FONSECA 2006 pham chat sau thu hoach bi anh huong boi do am cua bap cai

JFS

M: Food Microbiology and Safety

Postharvest Quality and Microbial Population of
Head Lettuce as Affected by Moisture at Harvest
JORGE M. FONSECA

ABSTRA
CT
ceber
g lettuce was ev
aluated for yield, micr
obial population, and posthar
vest quality either follo
wABSTRACT
CT:: IIceber
ceberg
evaluated
microbial
postharv
followent irr
igation ter
mination (IT
e and after a rrainfall
ainfall ev
ent. Lettuce rreceiving
eceiving late (4 d
different
irrigation
termination

(IT)) schedules or befor
before
event.
ing differ
befor
e har
vest) IT sho
w ed incr
eased w
eight and diameter
obic bacter
ia counts
wer quality than
before
harv
show
increased
weight
diameter,, higher aer
aerobic
bacteria
counts,, and lo
low
plants subjected to early (16 d befor
e har
vest) IT
iddle (8 d befor
e har
vest) IT pr
oduced similar yields

before
harv
IT.. M
Middle
before
harv
produced
yields,, and
obial population in lettuce than late IT
ain, micr
obial population incr
eased b
y 1.5 and 3.0 log
lo
wer micr
increased
by
low
microbial
IT.. After rrain,
microbial
colony
-for
ming units (CFU)/g in outer and head leav
es rrespectiv
espectiv
ely
esults rrev
ev
ealed the impor

tance of
colony-for
-forming
leaves
espectively
ely.. The rresults
evealed
importance
managing moisture conditions at harvest to enhance overall quality of lettuce.
Keywords: Irrigation termination, Lactuca sativa L., shelf life, storage, yield

U

MS 20050590 Submitted 9/30/05, Revised 11/9/05, Accepted 11/17/05. The
author is with Univ. of Arizona.Yuma Agricultural Center, 6425 West 8th St.
Yuma, AZ 85364. Direct inquiries to author Fonseca (E-mail:
).

© 2006 Institute of Food Technologists
Further reproduction without permission is prohibited

thus, plants subjected to mild stress in the field may adapt better
for subsequent stress conditions occurring at harvest and during
postharvest storage (Galindo and others 2004).
For some applications, mild water stress does not produce significant differences in yield. With lettuce in particular, similar yields
were obtained with 25% reduction in water availability (Gallardo
and others 1996; Coelho and others 2005). Moreover, excess water
during the growth of plants has produced inferior quality products
and higher microbial population at harvest (Koivula and others
2004).

The effect of moisture at harvest, either due to IT timing or rain,
on microbial quality and shelf life of head lettuce has not been documented. The objectives of this study were as follows: (1) to evaluate the effect of different schedules of last irrigation on microbial
population of fresh iceberg lettuce; (2) to determine the effect of IT
timing on yield and postharvest quality of fresh iceberg lettuce;
and (3) to determine the impact of rainfall immediately before
harvest on microbial quality of iceberg lettuce.

Material and Methods
Plant material and cultivation practices
Iceberg lettuce cv. Honchos II and Sahara (Seminis) were grown
during the winter season 2003-2004 and 2004-2005, respectively, at
The Univ. of Arizona–Yuma Agricultural Center (Yuma, Ariz., U.S.A.)
in an alluvial clay loam soil. Crops were subjected to agronomical
practices as currently applied in commercial settings. Briefly, in
both trials crops were planted during October and harvested during the month of January; fertilization included 45 kg/ha of 10-4-00 at planting followed by 3 applications of 20-0-0-17 at 50 kg/ha.
The irrigations before the last irrigation, included overhead sprinkling for 10 h daily during the first 5 d after planting to establish
stands, and 2 furrow irrigations, 4 and 8 wk after planting. Accumulated rainfall volume during the plants’ growth was 2.1 cm for the
1st trial and 2.9 cm for the 2nd trial. Last rainfall event occurred 19
d before harvest in the 1st trial and 12 d before harvest in the 2nd
trial, which added 0.2 and 0.4 cm of water, respectively, to the soil.
Relative humidity at harvest was 35% in the 1st trial and 28% for the
2nd trial. Harvest of lettuce was conducted between 7 and 10 a.m.,
when temperatures ranged between 2 °C and 8 °C.
Vol. 71, Nr. 2, 2006—JOURNAL OF FOOD SCIENCE M45
Published on Web 2/15/2006

