Why do some grapes have thick skin?
Seedlessness in table grapes: a useful but complicated property
At best, table grapes should not have any seeds, because the consumer wants this, at least according to the opinion of the trade. This property is also known as parthenocarpy (without a nucleus) in technical jargon. So currently 40% of the imported grapes have no seeds. This is particularly true for small berry varieties such as the sultaninas, which mostly come from Turkey, or the “Flame Seedless” variety, which is widely grown around the world.
No generative reproduction without kernels
The lack of cores also plays a role in consumer surveys. A high proportion of consumers generally rejects grapes containing seeds. German table grapes or Esstrauben (wine grapes cut for direct consumption) have a hard time here, as they are usually seed-containing. It should be noted that the fruits of the grapevine, the grapes, naturally have kernels or seeds. These are used for natural reproduction by mammals, birds or simply by rotting of unharvested grapes. With all wild vines, this is the natural way of reproduction over long distances, since grape-consuming animals usually do not digest the kernels, but excrete at least some of the kernels intact. This is illustrated very nicely by a rotting heap of pomace left to its own devices, at the edges of which young vine seedlings usually sprout en masse. A natural reproduction in a spatially limited scope is also possible vegetatively via creeping and rooting shoots. The best example of this are overgrown root rashes on bushes that climb up like lianas and form roots when they come into contact with the ground. From the point of view of the phylloxera problem, however, this is not a welcome picture. The vine has different ways of natural distribution and therefore, as a culturally and historically "old" wild plant, has withstood warm periods and cold phases in a variety of ways.
In the cultivation of the vine, the propagation via seeds lost its importance, because the authenticity of the variety was lost as a result. The propagation took place - apart from breeding and random seedlings - exclusively vegetatively by cuttings; in times of phylloxera adequately through grafted vines. Seeds were therefore no longer a selection criterion for the reproductive success of a domesticated vine.
The first seedless vines arose purely by chance and were deliberately propagated further, as they apparently had positive properties that justified continued cultivation. Raisins and currants have been known since ancient times. They are made from the seedless, light-skinned Sultana grape, better known as "Thompson Seedless", and the blue-skinned, also seedless, small-berry Korinthiakirebe (black currant, named after the Greek city of Corinth). Traditionally in the Mediterranean area, because there are favorable conditions for raisin production through natural drying, and the vines deliver very ripe fruits and reliable yields. "Raisins" is the collective term for dried grapes, which are usually naturally seedless.
Only Zibeben can also contain kernels, as these used to be traditionally used as a natural added sugar for certain types of wine, mainly in Hungary they were added to the Tokay outbreak wine during fermentation in order to make the wines more alcoholic and therefore more durable. Raisins with seeds were probably a quality flaw early on, although they were by no means worse in terms of taste. However, the kernels interfered with consumption and processing in dishes, especially the dry kernels in raisins become very hard. On the other hand, the kernels hardly play a role in wine production, as they remain with the pomace as a solid mass and ensure good infiltration of the press cake during the pressing process and when it breaks. The risk of unwanted phenols being leached out through long mash fermentation times (especially with Pinot Noir) can be avoided through careful processing and, more recently, through special core discharge systems in mash fermentation tanks.
Therefore, the seedlessness can be seen as a natural defect that promised advantages in traditional selection breeding for raisin and table grape production. In the case of other soft fruits such as strawberries, raspberries or blueberries, where the kernels are eaten, core poverty also plays an important role in table fruit varieties. The same applies to watermelons, of which almost only seedless varieties have been on the market in recent years. Kernels should primarily not be a nuisance during consumption, should not be a nuisance during consumption or represent unsavory waste that must be taken out of the mouth and end up next to the fruit plate. Although grape seeds neither taste bitter when chewed nor put a lot of strain on the teeth when biting, so they could actually be eaten by most without any problems, they are often considered undesirable. Not to mention that grape seeds contain valuable ingredients. They are and were used as a raw material for the production of edible oil (grape seed oil), and they also have high levels of nutritionally important minerals, trace elements, vitamins and, above all, dietary fiber. Ground grape seeds (grape seed flour) are added as a binding agent in cake flours.
