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Predictive model of keeping quality of tomatoes
Posthart~est Biology and Technology, 2 (1993) 179-185 © 1993 Elsevier Science Publishers B.V. All rights reserved 0925-5214/93/$06.00
179
POSTEC 01020
Predictive model of keeping quality of tomatoes Janna J. Polderdijk, L.M.M. Tijskens, Jan E. Robbers and Henry C.P.M. van der Valk A TO-DLO Agrotechnological Research Institute, Wageningen, The Netherlands (Accepted 11 September 1992)
ABSTRACT Polderdijk, J.J., Tijskens, L.M.M., Robbers, J.E. and Van der Valk, H.C.P.M. (1993) Predictive model of keeping quality of tomatoes. Posthart'est Biol. Technol. 2, 179-185. A relation was established between the objectively-determined firmness of glasshouse tomatoes and subjective estimates of the keeping quality of the same fruit. Firmness was measured by determining the compression distance of fruit under a force of 3 N. Keeping quality was assessed daily by experienced personnel, and an endpoint was considered to have been reached when the tomatoes were too soft to sell. The following expression was established: keeping quality (days at 18°C) = -12.6 (days/mm)x compression distance (mm)+ b (days). Included in the intercept b were factors varying with the point in the growing season at which the samples were assessed, such as daylength. Fruit firmness was found to be relatively independent of the temperature of the fruit at which this factor was measured. The regression equation accounted for 74% of the variation in keeping quality.
Key words: Tomato fruit; Tomato keeping quality; Tomato firmness; Predictive model of tomato survivability
INTRODUCTION
In The Netherlands tomatoes are distributed through auctions, where the fruits are inspected for size, colour and external quality factors such as firmness, irregular colouring, sunscald, russetting, damage/bruising, gold spots, and silver stains. The defects are evaluated by experienced auction personnel, without the help of measuring equipment. The fruits are graded according to size and colour. A commodity can be further subdivided into quality classes based on the external factors, and on the homogeneity of the tomatoes. The keeping quality of tomatoes is a very important quality criterion, which is strongly connected with softening of the fruits. Therefore, firmness is one of the Correspondence to: J.J. Polderdijk, ATO-DLO Agrotechnological Research Institute, P.O. Box 17. 6700 AA Wageningen, The Netherlands.
180
J.J. P O L D E R D I J K ET AL.
most important quality parameters for shippers and sellers (Jordan et al., 1985). Firmness is monitored at the auction and in the trade channel by manual inspection only. The degree of red colouration is assumed to be representative of the stage of ripening and therefore is related to softening and also keeping quality. Tomatoes are graded and pre-classified by the growers and are placed in different sections at the auction, based on fruit colour. However, shipments of tomatoes of a particular colour class may still show insufficient keeping quality because of excessive softening. This means that the classification based on colour alone does not adequately match potential keeping quality. Colour and firmness at the time of picking are both ripening phenomena that are correlated to keeping quality (Hall, 1964; Stenvers and Stork, 1976). A previous study with Dutch glasshouse tomatoes produced the proposition that firmness measurement at the auction can be reliably used to exclude suspect tomatoes (Polderdijk, 1989). However, initial firmness alone was insufficiently accurate in predicting the keeping quality throughout the production season that continues from early March until the beginning of December. To develop a model for predicting the keeping quality of tomatoes during the whole season it is essential to allow for changes in the relationship between the initial firmness and the keeping quality of the fruits during the year. Moreover, knowledge about factors determining and influencing the firmness of tomatoes is essential for optimizing growing conditions and postharvcst storage techniques. In this paper the relationship between firmness of tomatoes at arrival at the auction and their keeping quality under standardized conditions is discussed. A model describing this relation is presented. In addition, the effect of fruit temperature on the firmness measurement was evaluated. In practice the fruit temperature of the tomatoes arriving at the auction is closely related to the temperature during transport. If fruit temperature directly influences the firmness measurement, it would have consequences for a predicting model based on firmness. MATERIALS AND METHODS
