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Mathematical analysis of lye peeling of tomatoes
JoumalofFoodEngineenng23 (1994) 225-231 Q 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0260-8774/94/$7.00 ELSEVIER
Research Note Mathematical Analysis of Lye Peeling of Tomatoes Levent Bayindirfi Food Engineering
Department,
Middle East Technical University, Iniinii Bulravi, 0653 1 Ankara, Turkey
(Received 16 June 1992; accepted 26 April 1993)
ABSTRACT Tome-temperu~re-concen~~t~n relatums for lye peeling of tomatoes were in~st~g~ted and a mathe~tic~l model was developed and rested. The time-temperature correlation was linear, white the time-concentration correlation wasexponential. Two sets oftreatments were used in the experiments: (a) treatment with sodium hydroxide alone; and (b) pretreatment with sorbic acid foilowed by immersion in sodium hydroxide. Treatment with 9% sodium hydroxide at WC for 60 s was su~cient to peel the tomatoes. The model developed predicted the results with a correlation coeficient of approximately 099.
NOTATION
c r2 t T
Concentration of sodium hydroxide (%) Correlation coefficient Time(s) Temperature (“C) Constant Constant Constant Constant
in in in in
eqn ( 1) eqn ( 1) eqns (2) and (3) eqns (2) and (3) 225
INTRODUCTION The tomato and its products have a large market in Turkey. One of the most important steps in the canning of whole and cut tomatoes is the removal of their skin. Steam and lye peeling methods are used worldwide for peeling tomatoes prior to canning. Steam peeling of tomatoes has been used in Europe for several years. Stephens et al. (1973) reported the effects of different immersion times in hot calcium chloride on the peeling efficiency of tomatoes, as an alternative to peeling by sodium hydroxide. Neumann et al. (1978) reported on peeling aids and their application to caustic peeling of tomatoes. They showed that a pre~eatment with octanoic acid was most promising. Their studies resulted in effective peeling by using 0.15% octanoic acid in a pretreatment solution for 30 s at 65°C rather than the caustic bath, and this minimized the amount of peeling aid in the recovered pulp. There have been a large number of time-temperature-concentration relations reported for peeling various products (Athanasopoulos & Vagias, 1987). In this study, a relationship was developed to describe the timetemperature-concentration relationship for tomatoes. MATERIALS AND METHODS Tomatoes of Adana variety, cultivated in the southern region of Turkey, were purchased from a local market. They were separated according to their size, maturity, damage, and disease. Undamaged and mature tomatoes of 4.5-5.5 cm diameter were used in the experiments. Sodium hydroxide solutions (6%, 9%, 12% and 15% (w/w )) were used in the experiments at 60, 70, 80 and 90°C for varying time intervals (Table 1). As the temperature was increased, shorter time intervals were chosen since peeling was expected to be more efficient at higher temperatures. Solutions, heated to the chosen temperature, were held constant by using a temperature-controlled water bath. The tomatoes were immersed in the hot solution of sodium hydroxide for a predetermined time and were then removed and cooled in tap water having a flow rate of 100 ml/s at 15°C for 60 s. Then the tomatoes were evaluated according to how much they had peeled. In order to find out the need,for pretreatment, 1% (w/w) sorbic acid solution was used at 65°C for 60 s followed by an immersion in 9% sodium hydroxide solution at 90°C for 40 s.
L. Bayindirli
227
TABLE 1 Time-Temperature-Concentration-Score Relation for Lye Feeling of Tomatoes
Concentration (%, NaOH)
Temperature (“C) Concentration (%, NaOH)
Temperature CC)
Concentration (%, NaOH) 12 15 6 9
Temperature (“C) Concentration (%, NaOH) 6 9 12 15 1s 1115 tf 15 1115 .. .L II* .*.
Temperature (‘C)
Samples were scored visually according to the degree of peel removal from the surface of the tomato as below: - very good peeling: peeling higher than 98% = score***** - good peeling: peeling higher than 7 5% = score**** - slide peeling: peeling higher than 50% = score*** - bad peeling: peeling higher than 25% = score** - no peeling: peeling higher than 0% = score* The results are tabulated in Table 1. In each column, the minimum time for very good peeling (i.e. peeling higher than 98% and obtaining a score of *****) is shown as a shaded area.
