Uncovered forklift openings and through the “hot” fruit where it warms (purple arrows). It‘s then drawn through the slots in the plywood (red arrows) and upward towards the evaporator coils to be re-cooled. Photo by courtesy of Hugh Fraser, OMAFRA
The key to keeping fruits and vegetables alive after they’re harvested is lowering the temperature of the produce and forced-air cooling systems.
“A lower produce temperature reduces ethylene production, damage from micro-organisms, moisture losses and bruising injury,” says Hugh Fraser, extension agricultural engineer with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) in Vineland.
Three kinds of forced-air cooling systems are available, but the one he likes best for cooling produce packed into bins that have slots in the bottom is the serpentine cooling system. Although this is a simple cooling system, it can be difficult to explain in words, he said.
Specially sized, high capacity fans inside the cold storage unit pull refrigerated air through a specially designed duct that a column of bins in multiples of two are placed against; usually four or six bins high. The duct has slots lined up perfectly with every-other forklift opening of the bin column [bin number two, four and six in the case of a column six bins high]. With the fans operating, the forklift openings of these same bins are covered with a tarp so refrigerated air cannot enter that forklift opening across from the plenum slots.
Because the duct is under suction, these tarps are sucked tightly against these forklifts openings. Refrigerated air is then compelled to enter through the remaining three forklift openings and travel up bins one, three and five, or down bins two, four and six, through the produce via the slots in the bins’ floor. Hence, the name serpentine forced-air cooling.
For each column of bins, atop the plenum wall is one centrifugal squirrel cage fan that draws a minimum of one cubic foot per minute per pound of produce (CFM/lb) at 10 mm of static pressure.
Multiple stacks should be packed tightly together with slots/holes covered on the outer most columns to ensure there is little to no short-circuiting of refrigerated air.
“The outer bin sides can be tarped, but in reality, you don’t get a lot of short circuiting through there and there is little difference in cooling,” Fraser said.
One advantage of this system is that it takes the least amount of floor area per kilo of fruit. The second advantage is that the cooling air travels through the depth of the bin and not the width, which puts less load on the fan and speeds cooling.
“This is very efficient and is the best option for cooling bulk produce before packing,” he said.
The disadvantages are it works only with bins that have a slotted floor and any side vents should be blocked.
“There can also be some short circuiting of air through the top bins. And because the cross-section of the fork lift openings are so small … you have a relatively small area through which to pull air. This restricts how many bins.”
Fraser said going two or three stacks deep would work just fine with correctly sized fans.
So how does it compare to the room cooling method?
An Ontario producer built a serpentine cooling system that Fraser used in a test with pears. In this test, one stack of six plastic bins were cooled the serpentine way and compared with room cooling in three plastic bins sitting in an open area of the cooler surrounded by cold, blowing refrigerated air.
Researchers simultaneously monitored the internal temperatures of pears in both the serpentine forced air-cooling and room cooling systems.
They found the serpentine system cooled the pears five times faster and reached a 7/8 cooling time of 3.6 hours versus 18 hours for the room cooling system. “We were actually very ‘kind’ to the room-cooled pears because if these were packed into a cold storage the way they normally would be, it could be a lot longer than 18 hours. I’ve done tests like this with apples and it can be a day and a half before they all get cooled,” he said.
Fraser identified the six components needed to make a forced-air serpentine system.
Need one squirrel-cage centrifugal fan per column of bins. The fan should be able to draw one cubic foot of air per minute (CFM) per pound of fruit at 10 mm (0.4 inches) of static pressure. This translates into one 3,000 CFM fan for six stacked bins of fruit holding about 500 pounds.
“The good news is that every piece of fruit in these bins is at virtually the same temperature when you are done,” he said.
Be wary of any undue restrictions on the fan due to holes that are too small in the bin, or cross-section of forklift opening. You want about two square feet of opening for every 1,000 CFM of airflow.
“So with a 3,000 CFM fan you would like about six square feet of cross section so you are not choking the fan and putting an undue load on it,” he said.
Most plastic bins used by Ontario tender fruit producers are ideal for serpentine cooling because they have straight side walls, they fit like LEGO™ blocks and have long, slotted floor vents. A full 25 per cent of the area perpendicular to the airflow is open.
Short circuit prevention
“If you want to do this right, you have to make sure the air flows through the produce so you have to make sure there are no spots where the air is going to sneak through,” Fraser said.
Put bins together nice and tight, use bumper pads to fill gaps and cover forklift openings correctly. Use foam on the wall for good seal. Use a static pressure gauge to measure if the system is tight.
Ensure you have enough refrigeration capacity to pull the heat out quickly. The hot air pulled from the fruit is directed toward the evaporator coils where it is cooled, then blown back into the cold storage where it is picked up again by the serpentine system.
Monitoring is simple and tells you how effective your system is. In one particular case, the cold air going into the bins measured at 36 F, while at the fan, the hot air coming out was about 51 F.
“That hot air is about half way between the cold air going in and what the fruit temperature actually is. In this particular case, the fruit averaged about 64 degrees which is close to half way,” he said.
By monitoring the temperature of the cold air going in and the hot air coming out, you can predict when the thermostatically controlled fan should be turned off.
“I can’t emphasize enough that virtually every piece of fruit in that bin [should be] about the same temperature when you are done,” he said.