Relation between initial firmness and keeping quality Commercially grown round glasshouse tomatoes (Lycopersicon esculentum L.) were purchased 19 times from two auctions in the Netherlands from autumn 1989 until late summer 1990. The commercial supply at the auctions consisted of the cultivars Calypso (70%), Liberto (20%) and the remaining 10% included Counter, Spectra, and Criterium. These cultivars are uniform in appearance and are considered equivalent. In the experiments differences between cultivars were ignored. At each sampling date representative samples of 15 sound fruits (diameter 47-57 mm) each were taken from ten different growers at the moment of delivery at the auction around noon to be sure the tomatoes measured were picked that day. In order to obtain the largest possible range in initial firmness, soft and firm
P R E D I C T I V E M O D E L OF T O M A T O K E E P I N G Q U A L I T Y
181
samples were selected by manual firmness evaluation. Fruits were transported to the laboratory within 2 h. Subsequently, the fruit firmness was measured using an Instron Universal Testing Machine. The tomatoes were compressed between two flat plates, diameter 50 ram, with a compression rate of 20 m m / m i n up to a maximum force of 3 N. In contrast with forces used by Gormley and M a h e r (1987) and Gough and Hobson (1990) this small force has been shown to be nondestructive (Stenvers et al., 1973), and the m e a s u r e m e n t did not reduce keeping quality (data not shown). The measuring point on the tomatoes was at the equator of the fruit, between the radial walls of the pericarp. The tested commercial cultivars have two or three locules which can easily be recognized by lines on the surface, starting at the blossom end of the fruit. Using these marking lines load points on both sides of the fruits were between the radial walls. As loading between radial walls was easy with the cultivars used, measuring compression along the longitudinal axis, as done by Hobson and Ambler (1988) was not necessary. Besides, their method does not match accepted manual firmness evaluations, and requires the removal of the calyx, which is unacceptable for some markets of round tomatoes. Ripening stage was defined by colour classification, using the Dutch rating scale 1 to 12 (1, green; 7, pink; 12, deeply red). Colour values measured according to this scale showed a correlation of 0.97 with the Hunter Lab/a value. For assessing the keeping quality, the tomatoes were stored at 18°C and a nn of 80%. Keeping quality was defined as the time in days at the constant temperature and R H between the start of storage and the time the fruits were unacceptable. Unacceptable means too soft to sell. Daily inspection was executed through manual evaluation of the firmness of the equator of the fruits by trained specialists. Regularly, firmness of the discarded fruits was also measured with the Instron Universal Testing Machine. The compression distance of these fruits was 1.47 mm (sD = 0.16). Other quality criteria than firmness (colour, taste, microbial decay, bruises, gold spots etc.) were not used to determine acceptability. The keeping quality and the compression distance were averaged for 15 fruits per grower. The data of 10 growers per testing date were used for regression analysis of initial firmness and keeping quality. Sampling dates were depicted in week numbers, where the first week of the year is number 1.
Influence of fruit temperature on firmness To evaluate the effect of the fruit t e m p e r a t u r e on fruit firmness measurement, fruits were kept overnight at 15°C, and the compression distance was measured the next morning. Subsequently, fruits were transferred to a climate chamber with a temperature of 18°C. The measuring spot was marked, and each m e a s u r e m e n t was repeated two more times at the same tomato. Repetitive measurements with this force did not result in significant changes in the compression distance readings (15 readings of a pink tomato on the same loading point showed an average of 0.60 m m with a SD = 0.02, and of a red tomato 1.27 m m with a SD = 0.05). Internal t e m p e r a t u r e of the fruits was measured in dummy fruits. It was found that 3 h was
182
J.J. P O L D E R D I J K
E T AL.
