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: jfonseca@ag.arizona.edu) © 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 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 d after planting to establish stands, and furrow irrigations, and 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 and 10 a.m., when temperatures ranged between °C and °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) OSa Whole plant weight (g) Head weight (g) Head diameter (cm) Water activity 1403.25 940.22 13.22 0.996 1406.13 952.54 13.03 0.998 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 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 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 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 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 °C to °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 trained people (3 men and 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 was excellent quality, good quality, the salability point, fair (becoming objectionable), poor, and 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 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 readings, including to the middle of the leaf stem, at cm from the edge of the leaves, and at cm from the edge of the leaves Water loss during postharvest storage was monitored by measuring the difference in weight between day 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 indicating severe deterioration Grade was assigned to lettuce that was only slightly affected Grade was associated with salability point, and grades and 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 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 replicates Sampling of lettuce before and after rainfall was conducted at random in different fields in Yuma, Arizona, using plants per each of 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 URLs and E-mail addresses are active links at www.ift.org Effect of moisture on quality of lettuce Table 2—Effect of timing of the last irrigation on lettuce quality parameters after d in storage at °C to °C Irrigation termination timing (days before harvest) Quality factors 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 6.65 0.85 1.6 1.3 1.2 64.31 –15.68 28.54 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 to scale with = excellent, = salability point and, = completely deteriorated Brown stain and pink rib were evaluated with a to scale with = no symptoms, = salability point, and = 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 °C to °C Irrigation termination timing (days before harvest) Quality factors OVQ Decay Water loss (%) Brown stain Pink rib L* a* b* OSa 5.70 2.60 2.32 2.52 1.44 62.00 –15.80 28.35 5.75 2.55 2.60 2.21 1.41 61.40 –15.68 28.47 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 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 treatments receiving late IT Significant URLs and E-mail addresses are active links at www.ift.org 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 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 to scale with = excellent, = salability point and, = completely deteriorated Brown stain and pink rib were evaluated with a to scale with = no symptoms, = salability point and, = 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 °C to °C Irrigation termination timing (days before harvest) Quality factors 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 5.55 2.66 3.10 3.45 2.49 60.50 –16.68 28.88 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 to scale with = excellent, = salability point and, = completely deteriorated Brown stain and pink rib were evaluated with a to scale with = no symptoms, = salability point and, = 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 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 log increase a week after rainfall The outer leaves showed an increase in microbial population of 1.5 log in d but declined after d (Figure 2) The same pattern was observed in 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 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 URLs and E-mail addresses are active links at www.ift.org 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, 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 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 URLs and E-mail addresses are active links at www.ift.org 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 (Capsicum 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 metabolism 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 development 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 Microbial quality of linseed and fiber hemp plants during growing and harvesting seasons Agric Food Sci 13:327–37 Leenhardt D, Loflie F, Bruckler L 1998 Evaluating irrigation strategies for lettuce by simulation 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 HortScience 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 assessment to the development 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 signaling compounds on the plant metabolic events during defense response, resistance to stress factors, formation of secondary metabolites and their molecular 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 Postharv Biol Technol 25:193–8 Yun B, Loake GJ 2002 Plant defense responses: current status and future exploitation 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 Management 79:28–42 Vol 71, Nr 2, 2006—JOURNAL OF FOOD SCIENCE M49 M: Food Microbiology & Safety Effect of moisture on quality of lettuce ... salability point, and grades and implied that heads were commercially unacceptable Microbiological analysis Aerobic plate count (APC) was carried out on the day of harvest to determine the microbial... 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... 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