Post Harvest Berry Handling
In this video, learn how to properly handle strawberries, blueberries, blackberries and raspberries during and after harvest in order to maintain the highest levels of quality, food safety and value.
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- [Voiceover] Proper Post-harvest Handling of Berries.
Berries are wonderful fruits.
They're delicious, nutritious, and they bring a good price at the market.
Nevertheless, berries have a short shelf life after they've been harvested.
If they aren't properly handled during and after the harvest, they lose value, nutritional value, as well as monetary value.
In this video, I will review proper post-harvest handling of berries, and provide recommendations on how to maintain the quality of raspberries, strawberries, and other berries after they are picked.
Harvested products are living systems that age over time.
Your goal is to slow down the aging process, but you cannot stop it completely.
We're going to discuss how to delay the aging after harvest, and some other special considerations.
Strawberries, raspberries, black raspberries, blackberries, and blueberries, are picked fully ripe for best appearance and eating quality.
Because they are so delicate and easily damaged, they're usually picked directly into final containers to minimize handling, so grading and sorting of damaged or decaying fruit happens as the crop is picked.
Relatively high respiration and transpiration rates as well as susceptibility to molds, mean berries should be cooled rapidly following harvest.
Even when they are handled under optimal harvesting temperature and humidity conditions, their shelf life is still short compared to most fruits and vegetables.
Finally, even though we all know that consumers should be washing the berries before consuming them, we also know that this doesn't always happen.
That makes farm food safety procedures to minimize the risk of spreading foodborne pathogens especially critical.
Scientists have identified seven general processes or factors that play a role in maintaining quality and shelf life in harvested fruits and vegetables.
These include: respiration, transpiration, physical damage, decaying and infestation by insects, color changes, ethylene production or sensitivity, and chilling injury.
In the case of berries, the most important are the first four, respiration, transpiration, physical damage, and decay and insect infestation, and to a lesser degree, the fifth, color changes.
Next, I will review each one of these in greater detail.
The first process is respiration.
Respiration occurs in each and every living thing, in us, in apples, in mosquitoes, and so on.
Within the plant cell, sugar molecules are broken down and combine with oxygen, and this chemical reaction releases carbon dioxide, water, and energy.
This energy is used to keep the plant part alive, but some of the energy is in the form of heat, so respiration creates heat.
Fruit and vegetable products have different rates of respiration.
This table shows the different rates of berries at three different temperatures.
Blueberries, strawberries, and then blackberries and raspberries.
Held at 32, 50, and 68 degrees Fahrenheit.
The lower numbers in the table indicate that less heat is being created by respiration, and the higher numbers mean more heat.
For example, let's compare the rates for blueberries.
At a colder temperature, the respiration rate is 1,300 BTUs per ton per day, but at 68 degrees Fahrenheit, the rate is 15,000.
We can also see how these three berries compare at each temperature.
The blackberries and raspberries have the highest rate of all the berries, no matter what temperature they're held at.
At 68 degrees Fahrenheit, it's almost like they're burning up at 39,000.
How can we slow the rate of respiration?
We can cool the fruits quickly in order to lower the respiration rate and preserve the quality.
There are two methods to cool strawberries, blackberries, blueberries, and raspberries.
The first is room cooling, in which you simply put the containers in a refrigerated room.
This method doesn't cool the fruits very rapidly though.
A better method is forced air cooling, in which fruit is placed in a refrigerated room which has fans that move cold air around and through the packages of fruit.
While it's important to cool berries and strawberries, you should never use ice or water to cool berries, never.
It's not difficult to construct a forced air cooling unit.
For example, this is one design by researchers at North Carolina State University.
You can find these and other good examples of small scale forced air coolers on the internet.
The next process that we need to understand is transpiration, and this is pretty simple.
Plants lose water while they grow, and they keep losing water even after they're harvested.
This loss of water causes the fruit to soften and shrivel.
How can we slow transpiration?
We need to keep storage areas humid, we need to lower the temperature, just like for slowing respiration, and also, reduce the air movement over the fruit.
Finally, use protective packaging to hold in humidity and minimize water loss.
Now, do you notice any problems with this list?
Do you see a conflict?
Well we talked about using forced air to cool the berries to lower respiration, but air movement around the fruit increases transpiration, so when using forced air, we need a compromise, an equilibrium, so that we can cool the berries, but not dry them out too much.
The next process or factor is physical damage.
Fruit can be physically damaged in different ways.
For example, by insects or birds, or rain, or hail, or it can be damaged during the harvesting process.
One of the reasons that berries are grown in high tunnels is to protect them from damage.
The photo on the left shows strawberries being grown in the state of Guanajuato in Mexico under very large tunnels.
Berries can also suffer physical damage post-harvest when they are put in containers that are too large, like this picture on the right.
Berries are also susceptible to decomposition and decay, caused by pathogenic organisms, such as botrytis, rhizopus, and anthracnose.
Berries can also be infested by insects, like the spotted wing drosophila.
Disease and insect infestation can make a crop completely unsalable.