M: Food Microbiology & Safety

Introduction
nderstanding the dynamic of the microbial population of lettuce is important for growers to deliver safe food to consumers. Water used for irrigation of food crops is commonly not treated

and likely contains high microbial counts (Stine and others 2005).
Commonly, postharvest washing and sanitizing methods fail to reduce microbial populations by more than 99% (Sapers 2001), which
reveals the importance of ensuring lettuce without pathogens and
with low microbial population at harvest. Cases of illness outbreaks
associated with contaminated food have resulted in catastrophic
damages to the industry. Several outbreaks of pathogenic bacteria
have been associated with the consumption of lettuce (Kapperud
and others 1995; Acker and others 1998; Hilborn and others 1999).
Increased microbial populations and higher risk of contamination are expected when vegetable surfaces are in direct contact with
irrigation water (Stine and others 2005). It is also possible that water availability in the soil increases the microbial population in plant
tissues due to higher turgor of plants, higher plant transpiration
rate, and subsequent moisture accumulation on the leaves surface
(Coelho and others 2005). Therefore, it is hypothesized that the
longer the term between last irrigation and harvest, the lower the
microbial population in the harvested product. Irrigation termination (IT), a concept indicative of the timing of the last irrigation, is
used to improve quality of agronomical crops such as soybean
(Heatherly and Spurlock 1993) and cotton (McConnell and others
1999), but its impact on vegetable quality has not been examined.
Although early IT could yield vegetables with lower microbial
population and better quality, early IT from lettuce fields potentially decreases products’ weight and affects grower profits. In a study
with broccoli, the highest quality during postharvest storage was
obtained when the plants were subjected to water stress during the
late stage of growth ( Wurr and others 2002); however, authors
found a significant decline in yield with all the water stress treatments. The improvement in quality appears to be a result of plants
developing adaptation mechanisms to cope with limiting factors;


Effect of moisture on quality of lettuce . . .
Table 1—Yields parameters of iceberg lettuce as affected by timing of the last irrigation
Irrigation termination timing (days before harvest)

4 OSa
Whole plant weight (g)
Head weight (g)
Head diameter (cm)
Water activity

1403.25
940.22
13.22
0.996

4
1406.13
952.54
13.03
0.998

8
1413.69
944.98
12.97
0.996

16
1254.93
846.70
12.73
0.995

LSD (P < 0.05)

111.79
76.48
0.45
0.0028

a OS indicates that last irrigation was conducted with overhead sprinkles; other treatments were watered with furrow irrigation. Values of plant weight are the

mean of 40 samples. Values for head weight and head diameter are the mean of approximately 180 samples.

Irrigation termination setup
The last irrigation for the 1st trial was set either 24, 16, or 6 d
before harvest. These treatments resulted in soil water content at
harvest of 15.5%, 17.2%, and 17.7%, respectively, for 0- to 30-cm
depth. The 2nd trial included treatments with IT applied 16, 8, and
4 d before harvest, corresponding with soil water content of 14.5%,
16.2%, and 18.2%. All treatments received the same amount of
water during the season, approximately 30 cm, and were harvested the same day. Furrow was used for the last irrigation because it
is the most common method of irrigation in Arizona; however, an
overhead sprinkling treatment was also added to the study (late IT)
because it is an irrigation method used in other regions.
The experimental site was divided into plots, each consisted of
three 182-m-long beds. Lettuce for yield, postharvest quality, and
microbial evaluations were selected from the middle bed.

Evaluation of rain effect
The area of Yuma, Ariz., received intermittent rain during the
time of this study. Lettuce was monitored for bacteria population
24 h before harvest and 2, 7, and 12 d after 5 rainfall events. The
lettuces were selected at random from different fields in the Yuma
valley.