sufficient to equilibrate with the external temperature. After 3 h (when the fruit temperature was 18°C), firmness was measured, and the fruits were transferred to a chamber at 23°C. Again the firmness was measured after 3 h. The procedure was once again repeated at 28°C. As a control for maximum ripening and (thus) softening, fruits were transferred directly from 15°C to 28°C for 9 h. After 9 h the firmness of the control tomatoes was measured. At each temperature two fruits per colour stage (stages 4, 6 and 9) were measured. Compression distance was calculated as a percentage of the initial compression distance at 15°C. RESULTS Determining the keeping quality of tomatoes by manual evaluation of the firmness has proven to be reliable and reproducible. Tomatoes were considered unacceptably soft by three experienced specialists with a variation of one day at most. The compression distance, measured as the difference between the initial point of contact with the fruit and the diameter of the compressed fruit under a maximum force of 3 N, was used as a firmness indicator. The compression distance was also calculated as strain, that is the ratio between compression distance and the initial diameter of the fruit. This factor should correct for differences in initial size, but was not involved in the final model, and will therefore not be discussed further. The linear regression between the compression distance at 3 N and the keeping quality of the 2850 individual tomatoes throughout the whole production season accounted for only 12% of the variation. However, keeping quality of tomatoes at separate sampling dates during the harvest period correlated well with the initial firmness of the same fruits. As a representative example, the means of 10 batches of fruit collected in week 23 are shown in Fig 1. Firm fruits (with low compression distance) exhibited a relative long keeping quality, whereas soft fruits (high compression distance) showed a low keeping quality. Similar correlations between compression distance and keeping quality were found throughout the test period for each sampling date. The slopes of the calculated individual regression lines of each sampling date showed only little variation while the intercepts with the y-axis varied strongly throughout the season. In spring and autumn the intercepts are relatively high compared to the intercepts in summer. Therefore, a multiple linear regression model was calculated using the data from the 19 sampling weeks. To distinguish the sampling dates, corresponding week numbers were included as a factor for the regression analysis. This model consists of a common slope for all weeks, and a week-dependent intercept, fitted, using indicator variables (Fig 2). The common slope was estimated as - 1 2 . 6 (standard error 1.06). Keeping quality (days) = - 12.6 (days/mm)compression distance (mm) + intercept (days)
PREDICTIVE MODEL OF TOMATO KEEPING QUALITY 20
.~
I;';3
r
15
D
g
~0
o.
5
I
I
I
0.3
0.6
0.9
1.2
Compression distance (mini Fig. I. Relation between average compression distance and average keeping quality of l(I samples of 15 tomatoes each, sampled at week 23 in 1990.
If the mean data are fitted with multiple linear regression analysis according to the above model, 76% of the variability is accounted for. Initial colour of the fruits at the time of harvest did not contribute significantly to the model. Models were constructed with initial colour instead of compression distance. Of these models the highest % of the variability that was accounted for was only 57.5. Firmness values of green, pink, and red tomatoes were not influenced by the temperature of the fruits at the time of measurement (Table 1). Compression distances of fruits with internal temperatures of 15, 18, 22, and 28°C were not significantly different. As fruits stored at 28°C for the whole 9 h testing period showed an increase in compression distance up to 16%, increases in compression
I
i
30
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26
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O"~
18
1990
14
/// ~
1989
10 0
I
I
[
I
L
10
20
30
40
50
Week number Fig. 2. Relation between harvesting date of tomatoes (week number) and y-axes intercept of regression line of compression versus keeping quality (SD = 1.0-1.4) after calculating the multiple linear regression model.
184
J.J. POLDERDIJK ET AL.
TABLE 1 R e l a t i v e c o m p r e s s i o n d i s t a n c e of t o m a t o e s with initial t e m p e r a t u r e of 15°C c o m p a r e d to the relative c o m p r e s s i o n d i s t a n c e of the s a m e t o m a t o e s s u b s e q u e n t l y held for 3 h at 18°C, 22°C a n d 28°C; a n o t h e r r e p l i c a t e was held for 9 h at 28°C
Green Pink Red
15°C t = 0
18°C t = 3h
22°C t = 6h
28°C t = 9h
28°C t = 0-9h
100 100 100
102 10l 101
100 101 102
101 103 102
116 109 112
distances up to only 3% can fully be attributed to the softening by the progressive ripening during the course of the experiment.
