Preharvest Factors Influencing Fresh-cut Biology and Quality Lettuce Deirdre Holcroft Holcroft Postharvest Consulting, Inc Factors affecting Quality Lettuce Browning Preharvest Variety Irrigation/rain? Fertilization Weather Maturity Plant spacing Pests Diseases Physiological disorders Postharvest Cut to cool times Rapid cooling Processing Sanitation Packaging etc. Wounding Synthesis of phenolics PAL enzyme Oxidation of phenolics PPO enzyme Browning (Pinking) Lettuce Decay Appears watersoaked and wilted with translucent, dull brown to dark or black areas Physiological decay causes leakage of cell contents high microbial counts. Irrigation regimes (IR) on Iceberg lettuce Applied 5 irrigation regimes: 1. 50% (severe deficit) 2. 25% (slight deficit) 3. Conventional irrigation (?) 4. +25% (slight excess) 5. +50% (severe excess) But adjusted to four: Irrigation regime (mm) = Irrigation + Rain ET R1 R2 R3 R4 0 100 mm 101 200 mm 201 300 mm 301 400 mm 6 harvests over 3 years 1
Irrigation regimes on Iceberg lettuce 1. Excess irrigation Increased head compactness Increased cut edge browning decreased visual quality decreased yield 2. Deficit irrigation preserved: quality shelf life of cut lettuce IR on fresh wt. (g) and yield fresh wt. IR Head wt. (g) (kg/ha) (t/acre) R1 650 52,000 23 R2 710 56,800 25 R3 575 46,000 21 R4 625 50,000 22 IR on head compactness More compact IR on Visual Quality 9 = excellent, 5 = limit of marketability, 1 = inedible. 8 = full compactness 6 = max. acceptable 5 = optimal 4 = min. acceptable 1= no compactness visual quality IR on Off Odors 5 = severe, 3 = moderate (limit), 1 = none off odors Note: Midrib tissue stored in air was used to assess browning 5 = severe Cut Edge Browning Increased irrigation: rapid browning more severe browning Increase in PAL and PPO activity 3 = moderate (limit) 1 = none 2
Irrigation regimes on Iceberg lettuce 1. Excess irrigation Increased head compactness Increased cut edge browning decreased visual quality decreased yield 2. Deficit irrigation preserved: quality shelf life of cut lettuce 3. Irrigation is a tool for optimal yield and quality 4. Control 25% is recommended for Iceberg Irrigation regimes on Romaine lettuce Tested 6 irrigation regimes 1. 35% SIR 2. 5% SIR 3. SIR 4. +5% SIR 5. +35% SIT 6. +75% SIR 6 harvests over 3 years SIR = standard irrigation regime Conclusions Excess irrigation: accelerated cut edge browning when stored without MAP; but not when in low O2 Increased microbial growth Deficit and excess irrigation: reduced yield increased resp. rate Very different results to Iceberg Luna et al. 2013 Irrigation Type and Termination on Iceberg Lettuce (whole head) Treatment Head wt. (g) (kg/ha) (t/acre) 4 d, OS 940 75,200 34 4 d, Furrow 953 76,240 34 8 d, Furrow 945 75,600 34 16, Furrow 847 67,760 30 Lower yield Better visual quality Less water loss Less brown stain Irrigation termination and irrigation type : 4 days before harvest; overhead sprinkler 4 days before harvest; furrow 8 days before harvest; furrow 16 days before harvest; furrow Irrigation Termination on Microbial Quality Irrigation closer to harvest results in higher micro load; OS even higher Note: Whole head quality not fresh cut Effect of Rainfall on Microbial Quality Factors affecting fresh cut lettuce micro counts after rainfall, especially in head leaves. 3 log increase! Investigated these factors on visual quality of fresh cut lettuce Transplant date significant Maturity significant Overhead vs.. drip irrigation NS N fertilization NS K:N ratios no clear pattern Ca no clear pattern Rainfall before harvest had the greatest effect on discoloration Not temperature or sunshine hours. Hilton et al., 2009 3
