Lettuce receiving late 4 deceiving late 4 d befor before hare hare harvvvvvest IT shoest IT shoest IT showwwed incred incred increased weased weased weight and diametereight and diameter
Trang 1Vol 71, Nr 2, 2006—JOURNAL OF FOOD SCIENCE M45
© 2006 Institute of Food Technologists
Postharvest Quality and Microbial Population of Head Lettuce as Affected by Moisture at Harvest
JJJJJORGE M FONSECA
ABSTRA
ABSTRACTCTCT: I: I: Iceberceberceberg lettuce was evg lettuce was evg lettuce was evaluated for yield, micraluated for yield, micraluated for yield, microbial population, and postharobial population, and postharobial population, and postharvvvvvest quality either folloest quality either folloest quality either follow-
w-ing differ
ing different irrent irrent irrigation terigation terigation termination (ITmination (ITmination (IT) schedules or befor) schedules or befor) schedules or before and after a re and after a re and after a rainfall evainfall evainfall event Lettuce rent Lettuce rent Lettuce receiving late (4 deceiving late (4 d
befor
before hare hare harvvvvvest) IT shoest) IT shoest) IT showwwed incred incred increased weased weased weight and diametereight and diametereight and diameter, higher aer, higher aer, higher aerobic bacterobic bacterobic bacteria countsia countsia counts, and lo, and lo, and lowwwer quality thaner quality than
plants subjected to early (16 d befor
plants subjected to early (16 d before hare hare harvvvvvest) ITest) ITest) IT M M Middle (8 d beforiddle (8 d beforiddle (8 d before hare hare harvvvvvest) IT prest) IT prest) IT produced similar yieldsoduced similar yieldsoduced similar yields, and, and
lo
lowwwer micrer micrer microbial population in lettuce than late ITobial population in lettuce than late ITobial population in lettuce than late IT After r After r After rain, micrain, micrain, microbial population incrobial population incrobial population increased beased beased by 1.5 and 3.0 logy 1.5 and 3.0 log
colony
colony-for-for-forming units (CFU)/g in outer and head leavming units (CFU)/g in outer and head leavming units (CFU)/g in outer and head leaves res res respectivespectivespectivelyelyely The rThe rThe results results results revevevealed the imporealed the imporealed the importance oftance of
managing moisture conditions at harvest to enhance overall quality of lettuce
Keywords: Irrigation termination,
Keywords: Irrigation termination, Lactuca sativa Lactuca sativa Lactuca sativa L., shelf life, storage, yield L., shelf life, storage, yield
Introduction
Understanding the dynamic of the microbial population of
let-tuce is important for growers to deliver safe food to
consum-ers 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
re-duce 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
contamina-tion are expected when vegetable surfaces are in direct contact with
irrigation water (Stine and others 2005) It is also possible that
wa-ter 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
termina-tion (IT), a concept indicative of the timing of the last irrigatermina-tion, 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
potential-ly 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
treat-ments The improvement in quality appears to be a result of plants
developing adaptation mechanisms to cope with limiting factors;
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 signif-icant 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 doc-umented The objectives of this study were as follows: (1) to eval-uate 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 dur-ing the month of January; fertilization included 45 kg/ha of
10-4-0-0 at planting followed by 3 applications of 210-4-0-0-10-4-0-0-10-4-0-0-17 at 510-4-0-0 kg/ha The irrigations before the last irrigation, included overhead sprin-kling for 10 h daily during the first 5 d after planting to establish stands, and 2 furrow irrigations, 4 and 8 wk after planting Accumu-lated 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
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:
jfonseca@ag.arizona.edu).
Trang 2M: Food Microbiology & Safety
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
harvest-ed the same day Furrow was usharvest-ed 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
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%
rel-ative 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
con-ducted using a 9-point hedonic scale in which 9 was excellent
qual-ity, 7 good qualqual-ity, 6 the salability point, 5 fair (becoming
objection-able), 3 poor, and 1 extremely poor (Artes and Martines 1996) The
indication of maximum shelf life of products was when visual
qual-ity 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
mea-surement 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
stor-age 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
orders was carried with a scale of incidence and severity of any dis-order (Martinez and Artes 1999) This index ranged from 1, indicat-ing no symptom, to 5 indicatindicat-ing severe deterioration Grade 2 was assigned to lettuce that was only slightly affected Grade 3 was as-sociated 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 ac-cording to the film manufacturer’s recommendation, and submit-ted 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 anal-ysis 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 con-ducted 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 ⱕ 0.05 to determine statistical significance.
