Research Article

Received: 2 December 2010 Revised: 5 January 2011 Accepted: 6 January 2011 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jsfa.4312

Changes in pH, acids, sugars and other quality

parameters during extended vine holding
of ripe processing tomatoes
Gordon E Anthon,a∗ Michelle LeStrangeb and Diane M Barretta

Abstract
BACKGROUND: Two important quality attributes of processing tomatoes are pH and titratable acidity. These and other quality
attributes can be affected by tomato fruit maturity and over-maturity. We have determined the magnitude of these maturity
effects in four processing tomato cultivars commonly grown in California.

RESULTS: Allowing tomatoes to remain on the vine for up to 4 weeks after ripening resulted in an increase in fruit pH of between
0.01 and 0.02 unit per day for the four cultivars examined. The increase in pH was paralleled by a decrease in titratable acidity,
due to a loss of citric acid. Glucose and fructose concentrations also declined with increasing maturity after ripening. Other
quality parameters (color, lycopene, total pectin, pectin solubility, and Bostwick consistency) all showed little change.

CONCLUSION: Vine holding of ripe fruit adversely affects quality, especially pH and titratable acidity. Recent problems with
high tomato juice pH encountered by tomato processors in California could be the result of increased average fruit maturity at
harvest.


c 2011 Society of Chemical Industry

Keywords: tomato; pH; maturity; quality; citric acid

INTRODUCTION In recent years processors in California have noted that the pH

Average annual production of processing tomatoes in California of products such as hot-break tomato paste have been increasing,
is about 12 million tons per year, accounting for more than 90% often requiring the addition of citric acid to obtain the required
of the US crop and about 40% of worldwide production. Since final pH. While processing conditions can affect the acid content
the 1960s harvesting has been by machine where all fruit from and pH of tomato products,2,3 changes in processing conditions do
not appear to be the cause of this recent pH rise. Rather, the rise in
an entire field is collected in a single pass. This harvesting system
pH can be observed in the raw fruit arriving at the processing plant.
results in a mixture of fruit with a range of maturities. After sorting
Typically each truck-load of tomatoes arriving at the processing
on the harvester to remove unripe green fruit, the tomatoes are
plant is sampled and the pH of the fruit determined. These data
transported by truck to processing plants where they are converted are available to the public (www.ptab.org). An analysis of this load
into a variety of products, including whole peeled tomatoes, diced data collected over the past 9 years (2001–2009) showed that the
tomatoes, and tomato paste. A major challenge in this production average pH of fruit arriving at processing plants has been rising
system is to coordinate the timing of the harvests with available by about 0.01 pH unit per year. Significantly, this pH rise can be
processing plant capacity. Often it is necessary to delay harvesting observed even when the data analyzed are limited to a single
until sufficient processing capacity becomes available. This ‘field cultivar grown in a single county. Thus the overall rise in pH is not
storage’ of ripe fruit can further affect the average maturity of the due to the introduction of new cultivars or to a geographic shift in
fruit being processed. production.4
Two important quality attributes of processing tomatoes are pH Several previous studies have looked at the effects of tomato
and titratable acidity (TA). Tomatoes are not a low-acid food and cultivar and fruit maturity on pH and other quality parameter.5 – 10
thus require less drastic thermal treatments than foods classified as
low acid (pH > 4.6) for the destruction of spoilage microorganisms
∗ Correspondence to: Gordon E Anthon, Department of Food Science and
to ensure food safety. It has been suggested that pH 4.4 is the
maximum desirable for safety and the optimum target pH should Technology, University of California, Davis, CA 95616, USA.
E-mail: geanthon@ucdavis.edu
be 4.25.1 Industrial processors of tomatoes in California typically
specify a pH of 4.2 or 4.3 in their processed products. The pH of a Department of Food Science and Technology, University of California, Davis,
tomatoes is determined primarily by the acid content of the fruit. CA 95616, USA
The acidity of the fruit is also important as a contributor to the b University of California Cooperative Extension, 4437-B S. Laspina St, Tulare, CA
flavor of the tomato products. 93274, USA