DISCUSSION
The intercept of the regression line between initial compression distance and keeping quality is indicative for the intrinsic keeping quality (KQI): that is the limit of keeping quality when the compression distance (CD) reduces to zero. KQI = lim~.o _, 0KQ(cD) The intrinsic keeping quality can be regarded as an indication for the potential maximal available keeping quality. This factor keeping quality shows a sinusoidal curve during the season. A relation with daylength or photoperiod seems obvious. Apparently, the variation of the intrinsic keeping quality can be largely attributed to week number. Prediction of keeping quality might become possible using initial firmness of the fruits delivered at the auctions and the calender, and as long as growing conditions remain uniform from year to year. However, superimposed on the smooth curve, irregular peaks and dips can be observed. Variables like temperature, radiation or EC value of the nutrient solution may have additional effects on the value of the intrinsic keeping quality resulting in these peaks and dips, and are thus subject of further studies on the postharvest biology of tomato fruits. The intrinsic keeping quality varies during the season and a description of this variation is a break-through in accounting for the changes as noticed in practice, but, it also makes it possible to predict storability. Knowing the relationship as described in the model it is obvious that tomatoes with the same initial fruit firmness may have different storage potentials during the season. The results of this study clearly show that the initial fruit firmness can be used to predict the keeping quality of a relatively uniform batch of tomato fruit. The rate of softening, the most important parameter determining the keeping quality, is high in the Summer and low in Spring and Autumn. The causes of the differences in softening during the season are most probably correlated with growing conditions and rate of fruit development.
P R E D I C T I V E M O D E L O F T O M A T O KEEPING Q U A L I T Y
I b;5
In order to use initial firmness of tomatoes at the auction as a parameter for keeping quality, the measurement must be reliable. Ambient temperatures substantially change during the season and consequently the temperature of the supplied fruits, However, temperature of the fruits, ranging from 15 to 28°C, had no significant influence on the compression distance as measured with the Instron Testing device. This implies that the firmness can be determined reliably under temperature conditions occurring at the auctions. ACKNOWLEDGEMENTS This research was financially supported by the Centraal Bureau voor de Tuinbouwveilingen (CBT) in The Netherlands. The authors thank Graeme E. Hobson, Littlehampton, U K for valuable comments. REFERENCES Gormley, T.R. and Maher, M.J. (1987) The effect of recompression on tomato fruit firmness. Ir. J. Food Sci. Techn, 11, 175-178. Gough, C. and Hobson, G.E. (1990) A comparison of the productivity, quality, shelf-life characteristics and consumer reaction to the crop from cherry tomato plants grown at different levels of salinity. J. Hortic. Sci. 65, 431-439. Hall, C.B. (1964) Firmness and color of some tomato varieties during ripening and according to harvest dates. Proc. Am. Soc. Hortic. Sci. 84, 507-512. Hobson, G.E. and Ambler, J. (1988) An improved automatic 'Firmness Meter' in use at the Glasshouse Crops Research Institute. Annual Report of the Glasshouse Crops Research Institute for 1986-87, 120-125. Jordan, J.L., Shewfelt, R.L., Prussia, S.E. and Hurst, W.C., (1985). Estimating the price of quality characteristics for tomatoes: aiding the evaluation of the postharvest system. HortScience, 20, 203-205. Polderdijk, J.J. (1989) Prediction of keeping quality of tomatoes. Acta Hortic. 244, 73-78. Stenvers, N., Rudolphij, J.W. and Bruinsma, J. (1973) Growth, ripening and storage of tomato fruits. I. The measurement of softening of ripening tomato fruits. Gartenbauwissenschaft 38, 517-531. Stenvers, N. and Stork, H.W., (1976). Growth, ripening and storage of tomato fruits. II. Evaluation of colour development as an indicator of tomato fruit ripening. Gartenbauwissenschaft 41, 167-170.
