Treatments to Maintain Fresh-cut Produce Quality & Safety Jim Gorny, Ph.D. Executive Director
Modes of Failure for Fresh-cut Produce Cut Edge Discoloration Tissue Softening General Tissue Senescence & Breakdown Microbial Decay (yeasts, molds, bacteria) Off Flavor and Aroma Development NOTE: Most fresh-cut products in the marketplace do NOT use chemical treatments to extend shelf-life.
Fruit and Vegetable Color Compounds Water Soluble Green (chlorophyll) Red to purple (anthocyanins) Brown, grey, black (phenolics) pink Fat Soluble Yellow, orange, red (carotenoids) e.g. lycopene, beta-carotene
Cut-Edge Discoloration Key Browning Reactions PHE PAL (Phenyalanine) Phenolics PPO ph 6-7 Oxygen Copper Quinones Condensation Rx s Colored Pigments Type 1 Fresh-cut Browning PPO present, low pre-formed phenolic content (e.g.lettuce) Type 2 Fresh-cut Browning Plenty of PPO and pre-formed phenolics present (e.g. apples)
Phenylalanine Ammonia Lyase (PAL) Key enzyme in phenolic biosynthesis. Mechanical injury (wounding) and ethylene can stimulate phenolic metabolism Phenolics are substrates for PPO; increased concentration stimulates browning. PHE PAL (Phenyalanine) Phenolics
PolyPhenol Oxidase (PPO) Two types - catalyze oxidation of mono-phenolics: o-diphenoloxidases di-phenolics:laccases Requires oxygen for reaction Optimal active ph range 6-7 Requires copper in prosthetic group PPO in plastids / Phenolic substrate in vacuole Genes have been cloned Phenolics PPO ph 6-7 Oxygen Copper Quinones Condensation Rx s Colored Pigments
OH o-dpo OH OH o-dpo O O R monophenol + 1/2 O 2 R o-diphenol + 1/2 O 2 R o-quinone OH O amino groups R OH p-diphenol Laccase complex + 1/2 O brown 2 R polymers O p-quinone
Cut-Edge Discoloration Strategies to Reduce Discoloration Phenolics PPO ph 6-7 Quinones Condensation Rx s Colored Pigments Reducing Agents (ascorbic acid) Low ph (organic acids) Chelators of enzyme co-factors (citric acid) Enzyme Inhibitors (sulfites)
Cut-Edge Discoloration Strategies to Reduce Discoloration: Reducing Agents Phenolics PPO ph 6-7 Quinones Condensation Rx s Colored Pigments Ascorbic Acid / Erythorbic Acid CH 2 OH HCOH O HO OH O CH 2 OH HOCH HO O OH O Considerations ph Concentration Ascorbate is consumed Cost Application means Safe sanitary use when recycled
Ascorbic Acid Effects on Pear Disc Color L*a*b Color 'L' Value 70 68 66 64 62 60 58 56 54 52 LSD=3.4 0% AA 0.1% AA 0.5% AA 1% AA 2% AA 0 1 2 Day
Cut-Edge Discoloration Strategies to Reduce Discoloration: -SH Amino Acids Phenolics PPO Quinones Condensation Rx s Colored Pigments +Cysteine Cysteine-Phenolic Adducts (Colorless) Cysteine-Phenolic Adducts (Pink) Considerations ph Off-aroma
Cut-Edge Discoloration Strategies to Reduce Discoloration: Enzyme Inhibitors Sulfites inhibit PPO, but banned on use in fresh fruits and vegetables. Still allowed on fresh-cut potatoes in USA. >10ppm must be labeled. Other PPO inhibitors 4-hexyl resorcinol. FDA GRAS status only for shrimp. Na 2 SO 3 NaHSO 3 SO 2
Cut-Edge Discoloration Strategies to Reduce Discoloration: MAP 1. Low Oxygen Atmospheres (<0.25%) Requires almost complete exclusion of oxygen to inhibit PPO activity 2. High Oxygen Atmospheres (>21%) 3. Edible Coatings 4. Carbon Monoxide MAP Inhibits PPO by binding to copper in prosthetic group. Approved in USA but not used due to worker safety issues.
