Keys to Fresher Produce
By Suanne J. Klahorst, Contributing Editor
Ethylene Scene
Venting
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Getting Fresh
If you've ever walked up to a peach tree on a hot summer day, plucked a ripe fruit off the branch, and tasted that sweet, juicy nectar, you know without a doubt that fruits ripen best on the tree. Remember what happened when you put a few of those deliciously ripe peaches into a sack? Then into a hot car in which they were jostled a bit along the way? You soon understood how rare and temporal the perfect peach could be.
Very few peaches are harvested at their peak of ripeness, and we've come to accept that certain compromises are necessary to reduce the costs of harvesting, shipping and handling delicate, ripe fruit. USDA's Agricultural Research Service (ARS), Kearneysville, WV, recently reported that it now costs Florida citrus processors as much to harvest by hand as it does to grow the fruit.
Growers prefer mechanical harvesting to hand-picking, even though it requires that fruit be harvested in an immature state. Another limitation is the need to spread the harvest over a longer period of time. Fruits can't always be harvested at their exact peak of readiness, nor are they consumed or processed immediately. For climacteric fruits - those that produce ethylene - harvesting before ripening provides more of an advantage than a disadvantage. Climacteric fruits include apple, apricot, banana, avocado, peach, plum, pear, nectarine, kiwi fruit, mango and papaya. Fruits that synthesize ethylene will respond to ethylene treatment during the ripening process. Controlling ethylene is one of the several ways that growers - and their customers - can take charge of the ripening process.
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Ethylene is a gas molecule consisting of two carbon and four hydrogen atoms (C2H4). The biochemical pathway for ethylene production starts with the amino acid methionine, which is converted to S-adenosyl methionine. An enzyme, ACC synthase, converts S-adenosyl methionine to 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene. A final enzyme, ACC oxidase, converts ACC to ethylene. After locating the genes coding for the enzymes, researchers invented another method for controlling ripening; this was demonstrated in tomatoes. When the gene coding for ACC synthase was deleted, the tomatoes didn't ripen until exposed to an external ethylene gas source.
Ethylene's role in the ripening process is no longer a mystery, either. In simple terms, the ethylene binds to a receptor site on the plant cell membrane and causes a chemical message to be transferred to the cell nucleus. The DNA in the nucleus begins the process of creating messenger RNA that eventually results in the synthesis of the enzymes required to bring about ripening. Several known methods prevent ethylene from binding to the receptor site, the simplest of which is to modify the atmosphere. Decreasing the percent oxygen in the surrounding atmosphere to below 8%, or increasing the carbon dioxide concentration above 1%, diminishes ethylene's effect. This is one of the key concepts used in modified-atmosphere (MAP) storage and packaging.
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After harvest, fruits and vegetables that need to be stored for any length of time before processing and distribution are chilled as soon as it is practical. This slows down respiration. Even at reduced temperatures, oxygen and carbon dioxide levels don't remain constant when fruits and vegetables are stored in a closed system. Even slow respiration causes an increase in carbon dioxide and a decrease in oxygen, so the gas levels need to be controlled to preserve quality. Venting and forced air circulation keep oxygen and carbon dioxide at acceptable levels during storage, not only for the fruit but also for the people who are in and out of the facilities throughout the day. Levels up to 5,000 ppm carbon dioxide are recommended to prevent ethylene ripening. (Normal atmosphere carbon dioxide is only 300 ppm.) OSHA sets regulations on human exposure to carbon dioxide. Specially designed facilities keep gas composition, humidity and temperature at the optimum levels for even ripening of valuable fruits. These facilities can degreen citrus fruits and soften melons as well, although these changes have less to do with flavor than total ripening.
In true ripening, the fruit becomes sweeter as well. Ethylene also can assist in the ripening of certain vegetables, such as chili peppers, which develop color during exposure to ethylene. Most vegetables, however, lose quality in response to ethylene. It leads to color loss in green snap beans and lettuce; yellowing in broccoli, cauliflower and cabbage; bitter flavor in carrots and parsnips; and sprouting in potatoes. The amount of ethylene that ripens fruits and spoils vegetables varies, with exposures as low as 1 ppm responsible for softening ethylene-sensitive fruits such as kiwi or avocado, or causing cucumbers to yellow.