J Sci Food Agric (2011) www.soci.org

c 2011 Society of Chemical Industry
 www.soci.org GE Anthon, M LeStrange, DM Barrett

In all tomato cultivars examined, pH increased as the fruit ripened were hand harvested weekly beginning 1 week after tagging for
from the green to pink to red stage, and continued to increase as a total of 5 weeks. Only undamaged fruit with minimal sunburn
the red ripe fruit remained on the vine. TA was at its maximum were included in the harvest.
at the beginning of the ripening process then decreased as the The second trial was grown at the University of California
fruit reached the ripe stage and continued to decrease with West Side Research and Extension Center in Fresno County,
over-maturity. Soluble solids have been shown to increase during California. The same four varieties as in the first trial were grown
ripening then remain constant with over-maturity.7 Contradictory from transplants and watered with subsurface drip irrigation
results have been obtained for the effect of maturity on juice and standard commercial practices. The first harvest was at
consistency where it has been shown that Bostwick values either 125 days after transplanting with three additional harvests at
increase5,6 or decrease with maturity.9 1-week intervals. For each harvest four 10-foot sections of the
Citric acid is the most abundant acid in tomatoes and the rows were selected at random and the entire mass of tomato fruit
largest contributor to the total TA.10,11 The decrease in TA with in that section collected. This fruit was then sorted by hand into
maturity and over-maturity is generally assumed to be due to a green, marketable red, and unmarketable damaged fruit (culls)
loss of citric acid, although direct measurements of changes in and the total mass in each category determined. Yields per acre
citric acid concentrations with maturity have not been reported. were calculated from these sample sections. A portion of the
Two other acids that contribute significantly to the TA are malic marketable red fruit was set aside and transported to UC Davis for
and glutamic acid. Malic acid is typically present at only one juice preparation and analysis.
tenth the level of citric acid, although the ratio of malic to citric
can vary considerably between different tomato cultivars.10,11 Preparation and evaluation of juices
Changes in malic acid levels with maturity and over-maturity Microwave hot-break juice was prepared from 1300 g samples of
have not been reported. Glutamic acid levels have been shown tomatoes from the field trials as described previously.13 For each
to increase 10-fold as the fruit ripens from the green to the red variety at each harvest date three separate juices were prepared.
stage.12 Further changes in glutamic acid with over-maturity have Juices were analyzed for lycopene, color, titratable acidity, and pH
not been reported. Glutamic acid is also an important contributor as described.13
to tomato flavor. Supernatants were prepared from these juices by centrifuging
Recently, new cultivars of tomatoes have been introduced which at 15 000 × g for 10 min. These supernatants were analyzed
have a greater ability to resist decay once fully ripened and can for individual sugars and acids using enzyme kits (R-Biopharm,
thus be left in the field longer after reaching the red ripe stage. Marshall, MI, USA). It has been shown that acid and sugar levels
These ‘extended field storage’ (EFS) cultivars are desirable for determined with these kits on crude tomato juice supernatants
the growers and processors because they allow greater flexibility give excellent agreement with HPLC analysis.14 Soluble solids
in the timing of harvests once the fruit has reached the ripe contents of the supernatants were determined with an Atago
stage, which helps in coordinating harvests with processing plant PR-32 refractometer, and phosphate concentrations by the Ames
capacity. Changes in fruit quality with EFS are likely to be similar to method.15
those seen in older non-EFS cultivars, although this has not been
studied. In this study we have examined both EFS and non-EFS
cultivars to determine the changes in sugars, acids, and pH that Statistical analysis
occur when ripe tomatoes are allowed to remain on the vine. Data are presented as means ± SE. An unpaired t-test with equal
variance was used to calculate the two-tailed P value to estimate
statistical significance of differences between means.
EXPERIMENTAL
Tomato plantings
Two separate field trials were grown. In the first trial, fruit of
RESULTS
Fruit color and lycopene
uniform maturity were obtained by tagging fruit in the field at the
pink stage, then this tagged fruit was hand harvested weekly for To assess the ripeness stage of the fruit the color and lycopene
5 weeks. In the second trial the entire mass of fruit from a randomly content of hot-break juice prepared from the fruit was determined.
selected 10 foot row section was harvested and sorted weekly for The a/b ratio, a measure of redness, increased between the first
four weeks. Four cultivars of processing tomatoes, Heinz 2401 harvest at 7 days after tagging, and the second harvest at 14 days,
(H2401), Sun Seeds 6368 (N6368), Heinz 9557 (H9557), and AB2 indicating that the fruit was not fully ripe until the second harvest
were examined. Two of these, H2401 and N6368, are considered (Fig. 1). The lycopene content of the juice similarly did not reach its
EFS cultivars. maximum level until the second harvest. The general correlation
Four cultivars of processing tomatoes, Heinz 2401 (H2401), Sun between lycopene and a/b ratio has been noted previously.16 Both
Seeds 6368 (N6368), Heinz 9557 (H9557), and AB2 were grown the a/b values and the lycopene content of the juice from N6368
in two separate field trials. Two of these cultivars (H2401 and fruit were lower than those of the other three cultivars at all stages
N6368) are considered EFS cultivars. The first trial was grown in of maturity.
Yolo County, California, at the Vegetable Crops Research Field
Station, University of California Davis. Plants were grown from pH and titratable acidity
transplants and watered with subsurface drip irrigation using The pH of hot-break juice increased with increasing maturity in all 4
standard commercial practices. At 96 days after transplanting, cultivars examined (Fig. 2A). The only obvious differences between
fruit at the pink stage were identified and tagged with small cultivars were the lower pH values for the H2401 tomatoes at all
adhesive stickers. Completely exposed fruit at the top of the stages of maturity. Both the EFS cultivars (H2401, N6368) and the
canopy was avoided to reduce the incidence of sunburn in the non-EFS cultivars (H9557, AB2) showed similar pH increases with
tagged set of fruit. A total of 500 of each variety were tagged. Fruit maturity. The pH increase between the first harvest at 7 days, and