There is certainly a certain amount of convenience and uncomplicated enjoyment in consumer desires for “convenience grapes”. It is annoying when core pieces get stuck between teeth. Smaller children in particular or older people with limited chewing ability are right to complain. There are old people's homes that only serve seedless grapes because the subsequent cleaning of teeth and teeth is too time-consuming and labor-intensive. Therefore, the grower should be interested in recognizing the consumer wishes, even if he may contradict them internally. Otherwise he leaves the business to other producers, especially foreign competitors, and loses out on the market in the long term. True to the motto: The customer is king.
Seedlessness is tied to the variety
Kernels cannot simply be sprayed away with the use of phytohormones or other artificial substances. The popular opinion that the use of gibberellic acid preparations can make grapes seedless does not apply. Rather, the use of gibberellic acid increases the thickness of the berry, as the growth spurt actually triggered by the berries is mimicked by repeated spraying during the berry growth. This very difficult treatment must be precisely tailored to the growing conditions and varieties. Although it is widespread practice in large producing countries, it is not permitted in Germany, where table grape production only plays a niche role.
In the case of pears, the use of gibberellic acid prevents the damaged fruit from falling off after the blossoms have frozen. Fruits still develop from the flower soil, as the signal is given to the tree that normal fertilization has taken place. In viticulture, gibberellins are only used as a flower spray for certain wine grape varieties in order to lower the fertilization rate and thus to obtain a loosened grape structure. The kernels give the signal for berry growth by releasing naturally formed gibberellins. The more pips that are formed, the larger the individual berries will be. A maximum of 4 cores can develop in the berry.
Well known to the winemaker are unfertilized virgin berries or honeyberries, which are caused by unfavorable flowering and which remain very small. The varieties “Riesling”, “Traminer” and “Huxelrebe” are particularly prone to this. In some years the majority of the berries are small and completely seedless. They mature and become very cute. With other varieties such as “Regent” or “Cabernet Sauvignon”, however, they remain green and completely immature. Depending on the weather, many seedless berries can form, but this is crucially dependent on the flowering conditions and not the rule. Virus-infected vines also tend to have virgin berries, which then reduce the yield considerably. The seedlessness in certain table grapes, on the other hand, is hereditary. Even under favorable growth and flowering conditions, they remain seedless, as they are no longer able to form seeds. You are sterile.
Older varieties are only partially satisfactory
Unfortunately, the traditional seedless varieties are not very suitable for our climate. Sultanina vines and currants are quite susceptible to winter frost and only grow bills under favorable conditions. Usually only the upper (apical) eyes are fertile, so that long wood has to be cut in order to generate any significant yield. Although both varieties mature well under favorable conditions, the high yield and low yields are not economical for table grape production. They are only authorized in assortments of grapes, educational trails or as a curiosity in the garden (house vine). In addition, the sensitivity to fungi (powdery mildew and downy mildew) is very high and the vines would have to be treated intensively accordingly. The botrytis pressure is also enormous in damp autumn weather. Breeding is therefore required to “create” new, cheaper varieties.
The “seedlessness” property is therefore extremely interesting for breeding on the one hand, and on the other it leads to a breeding dead end, since no further new varieties can emerge without the kernels of these new varieties. In particular, two seedless varieties cannot be crossed with one another. This makes it so difficult to combine taste and resistance properties with the seedless characteristic. In the past, a seedless variety was therefore used as a pollen donor. With luck, some of the progeny were seedless with positive properties (ripening time, berry color, fruit flavor, sweetness, berry size, berry shape, etc.) of the seeded mother variety.
Successful crossings with resistant American varieties have found a limited distribution. Similar to the breeding of fungus-resistant wine varieties (PIWI breeding), good resistance partners were crossed with the pollen from seedless table grape varieties. The Sultana vine was specifically crossed with the very fungus-resistant American hybrid "Ontario" in 1928 in the USA. A. Stout is given as the breeder. The varieties emerged from this seedling selection "Himrod", "Romulus" and "Lakemont Seedless". All four varieties were named after cities or places in New York State. Although all varieties have become more widespread, they do not satisfy those of commercial table grapes in terms of berry taste and consistency. In North America, some of the varieties are also used to produce wine. The resistance properties were inherited, but only to a limited extent. This means that cultivation without crop protection does not produce satisfactory grape quality.