Since ripening fruit produces ethylene during storage, ethylene absorbents and ventilation keep ethylene levels low during cold storage. Just before the fruit is shipped or processed, the temperature is increased and ethylene gas is circulated to ripen the fruit. Each type of fruit has an optimum ripening temperature and ethylene exposure time, which can range from 31° to 58°F and 12 to 72 hours. After ripening, the fruit is chilled again and can only be held for a few days before the quality begins deteriorating.
The skilled produce shopper has several methods for determining the ripeness of a fresh product - bending, squeezing, tasting, thumping, sniffing and asking strangers. Growers and processors use methodologies with less subjective parameters. Some of the physical characteristics of ripening that have been quantified in the laboratory are total-solids concentration, soluble-solids concentration, titratable acidity, various ratios of these three, and firmness determined by several types of instrumentation. Food processors who purchase and process fresh produce will benefit from being aware of the testing methods available to quantify ripening in the fruit or vegetable of interest, and will monitor quality accordingly.
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One of the fastest-growing segments among America's food processors is represented by the International Fresh-cut Produce Association (IFPA), Alexandria, VA. Sales of fresh-cuts were $6 billion to $8 billion in 1996, comprising about 10% of all produce sales, according to IFPA. Packaged salads alone topped $1.5 billion in 1997. This also is the industry segment most likely to lead the food-processing industry in implementing post-harvest technologies.
IFPA President Edith Garrett outlines the three principal means of ensuring quality in fresh-cut products: fresh, quality raw material; refrigeration; and packaging. The principles are similar to post-harvest preservation methods for intact fruits and vegetables, except that once these products are cut, the importance of these control points becomes even more acute. Not only must the grower harvest at the proper time, but the variety of the crops planted for the fresh-cut industry also is important, because of differences in susceptibility to browning and color loss after cutting. Packaging must function similarly to controlled-atmosphere storage, but with portability, protection and convenience. Fresh-cut products are packaged at the peak of ripeness, and the refrigerated temperatures in the distribution chain are critical to minimize the biochemical processes that lead to spoilage. An acceptable temperature range for fresh-cut products is lower than storage of the uncut raw material, typically 33° to 41°F. The colder the better, provided freezing can be prevented.
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Like controlled-atmosphere storage, MAP allows the package contents to respire while protecting against moisture loss. Permeability of the films is expressed as the oxygen transmission rate (OTM). "Within 24 to 48 hours, depending on the temperature and packaging material, fresh-cut produce will take in oxygen and give off carbon dioxide at a given rate," Garrett says. "Through the use of permeable films, oxygen is able to diffuse into the bag while carbon dioxide diffuses out, establishing a gaseous equilibrium that favors preservation of the product.
"Packaging alone cannot extend shelf life," she stresses. "One focus area of the IFPA is improving cold-storage controls. The foodservice operator, the food processor, and the retail grocer must be able to maintain the proper temperatures to get the maximum quality from fresh-cut products."
Other techniques hold promise. "Flushing with pure nitrogen is a strategy that eliminates oxygen immediately after packaging, which offers an immediate advantage over waiting two days for plant respiration to lower the level," says Alley Watada, Ph.D., post harvest physiologist, ARS, Beltsville, MD. Nitrogen flushing reduces the oxygen level from 21% in the normal atmosphere to less than 4% in the package. This is especially important for products that brown in the presence of oxygen and require a long shelf life, such as mixed greens.
A film with a permeability of 400 OTM can be used for 200 grams of honeydew packed in 5.75 in. by 7.75 in. trays when held at 41°F. However, if the temperature increases to 59°F, respiration rates would triple, and the 400 OTM film wouldn't transmit a sufficient amount of oxygen into the container to be effective.
New types of "smart" packaging films are composed of molecules that "sense" the temperature and allow for increased diffusion of gases as temperature increases. Smart packaging is not "one size fits all," however, and must be customized for the amount of product, the temperature range and the respiration rate of the individual products they protect. How low can the oxygen level get before the product is injured? "Again, that depends on the product," Watada says. "Honeydew suffers at oxygen levels below 4% while at 50°F, while 0.7% is acceptable for carrots. In general, the integrity of the plant tissue is maintained better when the oxygen level is lowered to the minimum acceptable level." Lack of oxygen is a concern for anaerobic microbial growth, such as Clostridium. However, good sanitation and good manufacturing practices control the introduction of these organisms. IFPA members also subscribe to Hazard Analysis Critical Control Point (HAACP).