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Tomato fruit maturity and pH www.soci.org

A 2.30 A 4.70
2.20
4.60
2.10
2.00 4.50
a/b

1.90

pH
4.40
H2401
1.80 H2401
AB2 4.30
1.70 AB2
H9557
4.20 H9557
1.60 N6368
N6368
1.50
0 10 20 30 40 4.10
0 10 20 30 40
Days after pink stage
Days after pink stage

B 120 B 65

100 60
Lycopene (µg g–1)

55
80

TA (µeq g–1)
50
60
45
H2401
40 AB2 40 H2401

H9557 AB2
20 35
H9557
N6368
30 N6368
0
0 10 20 30 40 25
0 10 20 30 40
Days after pink stage
Days after pink stage
Figure 1. Color and lycopene content of tomato juices prepared from
tomatoes of different maturities. (A) Color measured as the Hunter Lab a/b C 5.0
ratio. (B) Lycopene content.
4.9
4.8
the final harvest at 35 days, varied between 0.29 and 0.31 unit
4.7
for the four cultivars examined. This is equal to a pH increase of
approximately 0.01 unit per day. 4.6
Total TA of the hot-break tomato juices decreased with
pH

4.5
increasing tomato maturity (Fig. 2B). This decrease ranged from
4.4
17 µeq g−1 for the H2401 cultivar to 9 µeq g−1 for the AB2 cultivar.
The decrease in TA mirrored the increase in pH. The H2401 4.3
tomatoes, which had a notably lower pH than the other cultivars, 4.2 y = –0.7703 ln(x) + 7.3559
had the highest TA. A plot of pH versus TA for all 124 juice samples R2 = 0.8555
prepared in this study showed a clear relationship between pH 4.1
and TA (Fig. 2C). 4.0
20 30 40 50 60 70
–1)
TA (µeq g
Organic acids
The rise in pH and decrease in TA indicates that acid concentrations Figure 2. (A) Effect of maturity on tomato juice pH. (B) Effect of maturity
on titratable acidity (TA). (C) Relationship between pH and TA.
in the fruit are declining with maturity. The predominant acid in
tomatoes is citric acid. Citric acid levels declined with maturity in
all four tomato cultivars (Fig. 3A). Citric acid levels were 22–30%
lower at the final harvest than at the first harvest, for the four in all four cultivars then either declined or remained constant at
cultivars examined. As with the pH and TA measurements, the later harvests. Phosphate ions also contribute to the measured
H2401 tomatoes were notably higher in acid content than the TA. Free phosphate concentrations in these tomato juices ranged
other three cultivars, but showed a similar decline with maturity. from 3 to 4 mmol L−1 but showed no change with maturity or any
Glutamic acid levels were lower than citric acid and did not show consistent difference between cultivars (data not shown). These
an obvious decline with maturity in any of the cultivars (Fig. 3B). phosphate concentrations are in line with what has been reported
The change from the first to the last harvest ranged from +12% to previously.11
−16%. Malic acid levels were even lower than glutamic and also From the measured tomato juice pH, the expected TA of the
did not decline with maturity (Fig. 3C). From the first harvest (day juice can be calculated from the measured concentrations of
7) to the third harvest (day 22) malic acid levels actually increased citric acid, glutamic acid, malic acid, and phosphate, and their

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 www.soci.org GE Anthon, M LeStrange, DM Barrett

A 5.0 65
H2401
4.5 AB2
H9557 55
Citric acid (g L–1)

4.0
N6368

Predicted TA
3.5 45

3.0
35
2.5

2.0 R2 = 0.9703
0 10 20 30 40 25
Days after pink stage

B 2.5 15
15 25 35 45 55 65
Measured TA
2.0
Glutamic acid (g L–1)

Figure 4. Predicted versus measured TA.

1.5

the second and fifth harvests were small but significant (P < 0.05)
