HORT 413 Tree Fruit Production
Fruit maturation
There is a clear distinction between:
Fruit maturity:
Fruit ripeness:
How to determine the correct time of harvest &endash; depends on storage duration. There are a number of practical things to look at.
Days after full bloom (DAFB)
This is useful as a general reference for growers but is not very precise since there may be up to a 20-day spread in maturity from an average year to an early or late year. However this will give an indication of the approximate harvest date. The following table lists the approximate number of days from bloom to harvest of some common apple varieties in the midwest.
Anticipated days from bloom to harvest for a number of important apple cultivars.
|
Cultivar |
Days from Bloom to harvest |
|
Yellow Transparent |
70-90 |
|
Lodi, Pristine |
75-95 |
|
Mollies Delicious, Red Free |
120-125 |
|
McIntosh, Cortland, Gala |
125-145 |
|
Jonathan, Liberty, Grimes Golden, Empire, Red Delicious, Golden Delicious, Jonafree |
140-150 |
|
Mutsu |
145-170 |
|
York Imperial |
155-175 |
|
Rome, Winesap, Stayman |
160-175 |
|
Braeburn, Fuji, Goldrush, Enterprise, Granny Smith |
180-210 |
Starch content
Starch accumulates in the apple during fruit growth and is hydrolyzed to sugar as fruits mature and ripen. One of the changes that affects the prime eating time of the fruit is the conversion of starch to sugar. Try eating an apple a month before you'd normally harvest it and you'll sure know what starch tastes like. An iodine solution will turn starch black but is not affected by sugar, therefore this can be used to determine where the fruit is in the process of starch conversion to sugar. As the fruit mature and ripen, starch is converted to sugar so the black staining disappears. If you are harvesting fruit for immediate sale or very short term storage, there should only be a small amount of black staining (starch) present in the fruit. If the fruit are intended for storage, then harvest when there is a little more starch remaining in the fruit.
Which fruit are more mature, those with a rating of 0 or those with a rating of 6? Why?
Skin color
As fruit ripen, the green background color changes to a lighter green or even yellow. This can be a very useful harvest indicator, especially for varieties that require multiple pickings. A good example is Gala (see the photo below). The amount of red coloration is not a good indicator of fruit maturity. However as the green lightens, this tends to make the red color change from a dull red to a brighter shade of red.
Fruit firmness
Fruit become softer as they ripen, but many other factors can also cause changes in firmness. This is easily measured using a penetrometer available at most orchard supply companies.
Soluble solids (or sugar levels, brix)
As starch is converted to sugar during ripening, obviously the sugar level increases. Generally fruit with higher sugar taste better, but growing conditions can also affect sugar concentration in the fruit, so by itself this is not a good indicator of maturity. A refractometer is used to measure sugar concentration &endash; again, these are available from most orchard supply companies.
Seed color
Seeds turn from white to brown/black as fruit mature, so this can be a useful indicator to get you in the right ballpark.
Summary
For Indiana growers who are direct marketing and not planning to store the fruit for very long, use the low-tech approach: calendar date to get in the right ballpark and then taste of the fruit.
What affects fruit quality after storage?
Pre-storage conditions
1. Variety
2. Rootstock
3. Soil
4. Nutrition, esp. Ca
5. Climate
6. Fruit size
7. Diseases
8. Maturity at harvest
9. Delay between harvest and cooling fruit
10. Post-harvest treatments such as wraps, dips, coatings
Conditions within the store
Post-storage conditions
Concept of shelf life
After harvest, the best we can do is to maintain fruit quality.
Respiration
Uses O2, sugar
Generates CO2, water, heat
Aim to reduce respiration
Generally, the lower the temperature, the lower the respiration rate, but care must be taken to avoid cold injury or freezing damage to the fruit.
Controlled atmosphere (CA) storage
Air CA Nitrogen 79% 92% Oxygen 20% 3% CO2 0.03% 5%
Controlled atmosphere storage:
1. is expensive
2. slows respiration to a low rate
3. can maintain fruit quality for long periods
4. requires careful planning
Achieving CA conditions
1. fruit respiration to lower O2
2. flushing with N2
3. catalytic burners
4. ammonia crackers NH4 ---> N2 + H2O
5. membranes
6. molecular sieves
Control of CO2, O2 and C2H4 is important
Modified Atmosphere (MA) storage
Fruit respiration used to create or modify the storage atmosphere
Decreaed O2, increased CO2, increased water
Inexpensive but not very precise
The bottom line:
For most growers in Indiana starting out, probably all that isneeded is a small cool storage facility to provide some flexibility with marketing. Harvest fruit when it is almost fully ripe.