Yield evaluation and scoring quality systems

M: Food Microbiology & Safety

Ten lettuces were selected from each plot and were evaluated for
total whole plant weight (all aboveground tissue). A 10-m section
was selected within plants of the middle bed and all the lettuce was
measured for head weight and head diameter (equatorial). In the
2nd trial, water activity of composite samples of the 1st 5 head
leaves was measured with a WP4-T dewpoint potential meter
(Decagon, Pullman, Wash., U.S.A.). Batches of 32 lettuces were
harvested from each plot and carried to coolers for postharvest
evaluations. The lettuces were stored at 1 °C to 4 °C and >90% relative humidity. Eight heads of lettuce were brought from coolers to
laboratory for quality examination on days 0,7, 14, and 21. With a
test panel consisting of 4 trained people (3 men and 1 woman),
general appearance, bacterial decay, and physiological disorders of
the heads were evaluated. Overall visual quality (OVQ) was conducted using a 9-point hedonic scale in which 9 was excellent quality, 7 good quality, 6 the salability point, 5 fair (becoming objectionable), 3 poor, and 1 extremely poor (Artes and Martines 1996). The
indication of maximum shelf life of products was when visual quality scores of 50% of the lettuces dropped to below 6 in the OVQ scale.
Color (L*, a*, b*) was measured to the 2nd wrapping leaf using a
MinoltaTM CR-400 chromameter (Ramsey, N.J., U.S.A.). Each measurement was the average of 3 readings, including 1 to the middle
of the leaf stem, 1 at 1 cm from the edge of the leaves, and 1 at 2
cm from the edge of the leaves. Water loss during postharvest storage was monitored by measuring the difference in weight between
day 0 and the different periods of evaluation.
The comparison of the effect of treatments on physiological disM46

JOURNAL OF FOOD SCIENCE—Vol. 71, Nr. 2, 2006

orders was carried with a scale of incidence and severity of any disorder (Martinez and Artes 1999). This index ranged from 1, indicating no symptom, to 5 indicating severe deterioration. Grade 2 was
assigned to lettuce that was only slightly affected. Grade 3 was associated with salability point, and grades 4 and 5 implied that heads

were commercially unacceptable.

Microbiological analysis
Aerobic plate count (APC) was carried out on the day of harvest
to determine the microbial load of the lettuce. For the irrigation
termination evaluation 9 heads per replicate were taken and for the
rain impact assessment a total of 20 lettuce were harvested per
evaluation time. Composite samples were taken aseptically using
forceps and palette knifes sterilized with 90% ethanol. Samples (7
g) of head leaves were diluted in 70 mL of 0.1 peptone water according to the film manufacturer’s recommendation, and submitted to agitation using a stomacher (Seward, London, U.K.) at 230
RPM for 45 s. Appropriate serial dilutions were prepared, ranging
from 10-1 to 10-7. Aliquots (1 mL) of the homogenate were placed
onto APC 3M-PetrifilmTM (St. Paul, Minn., U.S.A.) and incubated at
32 °C for 48 h, and the developing red colonies were reported as
colony-forming units (CFU). Colony counts were calculated as
CFU/g and then converted into log value for statistical analysis. The
inoculation of the samples was conducted in duplicate. Samples of
the experiment conducted during the 1st y were also sent for analysis to Bio Research Laboratories, Inc. (Redmond, Wash., U.S.A.).
The bacterial analysis for the rainfall study was all conducted at The
Univ. of Arizona Vegetable Quality Laboratory.

Experimental design and statistics
The experiment evaluating the effect of IT was arranged in a
completely randomized design and each treatment consisted of 3
replicates. Sampling of lettuce before and after rainfall was conducted at random in 4 different fields in Yuma, Arizona, using 5
plants per each of 4 replicates. Data were subjected to analysis of
variance (ANOVA) at P Յ 0.05 to determine statistical significance.
When ANOVA indicated a significant difference, mean separation
was carried out by LSD test (P Յ 0.05).