1.0 H2401 and ranged from 10 to 12% for the four cultivars. Fruit of the AB2
AB2 cultivar had the highest sugar levels while H2401 had the lowest.
0.5 H9557 Soluble solids content, measured with a refractometer and
N6368 expressed in ◦ Brix, is a commonly used measure for assessing
0.0 tomato quality. The soluble sugars glucose and fructose are the
0 10 20 30 40
largest contributor to the total soluble solids. Between the first
Days after pink stage
and last harvests soluble solids declined by between 4% and 7%
C 0.8 for the four tomato cultivars, in agreement with the measured
declines in glucose and fructose (Fig. 5B). Overall soluble solids
0.7
showed a good correlation with the sum of glucose plus fructose
0.6 concentration (Fig. 5C).
Malic acid (g L–1)

0.5
Other quality attributes changed relatively little over the 4 weeks
of field holding. Consistency, measured as Bostwick values,
0.4 increased with later harvest times (Fig. 6A). For all four cultivars the
0.3 H2401 average Bostwick values at the fifth harvest were higher than those
AB2 from the first harvest (6–15% higher depending on the cultivar) but
0.2
H9557 for no individual cultivar was this difference significant at P < 0.05.
0.1 Pectin content and composition are known to be important
N6368
0.0 contributors to the consistency of tomato juice. Between the first
0 10 20 30 40 and fifth harvests changes in total pectin, measured as galacturonic
Days after pink stage acid (GalUA), were small (<5%) and not significant at P < 0.05
Figure 3. Changes in acids with maturity. (A) Citric acid. (B) Glutamic acid. (Fig. 6B). Similarly, no significant changes were found in the water
(C) Malic acid. solubility or the degree of methyl esterification of pectin between
the first and fifth harvests (data not shown). Although no large
change in consistency or pectin content occurred within a cultivar
known pKa values. When the predicted TA was compared with there were obvious differences between cultivars. The cultivars
the measured TA there was a very good correlation between H2401 and H9557 had substantially higher pectin contents and
these two values for all 124 juice samples prepared in this study lower Bostwick values than the AB2 and N6368 cultivars. Overall
(Fig. 4). Predicted TA values were equal to 86% of the measured Bostwick consistency showed a significant correlation with pectin
TA values on average, indicating that only 14% of the measured content (Fig. 6C), which is in agreement with previous research
TA acidity was due to other acids not measured. These other findings.9
acids would include ascorbic, oxalic, as well as numerous amino
acids including aspartic and γ -aminobutyric. In some varieties of
Effects of delayed harvesting
tomatoes these latter two amino acids have been shown to occur
at levels comparable to that of glutamic acid in ripe fruit.12 By tagging individual fruit it was possible to successively harvest
fruit of uniform maturity to determine the effect of maturity on
acidity and pH. However, this approach does not address the
Sugar and soluble solids question of how large a change in pH will occur if an entire field,
Glucose and fructose levels were highest at the second harvest containing fruits of varying maturities, is harvested at successively
when the fruit reached full ripeness, then declined slightly over later dates. Nor does it allow for determining the effect of extended
the next 3 weeks of vine holding (Fig. 5A). The decreases between vine holding on the yield of intact red fruit. To answer these