Remember, our goal is to slow down the aging process of the berries once they're harvested to maintain high quality and shelf life.
In this next section, I'll outline some recommended practices for different stages in the production process, pre-harvest, at harvest, and post-harvest.
In the pre-harvest stage, we recommend selecting varieties that are known to hold up better after harvest, using high tunnels, netting, and mulches to protect berries in the field, controlling diseases, and insects in the field, especially spotted wing drosophila.
Don't let your berries take a bumpy ride from the field to the pack house.
Protect them from bruising by grading your farm lanes.
Purchase appropriate containers that provide support and protection for the berries, but that also allow air movement.
For food safety reasons, be as vigilant as possible about keeping animals and birds out of production fields.
Here are some examples of using protective netting around berry plants, like here to protect berries from birds.
Netting can help protect the berries from insects, as well as from rain and hail damage.
Again, these are the very large production tunnels used for strawberry production in Mexico.
This is a very effective method to protect strawberries in the pre-harvest stage.
During the harvest, we recommend that you pick berries when it's cool, but dry.
You do not want to be picking berries when they are wet, either from rain or dew.
Pick berries into their final containers.
Don't transfer them from picking baskets into pint or quart containers.
You should train pickers carefully to pick only ripe berries, and to avoid ones that are damaged or decaying.
Do not overfill the containers or the berries could be damaged by compression.
For food safety purposes, it's essential that workers are healthy, with clean hands, and are using clean containers.
In the post-harvest stage, it's important to keep berries shaded while in the field, and also during transport.
You should cool them as soon after picking as you can using the forced air or room cooling methods reviewed earlier.
The optimum temperature for berries is between 32 and 35 degrees Fahrenheit.
The optimum relative humidity is between 90% and 95%.
Once again, do not let the berries come in contact with water or ice.
Once berries have been cooled, avoid letting them warm up again, as this will shorten their shelf life.
Keeping berries cool during transport and at the market is essential.
One method that works well for smaller lots is to pack them in pre-cooled ice chests that have frozen gel packs wrapped in paper on the bottom layer.
For larger quantities, refrigerated trucks are ideal for keeping berries cool during transit.
CoolBots are a great way of keeping berries cool after harvest, whether at the farm or during transport.
A CoolBot is a regular window air conditioning unit that is converted into a refrigeration unit for a small walk-in cooler or structure.
Here's an example of a shed that was converted into a cooler using a CoolBot.
Container size and design are critical for good post-harvest handling of berries.
The containers on the left are too big.
They do not protect the berries well, and the ones on the bottom are getting crushed.
The containers on the right are smaller and are able to protect the fruit better.
Use containers which allow for air movement, but don't pack the container so full that they damage the berries.
This photo shows berries for sale at a supermarket in the state of Jalisco in Mexico, and you can see that they're being displayed carefully in containers that have holes for ventilation.
These plastic containers help protect the berries and also slow down respiration.
The recommendations we have just gone over are general recommendations for all berries, so now I will discuss some of the ways that berries differ in their post-harvest handling needs, and we'll start with strawberries.
I've mentioned before that it is important to cool berries as soon after picking as possible.
Let's look at how delaying refrigeration might affect the shelf life of strawberries.
Research has shown that the shelf life reduces substantially for every two hours that you delay cooling the berries.
If you wait eight hours to refrigerate the berries you've picked, you've reduced the shelf life of your berries by 70%.
That is not what consumers are looking for.
Not only is it important to cool quickly, but once they are cooled, berries need to stay cool.
If you allow strawberries to rewarm after they've been cooled, the berries will darken in color.
Condensation can also occur, leading to increased molding.
You also need to keep the humidity high so the green sepals on those strawberries don't wilt.
The standard recommendation for strawberries is to harvest them from the white tip through to fully ripe, followed by rapid cooling to 32 degrees.
This results in the lowest decay and the longest shelf life, unless of course the berries rewarm.
Of all berries, raspberries have the shortest shelf life.
They have the highest transpiration rates, and can mold in just one single day.
For each hour in delay of cooling, you lose a day in shelf life.
Raspberries can be picked at the light pink stage and will ripen to normal sweetness and color.
Blackberries on the other hand must reach full color before picking.
They're a little less susceptible to decay and water loss.
Blueberries have the longest shelf life of all the berries, but proper picking is still essential.
Blueberries must be picked fully ripe for the best flavor.
A fully ripe blueberry is completely blue without any red tinge at all.
Red tinged berries will turn blue after picking, but they will not reach optimum sweetness or flavor.
Pick them without stems, and protect the berries from compression damage.
In conclusion, in order to maintain good quality, safety, and long shelf life, it's important to understand what is happening with berries after they are harvested.
Berries continue to live and to age after harvest, and they present some unique problems compared to some other fruits and vegetables.
We discussed how to manage respiration, transpiration, physical damage and decay, and insect infestation, as well as the importance of cooling, and cooling quickly.
Finally, we pointed out both the similarities and the differences in proper post-harvest handling of strawberries, raspberries, blackberries, and blueberries.
Frequently Asked Questions