Question from Fresh cut Workshop September 2012 Is there a correlation between rainy weather prior to harvest and enzymatic browning in cut lettuce? YES! Romaine Variety Evaluation Evaluation: 33 varieties over 2 years MA (0.2% O 2 ; 11% CO 2 ) CA (0.2% /5% O 2 ; 0% CO 2) Top performers: Clemente Darkland Green Forest Poor performers: Alpi Dark Green Romaine Queen of Hearts Sensitive to high CO 2 Triple Threat Romaine La Brilliante Iceberg) Ritmo butter (Varoquax et al., 1996) (Hayes & Liu, 2008) Romaine high CO 2 A.N.Other Lollo rosso Evasion and red oak Jamai Conventional soil production compare to soilless hydroponic system in the same open field Soilless produ ction resulted in : Faster maturation (62 vs. 102 days) Better visual quality Less cut edge browning Higher antioxidants and ascorbic acid Lower microbial counts especially coliforms at harvest and after 12 days Soil vs. Soilless Production Soil vs. Soilless Production Soil vs. Soilless Production on Microbial Quality Butter lettuce Daguan Soilless produ ction resulted in : Faster maturation (62 vs. 102 days) Poorer visual quality Cut edge browning was controlled by MAP Lower microbial counts All three lettuces had lower microbial counts in soilless systems, especially Lactic acid bacteria Total coliforms 4
Maturity indices for Romaine lettuce Over mature = more browning, more bitterness, shorter shelf life Less Mature = better texture, more prone to mechanical damage, more off flavors 1. Days from planting to harvest? Yuma about 100 days (winter) Salinas about 60 days (summer) 2. Leaf number (30 35 leaves) 3. Head height, weight, diameter 4. Head density 5. Core (stem) Romaine maturityindices Minimum head wt. (g) 300 400 Maximum core length (mm): fall/winter 50 spring/summer 80 Head length (mm) >200 Core Length (mm) on Quality 40 60 mm core = better visual quality and less browning >80 mm core = poor visual quality and more browning Gil et al., 2012 Gil et al., 2012 Iceberg Seasonal effects between Salinas, Huron and Yuma Winter production in Yuma Increased days from planting to harvest Very dense heads = poor quality Overall Conclusions Important factors in fresh cut lettuce quality: Rainfall/irrigation Maturity Variety And many factors still to be discovered Big data analytics Literature Cited Fonseca, J.M. 2006. Postharvest quality and microbial population of head lettuce as affected by moisture at harvest. Journal of Food Science 71(2), M45 M49. Gil, M.I. et al. 2012. Harvest maturity indicators of leafy vegetables. Stewart Postharvest Review 81: 1 9. Hayes, R.J. & Y.B. Liu 2008. Genetic variation for shelf life of salad cut lettuce in modified atmosphere environments. Journal of the American Society for Horticultural Science 1332: 228 233. Hilton, Clifford, Wurr & Burton. 2009. The influence of agronomic factors on the visual quality of field grown, minimally processed lettuce. Journal of Horticultural Science and Biotechnology 84, 193 198. Luna, Tudela, Martinez Sanchez, Allende & Gil. 2012. Long term deficit and excess or irrigation influences quality and browning related enzymes and phenolic metabolism of fresh cut lettuce (Lactuca sativa L.) Postharvest Biology & Technology 73, 37 45. Luna et al. 2013. Optimizing water management to control respiration rate and reduce browning and microbial load of fresh cut romaine lettuce. Postharvest Biology & Technology 80, 9 17. Selma, M. V., et al. 2012. Sensory quality, bioactive constituents and microbiological quality of green and red fresh cut lettuces Lactuca sativa L. are influenced by soil and soilless agricultural production systems. Postharvest Biology and Technology 631: 16 24. 5