Results and Discussion

T

he lettuce subjected to late IT showed higher whole plant and
head weight and higher water activity than lettuce harvested
16 d before harvest. Lettuce receiving late IT with overhead sprinkles also showed higher whole plant and head weight and larger diameter than lettuce subjected to early IT. Middle IT produced
plants and heads with similar weight at harvest than late IT. The
lettuce subjected to early IT showed a reduction of over 10% in head
weight in comparison with lettuce that had late and middle IT (Table 1).
The reduction of weight with the early IT treatment was expected. However, it was interesting that no difference in weight was
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Effect of moisture on quality of lettuce . . .
Table 2—Effect of timing of the last irrigation on lettuce quality parameters after 7 d in storage at 2 °C to 4 °C
Irrigation termination timing (days before harvest)
Quality factors

4 OSa

OVQ
Decay
Water loss (%)
Brown stain
Pink rib
L*
a*
b*

6.60
0.80
1.5
1.5
1.1
63.74
–15.80
28.77

4
6.65
0.85
1.6
1.3
1.2
64.31
–15.68
28.54

8
6.82
0.53
1.3
1.3
1.3
63.29
–15.65
27.80

16

7.23
0.45
1.1
1.2
1.2
63.27
–15.80
28.00

LSD (P < 0.05)
0.50
ns
ns
ns
ns
ns
ns
ns

a ns indicates that treatments were not different according to analysis of variance. OS indicates that last irrigation was conducted with overhead sprinkles;

other treatments used furrow irrigation. Values are the mean of 24 samples. OVQ was assessed with a 1 to 9 scale with 9 = excellent, 6 = salability point and,
1 = completely deteriorated. Brown stain and pink rib were evaluated with a 1 to 5 scale with 1 = no symptoms, 3 = salability point, and 5 = extremely
affected. Color units indicated L* = lightness, a*greenness and, b* = yellowness.

Table 3—Effect of timing of the last irrigation on lettuce quality parameters after 14 d in storage at 2 °C to 4 °C
Irrigation termination timing (days before harvest)
Quality factors
OVQ
Decay

Water loss (%)
Brown stain
Pink rib
L*
a*
b*

4 OSa
5.70
2.60
2.32
2.52
1.44
62.00
–15.80
28.35

4
5.75
2.55
2.60
2.21
1.41
61.40
–15.68
28.47

8
6.23
2.20

2.34
2.00
1.56
62.93
–15.65
27.95

16
6.4
2.20
2.02
2.03
1.41
63.00
–15.10
28.30

LSD (P < 0.05)
0.49
ns
0.56
0.42
ns
0.87
ns
ns

observed between the middle and the late IT treatments. Similar
situation was observed in the 1st year’s trial (data not shown). It is
possible that low temperatures during the last 2 wk before harvest

influenced these results by slowing down the plants metabolism,
but it was also revealed that excessive late IT is not necessary to
obtain maximum yields. Under medium to high levels of nitrogen
fertilization excess water have produced negative results or no difference in yield volumes under similar environmental conditions to
those in this study (Sanchez 2000). A strategy used to regulate water
availability in lettuce fields, consisted in restoring soil water content to field capacity as soon as it reaches a defined threshold
(Leenhardt 1998) seems not critical for the last irrigation of lettuce.
Adequate regulated deficit irrigation programs have increased
yield of other crops such as corn (Zhang and others 2005). Gallardo
and others (1996) suggested that in some stages of the lettuce
growth, complete water availability in the fields is not critical to
maximize yields volumes. Coelho and others (2005) recently obtained maximum diameter and weight of lettuce with a 25% reduction in the amount of water required to replenish total transpiration
of the plants.
Contrary to the pattern observed in the field, where higher benefits were obtained with late and middle IT, a week after postharvest storage, the OVQ of lettuce receiving early IT was higher than
that of lettuce subjected to late IT. The middle IT treatment produced OVQ that was not significantly different from either early or
late IT. No differences were observed in other quality parameters
(Table 2). Previously, the evaluation of quality parameters on the
day of harvest showed no differences among treatments (data not
shown).
After 14 d in storage, the OVQ of the early and middle IT were
higher than that of the 2 treatments receiving late IT. Significant
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differences were also observed in L* units, brown stain, and water
loss (Table 3). The brown stain and L* values denoted that tissue of
lettuce receiving late IT was more oxidized than that of other IT
treatments. Water loss was significantly higher in the late IT than
in the early IT. After 3 wk in storage, water loss was also higher in
the treatment receiving late furrow IT in comparison with early IT.
At this last evaluation, the overall quality of all treatments was low;

