Anti-browning treatment for loose leaf lettuce packing:
The enzymatic browning of lettuce (Lactuca sativa L.) is a main cause of postharvest quality loss, and is controlled by the enzyme phenylalanine ammonia lyase (PAL). However, effective browning inhibitors that prevent lettuce butt discoloration have not been commercially developed, so the effects of such inhibitors on PAL are largely unknown. Here, we not only developed an anti-browning treatment, but also explored the mechanisms of the PAL-associated browning of Iceberg lettuce by profiling all homologs of PAL genes at transcript level. The anti-browning treatment used a combination of 0.25 M acetic acid and 200 mL L−1 ethanol and was able to repress enzymatic browning and microbial growth for two weeks. Notably, the lettuce butt discoloration in stem disks was repressed by 0.5 M acetic acid by inhibiting PAL activity, and this inhibition of PAL activity was also observed in vitro using a crude PAL enzyme extract from lettuce stems. To investigate the anti-browning mechanism at the transcriptional level, we identified and cloned six predicted LsPAL genes in the Lettuce Genome Resource, and further found that four of these (LsPAL1 to LsPAL4) were wound-inducible in the lettuce stem. Among these four wound-inducible LsPALs, LsPAL4 showed the highest wound-induced fold-change, suggesting that LsPAL4 has a key role in lettuce browning. Interestingly, wound-induction of LsPAL genes was dramatically downregulated by application of acetic acid. Taken together, acetic acid treatment of lettuce stems repressed butt discoloration by repressing PAL both enzymatically and transcriptionally, and ethanol provided complementary antimicrobial activity. A combination treatment with acetic acid and ethanol therefore has commercial potential in lettuce head processing.
Lettuce packing & storage temperatures:
Lettuce is a perishable vegetable that its shelf life is limited by detrimental texture changes and enzymatic activities during postharvest storage. One of solutions that can be used in storage and distribution of this product is modified atmosphere packaging (MAP). But its effects on the mechanical properties of the product have not been studied. This information would be beneficial in inspection of eating quality and later processing such as cutting and shredding. This research aimed to study effect of active MAP technology on the postharvest mechanical properties of lettuce at different storage periods. It was found that MAP had a significant effect on the tension property of the lettuce leaf. MAP treatment preserved the brittleness of the specimens during storage, but the control ones lost their brittleness and tolerated greater tension. Stiffness and rupture energy were significantly affected by MAP with P values of 0.01 and 0.05, respectively. Also, stiffness of the MAP-treated samples was higher than the control at the same storage time. The findings are due to reducing respiration, transpiration and bacterial growth by MAP which delays nutrient decomposition and maintains the original color and texture.
Physiological Effects of Harvesting
Harvesting of fruits and vegetables increases their perishability through increased respiration rates, altered ethylene production rates, and increases in other biochemical reactions: discoloration and color, texture, aroma and flavor, nutritional quality The degree of processing and the quality of the equipment (i.e. blade sharpness), significantly affect the wounding response. Damage to cells near cut surfaces influences the shelf life and quality of the product. For example, lettuce cut by a sharp knife with a slicing motion has a storage life approximately twice that of lettuce cut with a chopping action. Shelf life of lettuce is less if a dull knife is used rather than a sharp knife. Strict temperature control is required to minimize the increased respiration rates of harvesting.
The Harvest Formula consists of the specifications for the Growing Room Atmosphere during Harvest Time and the Harvest Cutting methodology used. The Harvest Methodology will be specific to each Variety and may be automated or be manual.
Post-Harvesting Production Methods
Post harvest production methods are critical to extending shelf life. Cooling crops immediately after cutting removes sugars and other nutrients at the cut surfaces that favor microbial growth and tissue discoloration. Because of differences in composition, some crops such as cabbage are known as “dirty” products because they release substantially more organic nutrients with harvesting. For lettuce, discoloration of the cut surfaces is a major quality defect. Cutting stimulates enzymes involved in phenolic metabolism which in turn leads to the formation of undesirable brown pigments. To ensure packaged salad products with no brown edges, very low O2 (<0.5%) and high CO2 (>7%) atmospheres are used commercially. These conditions may lead to increases in acetaldehyde and ethanol concentrations, indicating a shift from aerobic to anaerobic or fermentative metabolism. These changes are greater in the iceberg lettuces than in romaine lettuces, and are correlated with the development of off-odors.