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A A 20
30
glucose + fructose (g L–1)

18

Bostwick (cm)
16
25
14
H2401
12
20 AB2
H9557
H2401 AB2
10
N6368 H9557 N6368
15 8
0 10 20 30 40 0 10 20 30 40
Days after pink stage Days after pink stage

B 5000
B
5.5
4500

Pectin GalUA (µg g–1)

Soluble Solids (°Brix)

5.0
4000

4.5 3500
H2401
4.0 3000
AB2

3.5
H9557 2500 H2401 AB2
N6368 H9557 N6368
2000
3.0 0 10 20 30 40
0 10 20 30 40
Days after pink stage
Days after pink stage
C 23
C 6.5
21
6
19
Bostwick (cm)
Soluble Solids (°Brix)

5.5 17

5 15
13 R2 = 0.6891
4.5
11
4
R2 = 0.7301 9
1500 2500 3500 4500
3.5
Pectin GalUA (µg g–1)
3
15 20 25 30 35 40 Figure 6. (A) Effect of maturity on Bostwick consistency. (B) Effect of
maturity on pectin. (C) Correlation between Bostwick consistency and
Glucose + Fructose (g L–1) pectin content.
Figure 5. (A) Changes in sugar (glucose + fructose) with maturity.
(B) Changes in soluble solids with maturity. (C) Correlation between soluble
solids and sugars.
yield by the end of the 4-week period. These results show the
superior field-holding ability of the EFS cultivars.
Delayed harvesting resulted in juice with a higher pH in all
questions a second field trial with a different protocol was used. varieties. Average pH ranged from 4.38 to 4.49 at the first harvest,
In this trial the same four cultivars of tomatoes were grown and then increased to a range of 4.70 to 4.89 at the final harvest
entire replicate 10-foot sections of rows were harvested in four 21 days later (Fig. 8A). The pH of the final harvest was more than
weekly harvests. The yield of marketable red fruit was determined 0.2 unit higher than that in the first trial, which may indicate that
and a sample of this red fruit was used to prepare hot-break juice the average maturity at the final harvest in the second trial was
for analysis. beyond the range of maturities examined in the first trial. As in the
The yield data showed a clear difference between the cultivars first trial the rise in pH was accompanied by a decline in TA due to
(Fig. 7). The two EFS cultivars, H2401 and N6368, had higher yields a loss of citric acid (Fig. 8B and C). Changes in soluble solids and
in all four harvests and showed no decline in yield until the fourth the concentrations of glucose, fructose, glutamic acid, and malic
harvest. In contrast, the yield of the non-EFS cultivars, AB2 and acid were all similar to what was observed in the first trial (data
H9557, dropped steadily with each harvest reaching near zero not shown).

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40 A 5.00
Marketable Red Yield (tons acre–1)

35 4.90
H2401
30 4.80 AB2
25 H9557
4.70

pH
20 N6368
4.60
15 H2401
AB2 4.50
10
H9557 4.40
5
N6368
4.30
0 120 125 130 135 140 145 150
120 125 130 135 140 145 150
Days from transplant
Days from transplant