179
POSTEC 01020
Predictive model of keeping quality of tomatoes Janna J. Polderdijk, L.M.M. Tijskens, Jan E. Robbers and Henry C.P.M. van der Valk A TO-DLO Agrotechnological Research Institute, Wageningen, The Netherlands (Accepted 11 September 1992)
ABSTRACT Polderdijk, J.J., Tijskens, L.M.M., Robbers, J.E. and Van der Valk, H.C.P.M. (1993) Predictive model of keeping quality of tomatoes. Posthart'est Biol. Technol. 2, 179-185. A relation was established between the objectively-determined firmness of glasshouse tomatoes and subjective estimates of the keeping quality of the same fruit. Firmness was measured by determining the compression distance of fruit under a force of 3 N. Keeping quality was assessed daily by experienced personnel, and an endpoint was considered to have been reached when the tomatoes were too soft to sell. The following expression was established: keeping quality (days at 18°C) = -12.6 (days/mm)x compression distance (mm)+ b (days). Included in the intercept b were factors varying with the point in the growing season at which the samples were assessed, such as daylength. Fruit firmness was found to be relatively independent of the temperature of the fruit at which this factor was measured. The regression equation accounted for 74% of the variation in keeping quality.
Key words: Tomato fruit; Tomato keeping quality; Tomato firmness; Predictive model of tomato survivability
INTRODUCTION
In The Netherlands tomatoes are distributed through auctions, where the fruits are inspected for size, colour and external quality factors such as firmness, irregular colouring, sunscald, russetting, damage/bruising, gold spots, and silver stains. The defects are evaluated by experienced auction personnel, without the help of measuring equipment. The fruits are graded according to size and colour. A commodity can be further subdivided into quality classes based on the external factors, and on the homogeneity of the tomatoes. The keeping quality of tomatoes is a very important quality criterion, which is strongly connected with softening of the fruits. Therefore, firmness is one of the Correspondence to: J.J. Polderdijk, ATO-DLO Agrotechnological Research Institute, P.O. Box 17. 6700 AA Wageningen, The Netherlands.
180
J.J. P O L D E R D I J K ET AL.
most important quality parameters for shippers and sellers (Jordan et al., 1985). Firmness is monitored at the auction and in the trade channel by manual inspection only. The degree of red colouration is assumed to be representative of the stage of ripening and therefore is related to softening and also keeping quality. Tomatoes are graded and pre-classified by the growers and are placed in different sections at the auction, based on fruit colour. However, shipments of tomatoes of a particular colour class may still show insufficient keeping quality because of excessive softening. This means that the classification based on colour alone does not adequately match potential keeping quality. Colour and firmness at the time of picking are both ripening phenomena that are correlated to keeping quality (Hall, 1964; Stenvers and Stork, 1976). A previous study with Dutch glasshouse tomatoes produced the proposition that firmness measurement at the auction can be reliably used to exclude suspect tomatoes (Polderdijk, 1989). However, initial firmness alone was insufficiently accurate in predicting the keeping quality throughout the production season that continues from early March until the beginning of December. To develop a model for predicting the keeping quality of tomatoes during the whole season it is essential to allow for changes in the relationship between the initial firmness and the keeping quality of the fruits during the year. Moreover, knowledge about factors determining and influencing the firmness of tomatoes is essential for optimizing growing conditions and postharvcst storage techniques. In this paper the relationship between firmness of tomatoes at arrival at the auction and their keeping quality under standardized conditions is discussed. A model describing this relation is presented. In addition, the effect of fruit temperature on the firmness measurement was evaluated. In practice the fruit temperature of the tomatoes arriving at the auction is closely related to the temperature during transport. If fruit temperature directly influences the firmness measurement, it would have consequences for a predicting model based on firmness. MATERIALS AND METHODS