228
Mathematical analysisoflyepeeling of tomatoes
RESULTS AND DISCUSSION The minimum required time for ‘very good peeling’ was the basic criteria selected for assessing each treatment and, as expected, this time was dependent of the temperature and concentration of sodium hydroxide solution. Immersion time was affected by the temperature for all ~oncen~ations and could be fitted to a straight line of the form:
T=a+#?t
(1)
The parameters obtained by regression analysis for each sodium hydroxide concentration are shown in Table 2 and the good fit is demo~trat~ in Fig. 1.
TABLE 2
O~ratio~~ Parameters and Constants for Lye Peeling of Tomatoes r2
Model
-0.02572
0+99
90.63
- o-035 33
@99
T= a+@ &n(l))
12
90.59
- 0.044 38
@98
15
91.53
- 0,067 82
0.99
Temperature (“C)
Concentration (%, NaOH)
a
60, 70, 80, 90
6
90.91
60, 70, 80, 90
9
60, 70, 80, 90
60, 70, 80, 90
I 0
I
200
I
400
I
600
I
800
Time bed Fig. 1.
B
i
1000
t
1200 1400
-
Time-temperature relation for lye peeling of tomatoes.
229
L. Bayindirli
The effect of sodium hydroxide concentration exponential relationship (eqns (2) and (3)).
was described
by an
C=exp(Q, + q,t)
(2)
ln C=(@+qpt)
(3)
Equation (3) is the linearized form of eqn (2) and both resulted in fits with correlation coefficients around 0.99 (Figs 2 and 3, Tables 2 and 3). Figures l-3 and eqns (l-3) clearly define the time-temperatureconcentration relations for lye peeling of tomatoes. It is also observed that as temperature and concentration increases, the time needed for
t -2 5 0 3
-3
+ -4 0 9ooc
-5 0
I 200
1 400
1 600
I 800
-
Equation
2
I I 1000 1200 Id 10
Time (see) Fig. 2.
Time-concentration
relation for lye peeling of tomatoes.
25 l 0
6O’C 7O’C l 80°C 0
90-c Equotion
-
0
200
400 .600
600
Time (set) Fig. 3.
Time-concentration
3
1000 1200 1400
-
relation for lye peeling of tomatoes.
230
Mathematical analysisoflye peeling of tomatoes
TABLE 3 Operational Parameters and Constants for Lye Peeling of Tomatoes Concentration (%, NaOH)
Temperature (“C)
6, 9, 12, 15
60
6, 9, 12, 15
Q,
(P
r?
Model
-1.24
-000130
70
0.99 lnC=Q,+qt (eqn (3)) - 1.42 - 0.001 69 0.98 or
6, 9, 12, 15
80
-1.22
-000441
6, 9, 12, 15
90
-0.47
-0.031 34 0.99
0.99
c=e(a+prj (eqn (2))
skin removal decreases. However, as seen from Fig. 3, the effect of temperature on peeling efficiency is higher than the effect of concentration. Before optimizing the concentration, a decision can be made with regard to temperature, in that working at 90°C is the most effective temperature among the experiments. When the temperature is fixed, it is easier to decide on concentration and at 9O”C, a concentration range of 6-15% was sufficient for peeling. Further, within that range 9% seem to be the most effjcient concentration as decreasing the concentration increased the time needed for ‘very good peeling’. Increasing the concentration, on the other hand, increased the amount of peeling aid used and also increased the amount of possible sodium hydroxide residue on the tomato, with only a small decrease in peeling time. Therefore, taking into consideration all the effects listed above, an optimum set of conditions for treatment of tomatoes was identified as 9% sodium hydroxide concentration at 90°C for 60 s. Once the time-temperature-concentration is fixed, then the necessity for pretreatment was investigated. Though the pretreatment decreased the treatment tQne for peeling by 33%, the overall process time increased, so use of a pretreatment step seems to be unnecessary. Finally, whatever the necessary time-temperature-concentration relation, the model represents the case quite well and such an equation is applicable to all varieties of tomatoes by simply inserting new constants into the equation for the variety and condition of the tomato used in the canning operation. ACKNOWLEDGEMENTS Thanks to Mr Hakan Kip who helped in collecting the data.
L. Bayindirli
231
REFERENCES Athanasopoulos, P. E. & Vagjas, G. A. ( 1987). Lye peeling of mandarins. J. Food Process Engng, 9,277-85.
Neumann, H. J., Schultz, W. G., Morgan, J. P. & Schade, J. E. (1978). Peeling aids and their application to caustic peeling of tomatoes. J. Food Sci., 43, 1626-7. Stephens, T. S., Saldana, G. & Brown, H. E. (1973). Effects of different submergence times in hot calcium chloride on peeling efficiency of tomatoes. J. Food Sci., 38,512-15.