When ANOVA indicated a significant difference, mean separation
was carried out by LSD test (P ⱕ 0.05)
Results and Discussion
The 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 sprin-kles also showed higher whole plant and head weight and larger di-ameter 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 (Ta-ble 1)
The reduction of weight with the early IT treatment was
expect-ed However, it was interesting that no difference in weight was
Table 1—Yields parameters of iceberg lettuce as affected by timing of the last irrigation
Irrigation termination timing (days before harvest)
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.
Trang 3M: Food Microbiology & Safety
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
dif-ference 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
con-tent 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
ob-tained maximum diameter and weight of lettuce with a 25%
reduc-tion in the amount of water required to replenish total transpirareduc-tion
of the plants
Contrary to the pattern observed in the field, where higher
ben-efits were obtained with late and middle IT, a week after
posthar-vest storage, the OVQ of lettuce receiving early IT was higher than
that of lettuce subjected to late IT The middle IT treatment
pro-duced 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
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 over-head 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 Pep-per grown in open field kept quality for longer time than pepPep-per 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 treat-ment 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 (Fernandez-Salguero 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 develop-ment 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
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)
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)
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.
Trang 4M: Food Microbiology & Safety
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
peri-ods of time between the last irrigation and harvest Lettuce receiving
late IT had microbial counts over 0.4 log higher than lettuce
subject-ed to early IT The microbial population of lettuce irrigatsubject-ed 4 d
be-fore harvest with overhead sprinkle irrigation was particularly
high-er than the othhigh-er treatments Ahigh-erobic bacthigh-eria 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
re-ported different risk of infections depending on type of crop,
irriga-tion method, and days between the last irrigairriga-tion, 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 in-creased 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 sam-ples were taken before and after rain (data not shown)
The results showed that rainfall occurring several d before har-vest 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 micro-organism gains access to the internal of the plant tissue through
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)
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.
Figure 1—Effect of irrigation termination schedule on
mi-crobial population of head lettuce All treatments were
furrow irrigated with exception of 1 treatment (4 OS) that
was irrigated with overhead sprinkles Bars indicate
stan-dard deviation.
Figure 2—Microbial population in heads and outer leaves
of iceberg lettuce after a rainfall event Bars indicate stan-dard deviation.
Trang 5M: Food Microbiology & Safety
leaf lesions (Brandl and Mandrell 2002) Sprinkling water to lettuce
plants produced loss of visual quality and elevated the risk of
con-tamination (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
out-er leaves declined more rapidly that in head leaves due to more
rapid drying and higher impact of sun UV light, 2 factors that
di-minish 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
dimin-ish bacteria growth Abiotic and biotic factors can function as
elic-itors 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
con-trols of the microbial quality are needed because efficacy of
sani-tizers is limited (Sapers 2001) Wet conditions in the soil could
facil-itate pathogen survival, bacteria uptake by roots, and their
migration to the head leaves (Solomon and others 2002b) The
find-ings in this study stimulate further research to validate results with
specific pathogens
Conclusions
The 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
irri-gation 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
References
Ackers ML, Mahon BE, Leahy E, Goode B, Damrow T, Hayes PS, Bibb WF, Rice DH.
1998 An outbreak of Escherichia coli O157:H7 infections associated with leaf
lettuce consumption J Infect Dis 177:1588–93.
Alferez F, Zacarias L, Burns JK 2005 Low relative humidity at harvest and before storage at high humidity influence the severity of postharvest peel pitting in citrus J Am Soc Hortic Sci 130:225–31.
Artes F, Martinez JA 1996 Influence of packaging treatments on the keeping quality of Salinas lettuce Lebensm Wiss Technol 29:664–8.
Banaras M, Bosland PW, Lownds NK 2005 Effects of harvest time and growth
conditions on storage and post-storage quality of fresh-quality peppers (Cap-sicum annuum L.) Pakistan J Bot 37:337–44.
Brandl MT, Mandrell RE 2002 Fitness of Salmonella enterica serovar Thomson
in the cilantro phyllosphere Appl Environ Microbiol 68:3614–21.
Coelho AF, Gomes EP, Sousa AP, Gloria MBA 2005 Effect of irrigation level on yield and bioactive amine content of American lettuce J Sci Food Agric 85:1026– 32.
Fernandez-Salguero J, Gomez R, Carmona MA 1993 Water activity in selected high-moisture foods J Food Comp Anal 6:364–9.