Relation between initial firmness and keeping quality Commercially grown round glasshouse tomatoes (Lycopersicon esculentum L.) were purchased 19 times from two auctions in the Netherlands from autumn 1989 until late summer 1990. The commercial supply at the auctions consisted of the cultivars Calypso (70%), Liberto (20%) and the remaining 10% included Counter, Spectra, and Criterium. These cultivars are uniform in appearance and are considered equivalent. In the experiments differences between cultivars were ignored. At each sampling date representative samples of 15 sound fruits (diameter 47-57 mm) each were taken from ten different growers at the moment of delivery at the auction around noon to be sure the tomatoes measured were picked that day. In order to obtain the largest possible range in initial firmness, soft and firm
P R E D I C T I V E M O D E L OF T O M A T O K E E P I N G Q U A L I T Y
181
samples were selected by manual firmness evaluation. Fruits were transported to the laboratory within 2 h. Subsequently, the fruit firmness was measured using an Instron Universal Testing Machine. The tomatoes were compressed between two flat plates, diameter 50 ram, with a compression rate of 20 m m / m i n up to a maximum force of 3 N. In contrast with forces used by Gormley and M a h e r (1987) and Gough and Hobson (1990) this small force has been shown to be nondestructive (Stenvers et al., 1973), and the m e a s u r e m e n t did not reduce keeping quality (data not shown). The measuring point on the tomatoes was at the equator of the fruit, between the radial walls of the pericarp. The tested commercial cultivars have two or three locules which can easily be recognized by lines on the surface, starting at the blossom end of the fruit. Using these marking lines load points on both sides of the fruits were between the radial walls. As loading between radial walls was easy with the cultivars used, measuring compression along the longitudinal axis, as done by Hobson and Ambler (1988) was not necessary. Besides, their method does not match accepted manual firmness evaluations, and requires the removal of the calyx, which is unacceptable for some markets of round tomatoes. Ripening stage was defined by colour classification, using the Dutch rating scale 1 to 12 (1, green; 7, pink; 12, deeply red). Colour values measured according to this scale showed a correlation of 0.97 with the Hunter Lab/a value. For assessing the keeping quality, the tomatoes were stored at 18°C and a nn of 80%. Keeping quality was defined as the time in days at the constant temperature and R H between the start of storage and the time the fruits were unacceptable. Unacceptable means too soft to sell. Daily inspection was executed through manual evaluation of the firmness of the equator of the fruits by trained specialists. Regularly, firmness of the discarded fruits was also measured with the Instron Universal Testing Machine. The compression distance of these fruits was 1.47 mm (sD = 0.16). Other quality criteria than firmness (colour, taste, microbial decay, bruises, gold spots etc.) were not used to determine acceptability. The keeping quality and the compression distance were averaged for 15 fruits per grower. The data of 10 growers per testing date were used for regression analysis of initial firmness and keeping quality. Sampling dates were depicted in week numbers, where the first week of the year is number 1.
Influence of fruit temperature on firmness To evaluate the effect of the fruit t e m p e r a t u r e on fruit firmness measurement, fruits were kept overnight at 15°C, and the compression distance was measured the next morning. Subsequently, fruits were transferred to a climate chamber with a temperature of 18°C. The measuring spot was marked, and each m e a s u r e m e n t was repeated two more times at the same tomato. Repetitive measurements with this force did not result in significant changes in the compression distance readings (15 readings of a pink tomato on the same loading point showed an average of 0.60 m m with a SD = 0.02, and of a red tomato 1.27 m m with a SD = 0.05). Internal t e m p e r a t u r e of the fruits was measured in dummy fruits. It was found that 3 h was
182
J.J. P O L D E R D I J K
E T AL.