Although these varieties have also gained a foothold in Germany, this was more due to the fact that no alternative seedless varieties that were more satisfactory were available up to now. The hybrid tone (raspberry or strawberry note, lambrusca taste) was also transferred to the cultivars in a weakened form. These notes, which are more reminiscent of drops or fruit gums, are atypical for table grapes and lead to rapid saturation. The berry skin also turns out to be tough and somewhat sour. Another problem is that the berries fall off easily when they begin to ripen, i.e. separate from the grape together with the berry stalk. The same thing happens when removing, so that the berry stalk often has to be plucked separately. The berry size is also rather small for seedless table grapes. The yield can vary greatly depending on the location and the cut and can be high (Lakemont). Himrod In contrast, it turns out to be a changeable carrier, sometimes heavy trickling can cause a total failure. Ripe is medium early at Himrod and medium late at Lakemont. For our climates, the low winter frost resistance is more decisive than the grape ripeness. At temperatures below –12 ° C, eye loss can occur, lower temperatures lead to serious damage to the trunk and the rods. This property is largely shaped by the Sultana grape, so locations away from the wine-growing areas are ruled out. When ripe, in conjunction with a large amount of water, the berries can easily burst and spoil quickly, and stalk paralysis is also an issue. Birds and wasps, the main pests of ripening berries, are not particularly fixated on these varieties. Obviously, it's because of the tough berry skin. The seed-containing variety “Muscat bleu” is much more attractive in this regard.
More seedless Americano varieties the red-skinned vines represent with other crossing partners Suffolk Red and Vanessa which are often sold in garden centers as resistant seedless grape varieties. The grape quality is comparable, but the yields are lower. Often the grapes trickle more heavily, the berries then remain particularly small. The variety Vanessa However, due to its appearance and low susceptibility to rot, it also has opportunities in commercial (organic) cultivation. Even if the berry skin is a bit tough, the subtle fruity note is convincing. Children especially love the sweet, small, seedless berries of this rose variety. The dark-skinned “Venus”, on the other hand, has an intense strawberry taste, which is reduced to a tolerable level when fully ripe. The shapely, medium to large loosely occupied grape with round, relatively large berries proves to be botrytis- and frost-resistant. The inner consistency of the berries is somewhat gelatinous.
Good quality seedless varieties are unfortunately more susceptible to fungi
If you accept that regulated and intensive plant protection is carried out, other varieties come into question, which are rated higher in quality than table grapes and yet meet our climatic requirements quite well. But here, too, the limited winter frost resistance is almost always the limiting factor in cultivation. These varieties convince with their berry taste as well as the pulp (crispier) and the berry skin (softer and thin-skinned).
The internationally grown variety Flame Seedless (red-skinned) still has the highest cultivation value of their group. The berries remain smaller than those of imported grapes, but ripen fully because the time of ripening is quite early. The berries have a firm "bite", are very sweet and pleasantly fruity in taste. The structure is loose berries, whereby the grape structure is quite fragile.
Other international varieties are Sublime Seedless and Suprior Seedlesswhich, however, have far too little yield (notes), so that no economic cultivation is possible here. In Italy, too, they have to be cut on long wood in order to provide sufficient yield. Despite the large berries (probably the largest completely seedless berries) and high grape quality combined with early ripening, this is not profitable. Approaches using special cutting systems and wide stick spacings are currently being examined so that the earnings situation can possibly be significantly improved. The vines are extremely vigorous, so that tree-thick trunks can form within 10 years if they are not damaged by winter frost.
The variety Sugraone Seedless is internationally recognized as one of the most valuable seedless table grape varieties. However, no cultivation results are yet available in Germany.
The “Perlette” variety produces very heavy grapes with lots of berries. Unfortunately, the compact grape structure leads to premature rot from inside the grape. The cultivation can therefore not be forced.
Other promising varieties of low-seed table grapes
Varieties with larger berries are usually not completely seedless, but have soft, rudimentary pseudo-seeds that are barely noticeable when consumed. The kernels can only be easily recognized by cutting berries. In botanical jargon, this is known as stenospermocarpy. From the cultivation and consumer point of view, however, they also fall into the category of seedless varieties. Newer varieties with larger berries are already being touted by German vine growers. However, the experience is often not yet sufficient to give an unrestricted cultivation recommendation. After a very positive year, disillusionment can often ensue, as the weather has a great influence on yield and quality. At least five evaluated test years are required in order to be able to make a certain consistency of the statements and to uncover weak points. A cultivation is therefore worthwhile to a limited extent for the time being, only after good experience should the respective variety be planted on a larger scale.If no berry color is specified, the berries are light-skinned, as this is preferred by the consumer (no noticeable stains on clothes or tablecloths). The varieties are to be named Tonia, which develops berries of comparable size, which become sweet and fruity (subtle nutmeg taste) when ripe. The variety has often performed extremely well at tastings, but not enough ripe berries are watery and poor in taste. A roof (foil roof) is required for optimal grape quality. Unfortunately, the wood's low frost resistance and yield loyalty leave a lot to be desired.