Harvesting: Leaf Size
Young leaf tissue will have higher respiration rates than mature fully developed leaves. 2 x 2 cm pieces from mature leaves have respiration rates almost double those of the intact leaves, but similar to rates of the small leaves. In the scientific literature, shredding mature leaves approximately doubled respiration rates. Different parts of a vegetable may have very different respiration rates as illustrated with data from broccoli. The respiration rates of iceberg and romaine lettuces cut as pieces (2-3 x 2-3 cm) are 20-40% higher than rates of the respective intact heads. The respiration rates of shredded lettuce and shredded cabbage are 200-300% greater than those of the intact heads and remain high throughout the storage period. Respiration rates and deterioration rates can be minimized by quickly cooling the product and storing at 5° C (41° F) or below. Selecting a Variety and a Nutritional Formula enables the maximum efficacy for leaf size respiration.
Modified Atmosphere Packaging (MAP)
Although temperature is the principal controlling factor for respiration rates, modified atmospheres will also reduce metabolic rates. Controlled atmospheres of 1-2% O2 + 10% CO2 reduced respiration rates of minimally processed strawberries, peaches and honeydew by 25 to 50% at 5° C. These same atmospheres also reduced ethylene production and softening of the fruit tissues. Control of the wound response is the key to providing good quality. Low temperatures minimize differences in respiration and ethylene production rates between the cut and the intact product. Low temperatures are also essential to retard microbial spoilage on cut surfaces. Variety, production conditions, stage of maturity, piece size, and storage conditions all contribute to variations in fresh-cut product physiology. Although MAP maintains visual quality by retarding browning, off-odors increase and lettuce crispness decreases during storage of the salad products.
Packaging technology is indispensable for most fresh harvested crops. The selection of the plastic film packaging material strives to achieve equilibrium between the oxygen demand of the product (oxygen consumption by respiration) and the permeability of the film to oxygen and carbon dioxide transmission. In practice, films are often selected on the basis of the oxygen transmission rate (OTR expressed in units of ml/m2-day-atm). Several product factors need to be considered in selecting film packaging: the rate of respiration of the product and the specific cut, the quantity of product, and the desirable equilibrium concentrations of O2 and CO2. Plastic film characteristics that need to be considered include: 1) the permeability of a given thickness of the plastic film to O2, CO2 and water at a given temperature; 2) total surface area of the sealed package; and 3) the free volume inside the package.
With current packaging technology, it is possible to have product of good visual quality even at temperature-abuse conditions. Although product temperatures of 20° C (68° F) are unlikely, short periods near 10° C (50° F) can readily occur. The visual quality of the product is only slightly reduced by holding at 10° C (50° F), but atmosphere composition, production of fermentative volatiles and off-odor development are notably different from product stored at 0° C (32° F). These data underscore the potential of low temperature storage in conjunction with appropriate MAP conditions. In the case of lettuce, the atmospheres effective in retarding cut edge browning are very different from the atmospheres recommended for intact lettuce heads (lettuce develops the disorder brown stain when to CO2 >2%). Green bell peppers provide another example in which modified atmosphere conditions beneficial for the fresh-cut product differ substantially from those recommended for the intact product. As more research is conducted on Varieties the temperature and atmosphere requirements will be very determined for each Variety. A Variety with a specific respiration rate can be selected for a specific film or a film can be selected and the Variety with a specific respiration rate can be selected.
Many types of films are commercially available and used for fresh-cut packaging, including polyethylene (PE), polypropylene (PP), blends of PE and ethylene vinyl acetate (EVA), and co-extruded polymers or laminates of several plastics. Besides the permeability characteristics described above, films must also satisfy other requirements (Zagory, 1995). They must have strength and be resistant to tears and splits (oriented PP or polystryene), punctures (low density PE), stretching (oriented PP or polyethylene terephthalate), slip to work on bagging machines (acrylic coatings or stearate additives), have flex resistance, clarity and printability, and in some cases resealability (Ziploc or sticky seals). Consumer tactile appeal and ease of opening are also important considerations. Film selection is a compromise between the strengths and weakness of the different materials. Many currently used films are co-extrusion or laminates of several kinds of plastics, each providing a specific benefit. Recent advancements in controlling the chain length of plastic polymers have resulted in high OTR films with superior strength, good clarity and rapid sealing. Rapid sealing is extremely important for high volume form-fill-seal packaging equipment. Bags are usually checked periodically on the processing line for seal integrity (in a water filled pressurized chamber) or “leakers”. There can be considerable variability in O2 concentrations in well-sealed salad bags, perhaps due to slight variations in film permeability during the manufacturing process.
Other packaging options include rigid impermeable trays covered with a permeable film or membrane patch. Micro-perforated films provide very small holes (40 to 200 μm) and allow elevated levels of O2 in combination with intermediate CO2 With temperature fluctuations, the permeability of most common films changes very little in comparison to the dramatic increases in respiration rates (oxygen demand) at warmer temperatures. With lack of oxygen, anaerobic metabolism occurs resulting in off-odors and other quality problems. Anti-fog films capable of dispersing water droplets to avoid condensation, incorporation of antimicrobials in films, and use of time-temperature indicators on or incorporated into plastic films are also relevant.
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