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The convenience of pre-washed and pre-cut produce for the consumer is evident from the expansion in retail grocery space now devoted to these products. But what about the foodservice and food-processing industries? Can they benefit from outsourcing fresh produce preparation? "Fresh-cut is a preferred alternative to canned, frozen or dehydrated produce for these key attributes: flavor, texture, color, nutrition and convenience," Garrett explains.
Many fruits and vegetables have characteristics that suit the fresh-cut distribution chain. If you've ever peeled an onion, you would easily agree that buying them already peeled offers advantages. Some produce poses a more formidable challenge to the fresh-cut industry; two examples are the all-American favorites: apples and potatoes.
Cut an apple or a potato and see how long it takes before it turns brown. You won't have to wait long. No one knows this better than Peter Karas, president, E-Z Spuds, Evanston, IL. For the past 23 years, he's been providing peeled and cut potatoes to foodservice establishments in the Chicago metropolitan area.
"The last McDonald's to use fresh potatoes for their french fries switched to frozen about 1970," Karas estimates. That was the first blow to the fresh-cut potato processors. The second was the FDA ban on the use of sulfites for preserving fresh-cut fruits and vegetables. "There is really no substitute for sulfite dips in terms of final fresh-cut potato quality," Karas says. "The alternative is a potato treated with a mixture of citric acid, ascorbic acid and sodium acid pyrophosphate. These potatoes can sometimes taste acidic, and the outside of the potato can develop a chemical burn over time, resulting in a rubbery texture that will not soften during cooking."
Karas saw several companies abandon the fresh-cut potato business during the sulfite ban. Meanwhile, the Fresh Potato Processors Coalition filed suit and the FDA reconsidered the sulfite ban on potatoes, since they're not eaten raw and the sulfites dissipate during cooking. Potatoes became the only fresh-cut product exempt from the ban on sulfites, providing they carry the proper labeling: "sulfites are added to retard oxidation." The potato coalition established a self-imposed maximum of 200 ppm residual sulfites.
Independent lab studies sponsored by Karas recently indicated that rinsing fresh-cut potatoes with water for 30 seconds can reduce sulfites below detectable levels. Another product rarely seen in the retail market is fresh-cut apples. Like potatoes, frozen and dehydrated alternatives are available, but millions of pounds of fresh apples are now being used in a variety of processed foods.
Scott Summers, director, quality and technical services, Treetop, Inc., Ingredient Division, Selah, WA, attended the "Pie Production" seminar at the American Institute of Baking, Manhattan, KS, December, 1997. He reports that among the attendees, there were equal numbers who preferred the taste of pies made with frozen apples as there were those who preferred fresh apple pie. All processors prefer large chunks of apples with good physical integrity. Using fresh apples eliminates the need for frozen storage, tempering and thawing prior to use.
In the Northeast and Central United States, growers provide apple varieties such as the Jonathan, the Jonagold (Jonathan/Golden Delicious), the Northern Spy or the Ida Red. In Treetop territory on the West Coast, the preferred pie apple is the Granny Smith. "A good pie apple is palatable, yet firm," Summers says. "Pressure is used to assess the conversion of starch to sugar in the apple, an indication of ripeness. The higher the acid content of the fruit, the greater the inhibition of polyphenol oxidase in the fresh-cut fruit, and the less browning occurs."
How do you prevent browning in a fresh-cut apple with a shelf life of five to 14 days? An effective system according to Summers is "combinations of citric acid, erythorbic acid, sodium erythorbate, sodium chloride and calcium chloride. Ascorbic acid can be used to increase the vitamin C content, but it is not as effective for prevention of browning as a combination of acidulants." Other solutions being investigated are edible coatings and "secret" formulas for surface treatments that don't require labeling.
Fresh-cut offers some valuable choices with improved flavor, texture, color and nutrition. New, as well as established, technologies in post-harvest preservation are creating distribution strategies that look very favorable for the use of more fresh ingredients. An assessment of refrigeration controls and shelf-life management is a necessity for foodservice or food-processing customers considering the use of these products. For food technologists and product development professionals who need more detailed information on handling, cold storage and other aspects of fresh products, IFPA, individual produce suppliers, universities, short courses, and ARS represent a few of the many resources available.
Suanne Klahorst is a free-lance food-science writer and food scientist with eight years of experience in the industrial enzyme industry. She is currently with the California Institute of Food and Agricultural Research at the University of California, Davis.
• Photo: Agricultural Research Service, USDA
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