R.P. Graham. 1976. Process for peeling fruits and veg- etables. U.S. Patent 3,950,556. 7. Willard, M.J. 1971. A grading system for peeled potatoes. Proc. 21st National Potato Util- ization Conference, July 28.