Apr 16, 2015Cool asparagus as quickly as possible once harvested
Randy Beaudry has respect for the makeup of what he believes is the most perishable of vegetables – asparagus.
“Asparagus is kind of an amazing vegetable,” said the professor in Michigan State University’s Department of Horticulture, who specializes in researching the quality of harvested plant products and pursuing maintenance measures targeted to improve them.
“(Asparagus) grows as fast as any vegetable grown as far as I’m aware. It has one of the highest metabolic rates of any vegetable grown on the planet, and changes more rapidly than any other vegetable I know of,” Beaudry said. “Asparagus is one of the most perishable vegetables on the planet.
“As a result, we have to try to maintain its condition with all of the tools we have at our disposal,” Beaudry said during a March presentation at the Oceana Asparagus Day in New Era, Michigan.
“It’s kind of amazing in terms if its activity, but it also is problematic because it’s running at such a rate you have to shut that down if you want to store it for any length of time. That starts with you guys, the crop and the season. Proper storage begins with understanding what is going on in the field.”
He said optimal conditions impacting postharvest handling and storage of asparagus include proper precooling methods, sanitation protocols, storage temperature, oxygen levels and carbon dioxide levels.
“Probably the number one tool for asparagus storage is to reduce the temperature of the product once it’s harvested,” he said. “We use low temperature storage. Removal of the field heat is the best tool to try to maintain the quality of that product.”
Beaudry said the “biggest error” that is made by growers is to “try and delay the movement of that asparagus to a place where it can be cooled. There are a number of different inputs that happen in terms of heat, or thermal inputs – some from the sun – which causes an increase of heat, so (it’s harmful) if it’s left out in the sun before it’s cooled down.”
Just a “simple delay in the harvest” can cause quality problems, he said.
“So when it’s harvested in the field and not moved, even if maintained in the shade, whatever the ambient temperatures are is not good enough.
“Removal of that field heat as rapidly as possible is probably our number one goal, and that delay in that cooling is probably what we would look at as the primary mistake that would be made.”
The effect of warmer temperatures on harvested asparagus also impacts nutrition and taste, Beaudry said.
“A number of issues are problematic here. (One is) loss of sugar – it’s a very highly metabolic product. It quickly uses up the sugar, so whatever we have in there for flavor is diminished.”
Ascorbic acid is a naturally occurring organic compound with antioxidant properties that is found in asparagus.
“Vitamin C levels are actually quite high in asparagus,” Beaudry said. “As they diminish pretty quickly in a number of days, we have probably a 50 percent loss, and that’s much more rapid under a high temperature regimen as well. So we have loss in the nutrient quality and loss of the flavor in the product.”
Beaudry said the asparagus spear “puts all crops to shame with its imperative need to grow. Similar to what bacteria do, it’s single minded. It pushes the stem above ground and generates seed and a good crop.”
He said asparagus stores long chains of fructose (fructans), which are degraded to sugars during the spring to support shoot growth. Sugar content in the crown declines through June.
When the spear is harvested, the supply of nutrients that enable the spear development is cut off. After harvest, the spear is dependent solely on what is present within itself.
Beaudry said the tip of the spear is where “much of the action is – new cell generation.”
“There is a decline in sugars from the base to the spear tip,” he said. “Sugars are depleted in the tip of the spear as they are consumed to support cell metabolism.”
With an increase in nitrogen (protein) from the base of the spear tip, Beaudry noted proteins accumulate in the spear tip as new cells are formed.
Once harvested, Beaudry said, preserving the quality of the spear depends on: temperature control, sanitation, humidity control and storage duration.
He said the effect of temperature on respiration and sugar loss can be dramatic.
“Asparagus metabolism changes with time,” he said. “The rate of respiration declines markedly as the sugars are used up.”
Sugar loss is greatest at the base of the spear, although the rate of respiration is similar. This indicates there is sugar transport from the base to the spear to the tip, and sugar conversion to dry matter, primarily in the lower sections of the spear.
“Asparagus is very sensitive to temperature,” he said. “There can be a 10-fold difference in the rate of metabolism. Sugars don’t stay in this plant like they do with something like an apple tree. Molecules are broken down rapidly to support all of the metabolism going into the spear.
“No new fructans are coming in after the time the first spear emerges. They’re being depleted over time.”
Temperatures also impact the fiber and toughness in asparagus.
“Asparagus develops lignified fibers that make it tough to eat and reduce useable spear length,” Beaudry said. “Lignification is affected by the postharvest temperature – the rate can be reduced by refrigeration.
“Temperature control is by far the best tool to control and maintain quality,” Beaudry said. “Storability declines as temperature increases.”
He said studies show asparagus held for one day at 70˚ F can result in losing eight days of storability at 32˚ F.
“Every hour spent out of refrigeration reduces storability,” he said. “Two to three hours (outside of refrigeration) equals one day lost.”
Methods growers should use to control temperature should include: Providing shade on sunny days if there is a delay in cooling; reduce the time the product is held at an elevated temperature; hydrocool as quickly as possible; hold in refrigerated storage at 32˚ F to 36˚ F; and recognize the potential for chilling injury can occur if the product is held for two weeks at 32˚ F.
Beaudry said it’s also important to sanitize hydrocooler water with chlorine or chlorine dioxide. Maintaining the highest possible humidity levels also is advised.