- Table grape varieties
Tonia - the variety is currently heavily advertised in the gardening trade. The grapes are large and seedless. However, because of the susceptibility to powdery mildew, cultivation is demanding and something for professionals.
Primus - this very early variety is particularly attractive to wasps. Single grapes can be perfectly protected by gauze bags, known in the trade as organza bags.
Millennium - it produces the heaviest grapes of a seedless variety. She also convinces with the taste.
Artemis - it is a fairly compact, seedless variety with pointed oval berries.
- The red-skinned variety Rhea has conspicuously pointed oval berries of medium size. Due to the late ripening (the color is much earlier), cultivation only makes sense in protected locations where it can mature well. The grape is very downy mildew (downy mildew) sensitive, even with advanced berry development. These disadvantages significantly limit the cultivation of the actually very shapely grape.
- Evita is a very young breed that develops spherical, medium-sized and very crisp berries. It has a fruity note of nutmeg, the soft kernels are barely noticeable when consumed. Ripening time is from the beginning of September, the location requirements are medium. When ripe in wet weather, however, the sensitivity to putrefaction is quite high, so that rain protection is generally advisable.
- It is a very early variety Primus, which has somewhat elongated berries. These are of different sizes, the large ones have soft pseudo-cores. The pulp consistency still has a lot of bite even when ripe and the aroma is very fruity with a pleasant fruit acidity. The variety has increased fungus resistance and the wood matures well. The yields have been satisfactory so far, and the early ripeness in particular makes them very valuable for cultivation. A wasp protection is absolutely necessary.
- The variety millennium makes quite large berries and heavy grapes. The pseudo-cores are not noticeable. So far, there is little experience in commercial cultivation, but the variety properties sound very promising.
- The variety Artemis has pointed oval berries that are compactly arranged on the frame. It is visually attractive, but rot can quickly spread. If it stays healthy, it can hang for a long time, as the berry skin is quite firm and the berries keep for a long time in storage. When ripe, the berries in the sun turn slightly reddish.
- Table grape varieties
Rhea - this red-skinned variety has conspicuously pointed oval berries of medium size.
Christine - the variety has small and soft seeds in relation to the size of the berries.
Moritz - varieties with few seeds or varieties with soft seeds like the red variety "Moritz" are superior to seedless table grapes in terms of berry size.
Heike - the seedless red variety has a loose grape structure.
- The varieties are very new Moritz, a blue, very large berries and seedless variety that ripens late.
- The variety Luis also has blue berries that are smaller and seedless. The ripening period is between September and October.
- Red berries indicate the varieties Heike and Anja on. Heike ripens earlier, as early as the beginning of September, and has sweet round berries. Anja is, however, oval in the berry shape and can be cut until October. The wood ripens well and the growth is upright.
- The variety has a high visual attractiveness Christine. The large, white-reddish berries have, in proportion, small and soft stones. With high levels of solar radiation, the risk of burns or strong pigmentation is quite high, so that an extension under the protective roof serves not only as a protective measure but also to improve quality. High yields ensure profitability. However, the late maturity limits the choice of location.
Especially the varieties with soft pseudo-seeds offer a good compromise between taste, berry size, non-disturbing seeds, sufficient ripeness of wood and good yields. The fungus and botrytis resistance is usually more favorable compared to completely seedless varieties. This will certainly be the focus of cultivation in the next few years. It is not yet possible to estimate at the moment which varieties from this group will have a high level of marketability. In any case, they are clearly superior to their predecessors such as "Himrod" or "Lakemont Seedless". Nurseries with the respective varieties can be requested from the author. However, seedlings are currently still in short supply.
- Götz, G. (2013): Seedlessness in table grapes: a useful but complicated property. Viticulture & Oenology Department (Viticulture Group), Service Center Rural Rhine Palatinate, Neustadt an der Weinstrasse.
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