however, decay was highest in lettuce receiving late IT with overhead sprinkles. In addition, brown stain values were found higher
and L* lower in the late furrow IT than in the other IT treatments
(Table 4).
The influence of pre-harvest mild stress on postharvest quality
is not well understood. In several cases, vegetables grown under
more favorable conditions have resulted in shorter shelf life. Pepper grown in open field kept quality for longer time than pepper
grown in the greenhouse (Banara and others 2005). Broccoli under
water stress showed increased postharvest quality (Wurr 2002). In
our study, the higher water loss rate of turgid tissue from the late IT
treatment suggests that the deterioration of quality in this treatment could be associated to higher water activity and subsequent
loss of water and condensation on surface, which resulted in an
ideal scenario to accelerate microorganisms growth and overall
decay. Although microorganisms are commonly inactivated when
water activity is lower than 0.995, differences in microbial growth
rate may be observed at higher water activity levels (FernandezSalguero and others 1993). Water loss immediately after harvest is
a predominant problem in most fresh vegetable applications. High
transpiration rate and subsequent water loss causes rapid development of physiological disorders during postharvest storage (Alferez
and others 2005). Agricultural practices, such as high nitrogen rate
fertilization, that enhance turgidity in plants often decrease shelf
Vol. 71, Nr. 2, 2006—JOURNAL OF FOOD SCIENCE

M47

M: Food Microbiology & Safety

a ns indicates that treatments were not different according to analysis of variance. OS indicates that last irrigation was conducted with overhead sprinkles;
other treatments used furrow irrigation. Values are the mean of 24 samples. OVQ was assessed with a 1 to 9 scale with 9 = excellent, 6 = salability point and,
1 = completely deteriorated. Brown stain and pink rib were evaluated with a 1 to 5 scale with 1 = no symptoms, 3 = salability point and, 5 = extremely
affected. Color units indicated L* = lightness, a*greenness and, b* = yellowness.

Effect of moisture on quality of lettuce . . .
Table 4—Effect of timing of the last irrigation on lettuce quality parameters after 21 d in storage at 2 °C to 4 °C
Irrigation termination timing (days before harvest)
Quality factors

4 OSa

OVQ
Decay
Water loss (%)
Brown stain
Pink rib
L*
a*
b*

5.10
3.6
3.03
3.24
2.21
61.13
–14.70
29.90

4
5.55
2.66
3.10

3.45
2.49
60.50
–16.68
28.88

8
5.47
2.68
3.04
2.82
2.64
62.93
–15.45
28.12

16
5.72
2.60
2.80
2.75
2.45
62.87
–15.83
28.10

LSD (P < 0.05)
ns
0.74
0.27

0.54
ns
1.12
ns
ns

a ns indicates that treatments were not different according to analysis of variance. OS indicates that last irrigation was conducted with overhead sprinkles;

other treatments used furrow irrigation. Values are the mean of 24 samples. OVQ was assessed with a 1 to 9 scale with 9 = excellent, 6 = salability point and,
1 = completely deteriorated. Brown stain and pink rib were evaluated with a 1 to 5 scale with 1 = no symptoms, 3 = salability point and, 5 = extremely
affected. Color units indicated L* = lightness, a*greenness and, b* = yellowness.

life, in part due to the significant reduction of stiffness associated
with loss of excess water during postharvest storage (Newman and
others 2005). Similar results, showing early IT as the treatment with
the highest quality and longer shelf life, was observed in the 1st
trial (data not shown).
The microbial population in lettuce increased with shorter periods of time between the last irrigation and harvest. Lettuce receiving
late IT had microbial counts over 0.4 log higher than lettuce subjected to early IT. The microbial population of lettuce irrigated 4 d before harvest with overhead sprinkle irrigation was particularly higher than the other treatments. Aerobic bacteria counts in head
leaves were higher than in outer leaves (Figure 1). These results
revealed the significant impact of moisture at harvest on microbial
population in lettuce. A recent Salmonella sp. risk assessment reported different risk of infections depending on type of crop, irrigation method, and days between the last irrigation, with the latter
being the factor affecting the highest (Stine and others 2005), which
coincides with this study.