Research Note Mathematical Analysis of Lye Peeling of Tomatoes Levent Bayindirfi Food Engineering
Department,
Middle East Technical University, Iniinii Bulravi, 0653 1 Ankara, Turkey
(Received 16 June 1992; accepted 26 April 1993)
ABSTRACT Tome-temperu~re-concen~~t~n relatums for lye peeling of tomatoes were in~st~g~ted and a mathe~tic~l model was developed and rested. The time-temperature correlation was linear, white the time-concentration correlation wasexponential. Two sets oftreatments were used in the experiments: (a) treatment with sodium hydroxide alone; and (b) pretreatment with sorbic acid foilowed by immersion in sodium hydroxide. Treatment with 9% sodium hydroxide at WC for 60 s was su~cient to peel the tomatoes. The model developed predicted the results with a correlation coeficient of approximately 099.
NOTATION
c r2 t T
Concentration of sodium hydroxide (%) Correlation coefficient Time(s) Temperature (“C) Constant Constant Constant Constant
in in in in
eqn ( 1) eqn ( 1) eqns (2) and (3) eqns (2) and (3) 225
INTRODUCTION The tomato and its products have a large market in Turkey. One of the most important steps in the canning of whole and cut tomatoes is the removal of their skin. Steam and lye peeling methods are used worldwide for peeling tomatoes prior to canning. Steam peeling of tomatoes has been used in Europe for several years. Stephens et al. (1973) reported the effects of different immersion times in hot calcium chloride on the peeling efficiency of tomatoes, as an alternative to peeling by sodium hydroxide. Neumann et al. (1978) reported on peeling aids and their application to caustic peeling of tomatoes. They showed that a pre~eatment with octanoic acid was most promising. Their studies resulted in effective peeling by using 0.15% octanoic acid in a pretreatment solution for 30 s at 65°C rather than the caustic bath, and this minimized the amount of peeling aid in the recovered pulp. There have been a large number of time-temperature-concentration relations reported for peeling various products (Athanasopoulos & Vagias, 1987). In this study, a relationship was developed to describe the timetemperature-concentration relationship for tomatoes. MATERIALS AND METHODS Tomatoes of Adana variety, cultivated in the southern region of Turkey, were purchased from a local market. They were separated according to their size, maturity, damage, and disease. Undamaged and mature tomatoes of 4.5-5.5 cm diameter were used in the experiments. Sodium hydroxide solutions (6%, 9%, 12% and 15% (w/w )) were used in the experiments at 60, 70, 80 and 90°C for varying time intervals (Table 1). As the temperature was increased, shorter time intervals were chosen since peeling was expected to be more efficient at higher temperatures. Solutions, heated to the chosen temperature, were held constant by using a temperature-controlled water bath. The tomatoes were immersed in the hot solution of sodium hydroxide for a predetermined time and were then removed and cooled in tap water having a flow rate of 100 ml/s at 15°C for 60 s. Then the tomatoes were evaluated according to how much they had peeled. In order to find out the need,for pretreatment, 1% (w/w) sorbic acid solution was used at 65°C for 60 s followed by an immersion in 9% sodium hydroxide solution at 90°C for 40 s.
L. Bayindirli
227
TABLE 1 Time-Temperature-Concentration-Score Relation for Lye Feeling of Tomatoes
Concentration (%, NaOH)
Temperature (“C) Concentration (%, NaOH)
Temperature CC)
Concentration (%, NaOH) 12 15 6 9
Temperature (“C) Concentration (%, NaOH) 6 9 12 15 1s 1115 tf 15 1115 .. .L II* .*.
Temperature (‘C)
Samples were scored visually according to the degree of peel removal from the surface of the tomato as below: - very good peeling: peeling higher than 98% = score***** - good peeling: peeling higher than 7 5% = score**** - slide peeling: peeling higher than 50% = score*** - bad peeling: peeling higher than 25% = score** - no peeling: peeling higher than 0% = score* The results are tabulated in Table 1. In each column, the minimum time for very good peeling (i.e. peeling higher than 98% and obtaining a score of *****) is shown as a shaded area.
228
Mathematical analysisoflyepeeling of tomatoes
RESULTS AND DISCUSSION The minimum required time for ‘very good peeling’ was the basic criteria selected for assessing each treatment and, as expected, this time was dependent of the temperature and concentration of sodium hydroxide solution. Immersion time was affected by the temperature for all ~oncen~ations and could be fitted to a straight line of the form:
T=a+#?t
(1)
The parameters obtained by regression analysis for each sodium hydroxide concentration are shown in Table 2 and the good fit is demo~trat~ in Fig. 1.
