Determination of ripeness and harvesting of apples

Determination of maturity

Determining the ripeness and harvesting of apples is an important agrotechnical operation by which we harvest the fruits of the whole year’s effort and work in the fall. But there is one big problem in harvesting, and that is that in Croatian orchards, on average, 10 to 25% of the fruits are declassified or destroyed, i.e. they go from a higher to a lower class due to improper and poor quality harvesting. This is a big problem that needs to be solved.

Apple fruits have two stages of maturity that are equally important for determining the time of harvest. These are physiological maturity and technological maturity. At physiological maturity, the fruits reach their largest size, and the seeds are fully developed and can germinate under favorable conditions. When the fruits achieve the best taste, smell and color of the skin and all other characteristics necessary for further use (processing, storage in a warehouse or cold storage), we can speak of technological maturity.

Fruits of early varieties are harvested 6 to 7 days before technological maturity, autumn varieties 7 to 14 days before technological maturity, and winter varieties are harvested at physiological maturity.

After harvesting, apple fruits are placed directly on the market or stored for later sale. When storing products, it is important to keep records of the stock of goods in the warehouse, the entry and exit of goods, as well as the final financial result of the production of the apple itself.

In order to correctly determine the ripeness and determine the date of apple harvest, it is necessary to know that apple fruits contain a lot of starch, a reserve substance that breaks down and turns into glucose. There are several tests or methods used to determine the date of the beginning of the harvest of apple fruits, but none of them is completely certain. The methods can be grouped into old and new modern methods.

Old methods include the method of the intensity of the basic color of the fruit – changing the basic color to yellow, yellow orange or red; seed color intensity method – the color of the seed in the fruit usually changes at the time of physiological maturity, and the white seed takes on a brown, dark, then black color. This is an uncertain indicator for determining the harvest date, because with summer varieties the seed is still immature while the fruit is already ripe, and with winter varieties it is the other way around. The method of changing the color of the meat – also an unsafe method by which the chlorophyll in the meat should be completely broken down so that the meat acquires a color without the green pigment as it ripens. And at the end of the method when the fruits start to fall from the tree – the method is unsafe because the fruits can also fall from the tree due to damage from pests or diseases.

The new, modern methods that are more reliable indicators of the optimal harvest time include the method of counting the days from full flowering to ripening – it is done for each variety separately, it is not completely safe because it depends on climatic conditions. For the variety Jonathan and its mutants, it is about 140 to 145 days, for Red Delicious and its mutants 145 to 155 days, and for Golden Delicious 150 to 155 days. The next method is to add up the mean daily temperatures from full bloom to harvest. For the Jonathan variety, this sum is 2440 °C. The following is a method of measuring the firmness of the flesh of the fruit. It is measured with a special device called a penetrometer and various chemical methods that determine the amount of sugar, acids, starch, etc.

Among the more reliable methods used today to determine the ripeness of apple fruits is the iodine-starch test or J-J-K (“iodine-iodine-potassium”) test. This test measures the amount of starch in the apple fruit with the aim of determining the right moment when the ratio of sugar and starch is the most favorable. The procedure for performing the iodine-starch test consists of taking a good average sample of 5 to 10 apples. The fruits are cut crosswise and dipped in a solution of iodine tincture (iodine-iodine-potassium solution). Iodine tincture will turn the starch dark blue to black. Iodine tincture can be obtained in pharmacies, and it is prepared in one of three ways:

– as a solution of 6.3 g of iodine and 10 g of potassium iodide in 1 liter of distilled water (put 10 g of potassium iodide in 200 ml of distilled water and mix well, then add 6.3 g of iodine and mix again. Finally, add distilled water until 1000 ml i.e. about 780 ml)

or

– as a solution of 6.3 g of iodine and 10 g of potassium iodide in 0.5 g of distilled water (put 10 g of potassium iodide in 200 ml of distilled water and mix well, then add 6.3 g of iodine and mix again. At the end, add distilled water up to 500 ml, i.e. about 280 ml. In a more concentrated solution, the reaction will be faster.

After a few minutes of reaction of the solution with starch, an assessment is made on the ripeness of the fruit based on the mottling of the sample. The breakdown of starch starts from the middle of the fruit towards the skin of the fruit, and it breaks down the slowest 2 to 3 mm below the skin itself and near the conducting bundles that go from the stalk. If the entire fruit is colored black and blue across the entire cross section, it means that the fruit is still green and still contains a lot of starch, so you should wait to harvest. When the fruit is almost completely unstained by the reagent, it means that the fruit has reached technological, consumption maturity, and the optimal time for storage has passed. In the event that light appears on the dark base of the entire cross-section of the fruit, and the parenchyma of the fruit is colored dark blue only under the skin, it means that the fruit contains both sugar and starch and as such is the best for harvesting and storage.

To determine the optimal harvest period, the so-called The Streif Index (SI) named after its author Dr. Josef Streif of the Bavendorf Research Station, University of Hohenheim in Germany. According to the Streif index, indicators of fruit maturity and quality are:

• fruit firmness in kg/cm2, measured with a penetrometer

• sugar content or more precisely soluble solids, refractometric value in Brix degrees, measured with a refractometer
• starch index according to the Ctifl scale (Centre tehnique interprofessionnel des fruits et legumes), which is determined by immersing half of a horizontal section of the fruit in a 1% solution of potassium iodide.

The formula for calculating the Streif index reads: Firmness/(JJK × starch index)

Manual picking of apples

The apple fruit is picked in such a way that it is covered with the whole hand, and the index finger is placed under the connection point of the stem of the fruit with the branch. The fruit is then slightly twisted and separated. The fruit must be picked with the stalk!

There are several ways in which we can pick apples in good quality, and we must use certain tools. One of the aids is a bag for picking, which we call a kangaroo bag, which holds about 8 kg of apples, and is placed on both shoulders of the picker so that the picker has both hands free for picking. In addition to the bag, it is also possible to use a crate with a movable bottom that stands on a box pallet or on a stand, i.e. a cart for such a crate. It is important that these aids enable us to make the path from the apple on the tree to the apple in the box pallet as short as possible. In a properly prepared plantation, the time to harvest one apple fruit is about 3 seconds. In addition to those two aids, carts can be used, with which the pickers can pull the box pallets themselves. A simpler way of harvesting for pickers is when carts are connected or a platform is used that can carry up to several box pallets, and the platform is self-propelled or pulled by a tractor.

Semi-automated apple picking

And, of course, the most efficient and high-quality method of harvesting is harvesting using special apple picking machines, the so-called self-propelled platforms. Such a semi-automated machine has belts that allow 6 pickers to pick simultaneously at all levels from the lower part of the canopy to the highest part of the canopy. In this way, the harvesting speed of 250 to 300 kg of apple fruits per hour is enabled, i.e. such a machine can have a harvesting capacity of up to 15 tons per day. This type of machine is definitely a necessity if you want to be competitive with fruit growers from other advanced European fruit growing countries.