Gallardo M, Jackson LE, Schulbah K, Snyder RL, Thompson RB, Wyland LJ 1996 Production and water use in lettuces under variable water supply Irrig Sci 16:125–7.
Galindo FG, Herppich W, Gekas V, Sjoholm I 2004 Factors affecting quality and postharvest properties of vegetables: Integration of water relations and me-tabolism Crit Rev Food Sci Nutr 44:139–54.
Goldhamer DA, Beede RH 2004 Regulated deficit irrigation effects on yield, nut quality and water-use efficiency of mature pistachio trees quality J Hortic Sci Biotechnol 79:538–45.
Guo XJ, Brackett RE, Beuchat LR 2001 Survival of Salmonellae on and in tomato plants from the time of inoculation at flowering and early stages of fruit de-velopment through fruit ripening Appl Environ Microbiol 67:4760–4.
Heatherly LG, Spurlock SR 1993 Timing of furrow irrigation termination for determinate soybean on clay soil Agron J 85:1103–8.
Hilborn ED, Mermin JH, Mshar PA, Hadler JL Voetsch A, Wojtkunski C, Swartz M,
Mshar R 1999 A multistate outbreak of Escherichia coli 0157:H7 infections
associated with consumption of mesclum lettuce Arch Inter Med 159:1758–64 Kapperud G, Roervik LM, Hasseltvedt V, Hociby EA, Iversen BG, Staveland K, Johnsen G, Leitao J 1995 Outbreak of Shigella sonnie infection traced to imported Iceberg lettuce J Clin Microbiol 33:609–14.
Koivula MJ, Kymalainen HR, Vanne L, Levo S, Pehkonen, Sjoberg AM 2004 Mi-crobial quality of linseed and fiber hemp plants during growing and harvest-ing seasons Agric Food Sci 13:327–37.
Leenhardt D, Loflie F, Bruckler L 1998 Evaluating irrigation strategies for let-tuce by simulation 1 Water flow simulations Eur J Agron 8:249–65.
Martinez JA, Artes F 1999 Effect of packaging treatments and vacuum-cooling
on quality of winter harvested iceberg lettuce Food Res Int 32:621–7.
Newman JM, Hilton HW, Clifford SC, Smith AC 2005 The mechanical properties
of lettuce: a comparison of some agronomic and postharvest effect J Materials Sci 40:1101–4.
McConnell JS, Vories ED, Oosterhuis DM, Baker WH 1999 Effect of irrigation termination on the yield, earliness, and fiber qualities of cotton J Prod Agric 12:263–8.
Puiupol LU, Behboudian MH, Fisher KJ 1996 Growth, yield, and postharvest attributes of glasshouse tomatoes produced under deficit irrigation Hort-Science 31:926–9.
Sanchez CA 2000 Response of lettuce to water and nitrogen on sand and the potential for leaching of nitrate-N HortScience 35:73–7.
Sapers GM 2001 Efficacy of washing and sanitizing methods for disinfection of fresh fruit and vegetable products Food Technol Biotechnol 39:305–11.
Solomon EB, Potenski CJ, Matthews KR 2002a Effect of irrigation method on transmission to and persistence of Escherichia coli O157:H7 on lettuce J Food Prot 65:673–6.
Solomon EB, Yaron S, Matthews KR 2002b Transmission of Escherichia coli O157:H7 from contaminated manure and irrigation water to lettuce plant tissue and its subsequent internalization Appl Environ Microbiol 68:397–400 Stine SW, Song IH, Choi CY, Gerba CP 2005 Application of microbial risk assess-ment to the developassess-ment of standards for enteric pathogens in water used to irrigate fresh produce J Food Prot 68:913–8.
Sudha G, Ravishankar GA 2002 Involvement and interaction of various signal-ing compounds on the plant metabolic events dursignal-ing defense response, resis-tance to stress factors, formation of secondary metabolites and their molecu-lar aspects Plant Cell Tissue Organ Cult 71:181–212.
Wurr DCE, Hambidge AJ, Fellows JR, Lynn JR, Pink DAC 2002 The influence of water stress during crop growth on the postharvest quality of broccoli Post-harv Biol Technol 25:193–8.
Yun B, Loake GJ 2002 Plant defense responses: current status and future exploi-tation J Plant Biotechnol 4:1–6.
Zhang B, Li F, Huang G, Cheng Z, Zhang Y 2006.Yield performance of spring wheat improved by regulated deficit irrigation in an arid area Agric Water Manage-ment 79:28–42.