sufficient to equilibrate with the external temperature. After 3 h (when the fruit temperature was 18°C), firmness was measured, and the fruits were transferred to a chamber at 23°C. Again the firmness was measured after 3 h. The procedure was once again repeated at 28°C. As a control for maximum ripening and (thus) softening, fruits were transferred directly from 15°C to 28°C for 9 h. After 9 h the firmness of the control tomatoes was measured. At each temperature two fruits per colour stage (stages 4, 6 and 9) were measured. Compression distance was calculated as a percentage of the initial compression distance at 15°C. RESULTS Determining the keeping quality of tomatoes by manual evaluation of the firmness has proven to be reliable and reproducible. Tomatoes were considered unacceptably soft by three experienced specialists with a variation of one day at most. The compression distance, measured as the difference between the initial point of contact with the fruit and the diameter of the compressed fruit under a maximum force of 3 N, was used as a firmness indicator. The compression distance was also calculated as strain, that is the ratio between compression distance and the initial diameter of the fruit. This factor should correct for differences in initial size, but was not involved in the final model, and will therefore not be discussed further. The linear regression between the compression distance at 3 N and the keeping quality of the 2850 individual tomatoes throughout the whole production season accounted for only 12% of the variation. However, keeping quality of tomatoes at separate sampling dates during the harvest period correlated well with the initial firmness of the same fruits. As a representative example, the means of 10 batches of fruit collected in week 23 are shown in Fig 1. Firm fruits (with low compression distance) exhibited a relative long keeping quality, whereas soft fruits (high compression distance) showed a low keeping quality. Similar correlations between compression distance and keeping quality were found throughout the test period for each sampling date. The slopes of the calculated individual regression lines of each sampling date showed only little variation while the intercepts with the y-axis varied strongly throughout the season. In spring and autumn the intercepts are relatively high compared to the intercepts in summer. Therefore, a multiple linear regression model was calculated using the data from the 19 sampling weeks. To distinguish the sampling dates, corresponding week numbers were included as a factor for the regression analysis. This model consists of a common slope for all weeks, and a week-dependent intercept, fitted, using indicator variables (Fig 2). The common slope was estimated as - 1 2 . 6 (standard error 1.06). Keeping quality (days) = - 12.6 (days/mm)compression distance (mm) + intercept (days)
PREDICTIVE MODEL OF TOMATO KEEPING QUALITY 20
.~
I;';3
r
15
D
g
~0
o.
5
I
I
I
0.3
0.6
0.9
1.2
Compression distance (mini Fig. I. Relation between average compression distance and average keeping quality of l(I samples of 15 tomatoes each, sampled at week 23 in 1990.
If the mean data are fitted with multiple linear regression analysis according to the above model, 76% of the variability is accounted for. Initial colour of the fruits at the time of harvest did not contribute significantly to the model. Models were constructed with initial colour instead of compression distance. Of these models the highest % of the variability that was accounted for was only 57.5. Firmness values of green, pink, and red tomatoes were not influenced by the temperature of the fruits at the time of measurement (Table 1). Compression distances of fruits with internal temperatures of 15, 18, 22, and 28°C were not significantly different. As fruits stored at 28°C for the whole 9 h testing period showed an increase in compression distance up to 16%, increases in compression
I
i
30
i
i
o
26
O O
oo 22 o. @ L_ o
O"~
18
1990
14
/// ~
1989
10 0
I
I
[
I
L
10
20
30
40
50
Week number Fig. 2. Relation between harvesting date of tomatoes (week number) and y-axes intercept of regression line of compression versus keeping quality (SD = 1.0-1.4) after calculating the multiple linear regression model.
184
J.J. POLDERDIJK ET AL.
TABLE 1 R e l a t i v e c o m p r e s s i o n d i s t a n c e of t o m a t o e s with initial t e m p e r a t u r e of 15°C c o m p a r e d to the relative c o m p r e s s i o n d i s t a n c e of the s a m e t o m a t o e s s u b s e q u e n t l y held for 3 h at 18°C, 22°C a n d 28°C; a n o t h e r r e p l i c a t e was held for 9 h at 28°C
Green Pink Red
15°C t = 0
18°C t = 3h
22°C t = 6h
28°C t = 9h
28°C t = 0-9h
100 100 100
102 10l 101
100 101 102
101 103 102
116 109 112
distances up to only 3% can fully be attributed to the softening by the progressive ripening during the course of the experiment.
