Maskinell skörd av jordgubbar
Mechanical harvesting of strawberries has been at the experimental stage since the early l960's, but has not attained any commercial success until recently. One of the main problems has been that the strawberries ripen successively during a couple of weeks, and that most harvesters are based on the once-over principle. This results in a decrease in yield at the same time as a relatively large quantity of green strawberries is harvested. It has frequently been considered profitable to hand-pick the first large berries and then harvest mechanically about one week later. Another approach frequently used by growers is to hand-pick several times and then harvest the remaining berries by machine. When a late harvesting of this kind is carried out, there are very few berries left, many of which are over-ripe or have not been picked because they are mouldy. In such a situation there are not only too few berries to pay for the machine-costs, but r also the berries are of such poor quality that extensive sorting is needed before they can be used. This type of mechanical harvesting has been found very unprofitable. The early trials are all centered on harvesting in normal row crops. During he l960's, prototype harvesters were developed at several universities in the USA and in Canada. All prototypes worked on the principle of stripping the berries from the plant. Fingers made of plastic or metal, placed at suitable intervals, combed through the plant and removed the strawberries. Tests were made of stripping forwards, backwards, and sideways, etc. Almost all possible approaches have been tested. Similar machines have been constructed also in Holland, Italy, the Soviet Union, and Denmark. The Danish machine is only a couple of years old, and this harvesting method is being tested actively in Denmark. One of the large problems with the stripping strawberry harvesters is their low capacity. Driving speeds are normally only 300-400 metres per hour. In order to be able to drive faster, the fingers which strip off the berries must also move faster, and then the number of berries thrown out and lost increases. Another problem is that the runners easily become entangled in the fingers and are pulled off, thus hindering the harvesting. In the late l960's, harvesters working on the principle of cutting off the entire plant started to appear. This method was developed simultaneously by the National Institute of Agricultural Engineering (NIAE) in England, and by growers and universities in Oregon, USA. The fundamental principle is as follows: strawberries and leaves are located on different stems, and if the plant is cut off close to the soil, the berries and leaves can be separated by blowing away the leaves and other light material. This harvesting method allows a considerable increase in the driving speed. Similar harvesters are being (or have been) developed also in Canada, Italy, and the Soviet Union. Other methods than the two described above have been tested: vibrating the stems of the plants, causing the ripe strawberries to fall off, for example; or lifting the berries with a net laid out on the ground in the spring. Whatever method is used, it has become apparent that growing technique must be modified if harvesting is to be carried out satisfactorily. There is probably no existing field that can be successfully harvested mechanically without modifications. In some cases, harvest-efficiency has been increased from 50 to 90%, simply by removing stones etc. and levelling out the beds. It became apparent that one of the largest problems is the lifting of the berries from the soil so they can be harvested. A large number of the berries lie on the outer edge of the row. Also, if the soil between the rows has been compressed by tractor wheels, this worsens matters. Great efforts have been made to breed varieties with an erect growth habit, but with little success. Various solutions have been tried for lifting the berries from the soil, different types of brushes and reels, for example. Fingers which comb through the plant (similar to the crop-lifter on a combine-harvester) are commonly used on the cutting harvesters. Air has also been used, both as a negative pressure (the vacuum cleaner principle) and by blowing compressed air from the sides of the row. Most of these methods have caused undesirable damage to the berries. Another problem with harvesting strawberries cultivated in rows, is the varying degrees of ripeness due to uneven exposure to sunlight. Berries on the outside of the rows ripen 3-4 days earlier than those in the middle. It is therefore difficult to find the optimum time of harvesting, with the maximum of ripe berries, but without any being over-ripe. A complete system of cultivation, of which the harvester is an integrated part, has been developed by Michigan State University in the USA. In this system, the strawberries are grown in a field without rows, thus covering the entire field. This solves a number of problems encountered in row-cropping. The increased number of plants (the aim is to have 150 established crowns per square metre in a fully-grown population) results in the straw-berries being raised higher from the soil, since the stems support the fruit. Moreover, the ripening is more uniform when all the berries are shaded beneath the foliage. Another important advantage is that the yield, according to American results, is twice as high as that obtained in the conventional row system. The rows cover only 40% of the field as opposed to the 100% coverage with the new system. This also means that the harvester doubles its capacity at the same driving speed and cutting width. Also, in the USA there are fewer problems with diseases as the berries do not lie directly on the soil. The fields are harvested with a machine developed at Michigan State University which cuts off the plant very close to the soil, preferably closer than 2 cm from the ground. To make this possible, it is vital that the field be absolutely level. The harvest is facilitated by crop-lifters which lift up the berries, and a reel which also lifts up the berries in front of the cutter bar, and feeds them into the harvester. Conveyors transport the straw-berries backwards into the machine, where they fall onto a grid conveyor. As they fall, they pass through a powerful current of air which blows away leaves and other light material. The grid conveyor transports the berries over two more fans which blow air from below, and raise the stems while the berries remain on the grid. A double-sickle shortens the stems to 3-4 cm. Bunches of berries are thus separated, giving individual berries with short stems. These fall onto a conveyor which transports them sideways out to storage boxes. Under normal conditions, the harvester has a capacity of approximately 2 tonnes per hour, and can harvest one hectare in approximately 6-8 hours. The harvesting efficiency is frequently higher than 90%. This figure can be compared with manual harvesting, where efficiency is often as low as 75%, ie one fourth of the strawberries are left in the field. Irrespective of whether the harvesting is done by stripping or by cutting, the berries must undergo further treatment before they can be frozen or processed in a factory. The harvester which gives the cleanest product is probably the one developed by Michigan State University. When these strawberries are to be used for making puree, for example, they can simply be tipped from the boxes into a tank of water for washing, after which all mouldy fruit is removed manually from an inspection belt. The remainder is fed into a puree-press. If the berries are to be IQF-frozen, a product which has a higher price, and probably also a wider market, both stem and cap must be removed. This can be done in a decapping machine. There are different types of decappers, one of which uses counter-rotating rubber rollers. When the stem is caught between two rollers, it is pulled off. On most varieties of strawberries the cap remains on the berries and they cannot be used for jam, for example. Another type of decapper has been developed at Michigan State University. This uses a conveyor with a number of smooth plastic-covered, counter-rotating rollers. When the stem is caught between two rollers it is held in place and transported past a band knife (a band saw without teeth) which cuts off the stem and cap. The capped berries can then be IQF-frozen, or used in making, for example, jam.
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Publisher: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
Permanent link to this page (URI)