TABLE 2
O~ratio~~ Parameters and Constants for Lye Peeling of Tomatoes r2
Model
-0.02572
0+99
90.63
- o-035 33
@99
T= a+@ &n(l))
12
90.59
- 0.044 38
@98
15
91.53
- 0,067 82
0.99
Temperature (“C)
Concentration (%, NaOH)
a
60, 70, 80, 90
6
90.91
60, 70, 80, 90
9
60, 70, 80, 90
60, 70, 80, 90
I 0
I
200
I
400
I
600
I
800
Time bed Fig. 1.
B
i
1000
t
1200 1400
-
Time-temperature relation for lye peeling of tomatoes.
229
L. Bayindirli
The effect of sodium hydroxide concentration exponential relationship (eqns (2) and (3)).
was described
by an
C=exp(Q, + q,t)
(2)
ln C=(@+qpt)
(3)
Equation (3) is the linearized form of eqn (2) and both resulted in fits with correlation coefficients around 0.99 (Figs 2 and 3, Tables 2 and 3). Figures l-3 and eqns (l-3) clearly define the time-temperatureconcentration relations for lye peeling of tomatoes. It is also observed that as temperature and concentration increases, the time needed for
t -2 5 0 3
-3
+ -4 0 9ooc
-5 0
I 200
1 400
1 600
I 800
-
Equation
2
I I 1000 1200 Id 10
Time (see) Fig. 2.
Time-concentration
relation for lye peeling of tomatoes.
25 l 0
6O’C 7O’C l 80°C 0
90-c Equotion
-
0
200
400 .600
600
Time (set) Fig. 3.
Time-concentration
3
1000 1200 1400
-
relation for lye peeling of tomatoes.
230
Mathematical analysisoflye peeling of tomatoes
TABLE 3 Operational Parameters and Constants for Lye Peeling of Tomatoes Concentration (%, NaOH)
Temperature (“C)
6, 9, 12, 15
60
6, 9, 12, 15
Q,
(P
r?
Model
-1.24
-000130
70
0.99 lnC=Q,+qt (eqn (3)) - 1.42 - 0.001 69 0.98 or
6, 9, 12, 15
80
-1.22
-000441
6, 9, 12, 15
90
-0.47
-0.031 34 0.99
0.99
c=e(a+prj (eqn (2))
skin removal decreases. However, as seen from Fig. 3, the effect of temperature on peeling efficiency is higher than the effect of concentration. Before optimizing the concentration, a decision can be made with regard to temperature, in that working at 90°C is the most effective temperature among the experiments. When the temperature is fixed, it is easier to decide on concentration and at 9O”C, a concentration range of 6-15% was sufficient for peeling. Further, within that range 9% seem to be the most effjcient concentration as decreasing the concentration increased the time needed for ‘very good peeling’. Increasing the concentration, on the other hand, increased the amount of peeling aid used and also increased the amount of possible sodium hydroxide residue on the tomato, with only a small decrease in peeling time. Therefore, taking into consideration all the effects listed above, an optimum set of conditions for treatment of tomatoes was identified as 9% sodium hydroxide concentration at 90°C for 60 s. Once the time-temperature-concentration is fixed, then the necessity for pretreatment was investigated. Though the pretreatment decreased the treatment tQne for peeling by 33%, the overall process time increased, so use of a pretreatment step seems to be unnecessary. Finally, whatever the necessary time-temperature-concentration relation, the model represents the case quite well and such an equation is applicable to all varieties of tomatoes by simply inserting new constants into the equation for the variety and condition of the tomato used in the canning operation. ACKNOWLEDGEMENTS Thanks to Mr Hakan Kip who helped in collecting the data.
L. Bayindirli
231
REFERENCES Athanasopoulos, P. E. & Vagjas, G. A. ( 1987). Lye peeling of mandarins. J. Food Process Engng, 9,277-85.
Neumann, H. J., Schultz, W. G., Morgan, J. P. & Schade, J. E. (1978). Peeling aids and their application to caustic peeling of tomatoes. J. Food Sci., 43, 1626-7. Stephens, T. S., Saldana, G. & Brown, H. E. (1973). Effects of different submergence times in hot calcium chloride on peeling efficiency of tomatoes. J. Food Sci., 38,512-15.