DISCUSSION
The intercept of the regression line between initial compression distance and keeping quality is indicative for the intrinsic keeping quality (KQI): that is the limit of keeping quality when the compression distance (CD) reduces to zero. KQI = lim~.o _, 0KQ(cD) The intrinsic keeping quality can be regarded as an indication for the potential maximal available keeping quality. This factor keeping quality shows a sinusoidal curve during the season. A relation with daylength or photoperiod seems obvious. Apparently, the variation of the intrinsic keeping quality can be largely attributed to week number. Prediction of keeping quality might become possible using initial firmness of the fruits delivered at the auctions and the calender, and as long as growing conditions remain uniform from year to year. However, superimposed on the smooth curve, irregular peaks and dips can be observed. Variables like temperature, radiation or EC value of the nutrient solution may have additional effects on the value of the intrinsic keeping quality resulting in these peaks and dips, and are thus subject of further studies on the postharvest biology of tomato fruits. The intrinsic keeping quality varies during the season and a description of this variation is a break-through in accounting for the changes as noticed in practice, but, it also makes it possible to predict storability. Knowing the relationship as described in the model it is obvious that tomatoes with the same initial fruit firmness may have different storage potentials during the season. The results of this study clearly show that the initial fruit firmness can be used to predict the keeping quality of a relatively uniform batch of tomato fruit. The rate of softening, the most important parameter determining the keeping quality, is high in the Summer and low in Spring and Autumn. The causes of the differences in softening during the season are most probably correlated with growing conditions and rate of fruit development.
P R E D I C T I V E M O D E L O F T O M A T O KEEPING Q U A L I T Y
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In order to use initial firmness of tomatoes at the auction as a parameter for keeping quality, the measurement must be reliable. Ambient temperatures substantially change during the season and consequently the temperature of the supplied fruits, However, temperature of the fruits, ranging from 15 to 28°C, had no significant influence on the compression distance as measured with the Instron Testing device. This implies that the firmness can be determined reliably under temperature conditions occurring at the auctions. ACKNOWLEDGEMENTS This research was financially supported by the Centraal Bureau voor de Tuinbouwveilingen (CBT) in The Netherlands. The authors thank Graeme E. Hobson, Littlehampton, U K for valuable comments. REFERENCES Gormley, T.R. and Maher, M.J. (1987) The effect of recompression on tomato fruit firmness. Ir. J. Food Sci. Techn, 11, 175-178. Gough, C. and Hobson, G.E. (1990) A comparison of the productivity, quality, shelf-life characteristics and consumer reaction to the crop from cherry tomato plants grown at different levels of salinity. J. Hortic. Sci. 65, 431-439. Hall, C.B. (1964) Firmness and color of some tomato varieties during ripening and according to harvest dates. Proc. Am. Soc. Hortic. Sci. 84, 507-512. Hobson, G.E. and Ambler, J. (1988) An improved automatic 'Firmness Meter' in use at the Glasshouse Crops Research Institute. Annual Report of the Glasshouse Crops Research Institute for 1986-87, 120-125. Jordan, J.L., Shewfelt, R.L., Prussia, S.E. and Hurst, W.C., (1985). Estimating the price of quality characteristics for tomatoes: aiding the evaluation of the postharvest system. HortScience, 20, 203-205. Polderdijk, J.J. (1989) Prediction of keeping quality of tomatoes. Acta Hortic. 244, 73-78. Stenvers, N., Rudolphij, J.W. and Bruinsma, J. (1973) Growth, ripening and storage of tomato fruits. I. The measurement of softening of ripening tomato fruits. Gartenbauwissenschaft 38, 517-531. Stenvers, N. and Stork, H.W., (1976). Growth, ripening and storage of tomato fruits. II. Evaluation of colour development as an indicator of tomato fruit ripening. Gartenbauwissenschaft 41, 167-170.