The effect of rainfall on microbial population was also evaluated.
The results showed that microbial population in lettuce heads increased after rain in outer and head leaves. The increase, however,
was more dramatic in head leaves, showing a 3 log increase a week
after rainfall. The outer leaves showed an increase in microbial
population of 1.5 log in 2 d but declined after 7 d (Figure 2). The

same pattern was observed in 4 different fields from which samples were taken before and after rain (data not shown).
The results showed that rainfall occurring several d before harvest decreases the microbial quality of lettuce. Moisture from rain
or from overhead sprinkling likely creates an ideal microclimate
that allows native microorganisms to proliferate and can facilitate
pathogen internalization through lesions. Contamination in the
field can become a risk of high magnitude, particularly if the microorganism gains access to the internal of the plant tissue through

M: Food Microbiology & Safety
Figure 1—Effect of irrigation termination schedule on microbial population of head lettuce. All treatments were
furrow irrigated with exception of 1 treatment (4 OS) that
was irrigated with overhead sprinkles. Bars indicate standard deviation.
M48

JOURNAL OF FOOD SCIENCE—Vol. 71, Nr. 2, 2006

Figure 2—Microbial population in heads and outer leaves
of iceberg lettuce after a rainfall event. Bars indicate standard deviation.
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leaf lesions (Brandl and Mandrell 2002). Sprinkling water to lettuce
plants produced loss of visual quality and elevated the risk of contamination (Solomon and others 2002a). Infiltration of Salmonella
sp. into growing tomatoes was observed to increase and remained
constant for 10 d (Guo and others 2002).
The conditions for growth of bacteria on the lettuce surface are
likely more favorable on head leaves due to the presence of a film
of condensate. On the other hand, the microbial population in outer leaves declined more rapidly that in head leaves due to more
rapid drying and higher impact of sun UV light, 2 factors that diminish microbial population (Coelho and others 2005). It is also
possible that outer leaves are subjected to intermittent water stress,
which can result in higher accumulation of metabolites that diminish bacteria growth. Abiotic and biotic factors can function as elicitors of defense mechanisms that induce plant resistance to a

broad array of plant pathogens (Sudha and Ravishankar 2002; Yun
and others 2002). It could be possible that similar response occur
against clinical bacteria.
Undoubtedly, an early IT can result in lower yields, but this
study showed that extremely late IT is not necessary to maximize
weight at harvest. Programs with regulated deficit irrigation have
been shown to produce similar yields (Goldhamer and Beede 2004)
and enhanced appearance (Puiupol and others 1996). A major
point revealed in this 2-year study is that wet conditions at harvest
result in increase microbial population. This means that if for any
reason pathogenic bacteria reach the surface of lettuce, late IT or
rain immediately before harvest may produce ideal conditions for
proliferation of the pathogens. The survival of microbes decline if
a period of time in dry conditions is allowed before harvest but
clearly, this cannot be achieved in times of frequent rainfall events.
When wet conditions in the field are inevitable, more rigorous controls of the microbial quality are needed because efficacy of sanitizers is limited (Sapers 2001). Wet conditions in the soil could facilitate pathogen survival, bacteria uptake by roots, and their
migration to the head leaves (Solomon and others 2002b). The findings in this study stimulate further research to validate results with
specific pathogens.

Conclusions

T

he results from this study showed that microbial population in
lettuce increases after rain and with late IT. Lettuce subjected
to overhead sprinkle irrigation showed inferior visual and microbial
quality than furrow irrigated lettuce. It was revealed that regulation
of moisture at harvest through appropriate scheduling of last irrigation could be a practical way to reduce microbial population of
iceberg lettuce and extend shelf life while keeping similar yields at
harvest. Although the potential decrease in weight produced with

an early IT is a concern of growers, it was shown in this study that
excessive late IT is not necessary to obtain maximum lettuce
weight at harvest.

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Effect of moisture on quality of lettuce . . .