Figure 7. Decrease in yield with later harvest date. B 60

55

DISCUSSION 50

TA (µeq g–1)
The results presented here, in agreement with several earlier 45
reports, show that tomato juice pH increases and TA decreases
40
with extended vine holding after fruit maturity. Hanna8 found that
35 H2401
the average increase in pH was about 0.3 unit over 30 days of
vine holding, which is similar to what we found here. Others have AB2
30
also reported increases in pH and declines in TA with increased H9557
tomato maturity.5,6,10 Citric acid is the most abundant organic acid 25
N6368
in tomatoes and the decrease in TA measured with increased fruit 20
120 125 130 135 140 145 150
maturity was due to a loss of citric acid from the fruit. During
ripening in tomatoes, as in other fruits, declines in acid levels are Days from transplant
accompanied by increases in sugars. At least a portion of this
change may be due to the metabolic conversion of acids into C
4.0
sugars by gluconeogenesis.17 Such a conversion of acids to sugars
did not appear to take place during extended vine holding of ripe
Citric acid (g L–1)

tomatoes because the decline in citric acid was not accompanied 3.5
by increases in the concentration of glucose and fructose. Rather
the loss of organic acids from the mature fruit appears to be
3.0
entirely through respiration. H2401
EFS and non-EFS cultivars showed no difference in their loss AB2
of citric acid with maturity. Nor was there any other obvious 2.5 H9557
difference between these types of cultivars other than the ability N6368
of the EFS fruit to remain intact longer after ripening, allowing
2.0
for a greater yield of marketable fruit in later harvests. Thus the 120 125 130 135 140 145 150
delayed harvesting made possible by EFS tomatoes will lead to pH Days from transplant
increases and possible problems for processors.
Figure 8. Increase in pH and decreases in TA and citric acid with later
The increase in pH with maturity can be compared with the
harvest date.
increase in pH observed in recent years at processing plants in
California. Data collected on incoming truck-loads of tomatoes
arriving at these processing plants has shown that over the 8 year differences caused by these cultural practices was a change in the
period from 2001 to 2008 the average pH of fruit has increased by relative maturity of the fruit at harvest.18 The difficulties in defining
about 0.01 unit per year for most common cultivars.4 For the four harvest maturity and its relationship to environmental conditions
cultivars examined here the pH increases with delayed harvest and fruit age have been discussed by others.19
were between 0.01 and 0.02 unit per day. Thus the observed The obvious approach for minimizing problems with high pH
increase in average pH could be explained if the average maturity would be to reduce the field holding of ripe fruit as much as
at harvest has been increasing by only 1 day per year. Over this possible. This may not be practical given that the advantages
same time period two new cultural practices have been widely field holding allows for coordinating harvests with processing
adopted: the use of transplants rather than direct seeding of plant capacity. Since the principal change in the fruit causing the
fields, and drip rather than flood irrigation. While it is possible undesirable rise in pH is a loss of citric acid, breeding for higher
that these new cultural practices are, by themselves, affecting initial citric acid content in the fruit, especially in EFS cultivars
fruit pH, it seems more likely that their effect is through changes where increased fruit maturity is more likely to occur, would be
in fruit maturation. This was the case in previous work where it desirable. The higher citric acid content of the H2401 cultivar shows
was shown that differences in fruit quality between conventional that such an approach is possible. Analysis of pH data collected
versus organic production could be explained if one of the principal during commercial production has shown that H2401 typically has

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the lowest average pH of any of the major cultivars.4 Alternatively, 8 Hanna GC, Changes in pH and soluble solids of tomatoes during vine
the pH in the final product may be reduced by adding citric acid storage of ripe fruit. J Am Soc Hortic Sci 78:459–463 (1961).
to the tomato juice during processing. Addition of citric acid has 9 Liu Y and Luh BS, Effect of harvest maturity on free amino acids,
pectins, ascorbic acid, total nitrogen and minerals in tomato pastes.
been routine for decades in California during production of whole J Food Sci 44:425–428 (1979).
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11 Paulson KN and Stevens MA, Relationships among titratable acidity,
pH and buffer composition of tomato fruit. J Food Sci 39:354–357
(1974).
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We would like to thank the California League of Food Processors composition and nitrogen metabolizing enzymes in ripening fruits
of Lycopersicon esculentum Mill. Plant Sci 159:125–133 (2000).
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nutritional differences in processing tomatoes grown under
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