ULO Effects on Sliced Apple Browning 86 84 Air Air+2% Ascorbic Acid 0.25% O 2 0.25% O 2 +2% Ascorbic Acid 100% N 2 100% N 2 +2% Ascorbic Acid LSD=0.96 82 L* Value 80 78 76 74 air transfer Gil, Gorny & Kader, 1998 0 1 2 3 4 Days at 10 o C
Effects of High Oxygen Atmospheres on Sliced Pears
ICEIN protein polysaccharide complex
0.35 Browning of Fresh-cut Lettuce Held for 4 Days at 15 C 0.30 20 A 437 nm 0.25 0.20 0.15 15 10 % Oxygen 0.10 0.05 0.00 Initial Air Argon Helium Nitrogen 5 2 0 Pilar Jamie and ME Saltveit, 2001
Cut-Edge Discoloration Strategies to Reduce Discoloration: Mild Heat (Saltveit, 2000; Murata et al, 2004) (50C 90s) @ 4C @ 4C
Tissue Softening TEXTURAL PROPERTIES (firmness: viscoelasticity, turgor) PLANT CELL Cell wall Plasma membrane Middle lamella STRUCTURAL COMPONENTS (cell wall, middle lamella, plasma membrane)
Tissue Firmness & Calcium Ca 2+ stabilizes cell membranes (turgor, membrane permeability and integrity) Ca 2+ forms ionic bonds between pectin molecules (firmness) COO - Ca 2+ - OOC Ca 2+ interacts with cell walls & middle lamella: Egg-box model : Ca 2+ interacts w/ cell walls & middle lamella calcium ions polygalacturonic acid chains (pectins)
Ca Lactate Effects on Pear Disc Firmness Gorny & Kader, 1996 Penetration Force (N) 10 9 8 7 6 5 4 3 2 LSD=1.03 0.0 0.5 1.0 1.5 2.0 % Calcium Lactate
Calcium Chloride Effects on Melon Firmness (Cantwell, 2001)
Tissue Firmness & Calcium Considerations ph Concentration Getting Calcium to where it is needed Off flavors (CaCl 2 bitter / Ca Lactate chalky)
Commercially Available NatureSeal Formulations Fruit Apples Avocados Grapes Limes Mangos Melons Pears Pineapple Vegetables Potatoes Beets Belgian Endive (chicory) Carrots Celery Eggplant Sweet Potatoes Parsnips Turnips Calcium Ascorbate @ near neutral ph
Tissue Senescence & Breakdown Senescence = programmed aging and cell death Plant Hormone Ethylene Accelerates Ripening and Senescence Wounding Plant Tissue Produces Ethylene Inhibiting Ethylene Production & Action may Extend Shelf-life
Tissue Senescence & Breakdown Strategies to Reduce Tissue Senescence & Breakdown: 1-MCP What is MCP? & Why should I care? MCP = 1- Methylcyclpropene Disrupts Ethylene Induced Ripening Applied at PPB or PPM Levels as a Gas or Liquid Approved on numerous crops Ethylene C C 1-MCP H C C CH H 3 H H
1-MCP and Ethylene are Present Cell Membrane 1-MCP Ethylene Ethylene receptor Signal (ON) Plant Cell Signal (ON 1-MCP prevents Ethylene binding) Temporarily Blocks Synthesis of Proteins and Enzymes related to senescence Senescence delayed: Extension of leaf life including photosynthetic activity
Color Changes (Water Soaking Development) in Fresh-cut Tomatoes Treated with Gaseous or Liquid 1-MCP 44 42 L* Value 40 38 36 Control Gas Liquid 34 0 7 14 Days at 5 C Luo, 2007
Sliced Tomatoes Stored for 14 Days Gas Liquid Control Luo, 2007
Microbial Decay Is microbial growth a primary cause of fresh-cut produce spoilage or a symptom of senescence? Pseudomonads fresh-cut vegetables Yeasts and Molds fresh-cut fruit Do lower microbial counts = longer fresh-cut shelf-life?
Microbial Decay Strategies to Reduce Microbial Decay & Enhance Microbial Safety Heat Treatments Hot Water Dips Steam Irradiation UV Light MAP Biosteam VSV
20 C Control 21 Days @ 4 C 76 C (Annous, 2005)
Heat Treatments Do not kill internalized microbes May have adverse quality effects (Suslow,, 2001)
(Annous, 2005)
Radiation Lethality Response of Various Organisms Humans Molds Pathogens Insects Plants Viruses Yeast 0.001 0.01 0.1 1 10 100 Radiation Dose kgy) (kgy( ) (Brackett. 1987)
Gorny & Kader, Unpublished
Microbial Decay Celery 14 Days @ 4C Parsley Green Onions (Fan 2007)
Microbial Decay Strategies to Reduce Microbial Decay: UV Light Larson & Johnson, 1999 Control Melons Overtly Spoil Before C.Bot toxin is produced. UV Treated Melons: 45 Total Samples 2 toxic @ 15C 9 toxic @ 27C Turbid Liquid, Slight Off Odor, No Package Bloating
Microbial Decay Strategies to Reduce Microbial Decay: MAP Low Oxygen Elevated (>10%) CO 2 Ozone Chlorine Dioxide CO 2 O 2 1 Cantwell, UCD
Chlorine Dioxide MAP System Microsphère PE Film Water Vapor Chlorine Dioxide Gas
Gaseous Ozone Romaine lettuce 1 day 6 C/95% RH Considerations Concentration Time Temperature RH Tissue Damage O 3 200 ppm*hr Control From: Smilanick
Flavor and Aroma Retention Off Flavors Ethanol, Acetaldehyde Sulfur Containing Compounds Generally Caused by anaerobic metabolism Spoilage organisms (lactic acid bacteria, yeasts) Flavor & Aroma Retention An enigma, wrapped in a riddle shrouded, by mystery. Complex interactions.
Summary Temperature control is the most important means of extending fresh-cut produce shelf-life!!!!!!!! Numerous hurdle supplemental technologies may be used to extend fresh-cut produce shelf-life. Apply new technologies to address specific modes of failure, for specific products. Be aware of possible treatment unintended consequences.
Jim Gorny, Ph.D. Executive Director University of California Department of Plant Sciences MS2 3045 Wickson Hall 1 Shields Ave Davis, CA 95616 Tel: 530.754.9270 Email: jrgorny@ucdavis.edu http://postharvest.ucdavis.edu/