Methods to determine berry mass, berry volume and bunch mass

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Methods to determine berry mass, berry volume and bunch mass

Danie van Schalkwyk, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Yield estimate, juice recovery, wine grapes.
Bunch mass is important to the cellar master/winemaker because it gives an indication of the available skin surface/juice volume and it may also be used to calculate the skin contact period (De Villiers & Theron, 1988). Bigger berries usually give better juice recovery, due to the bigger skin:juice ratio, while smaller berries give better colour in red cultivars and better flavour in cultivars with a grassy character.
Berry mass and size are largely influenced by factors such as genetic origin, berry set, number of berries per bunch, position of the bunch, number of pips per berry, number of bunches per vine (bud load), climate, water supply, fertilisation, soil type, rootstock cultivar and degree of ripeness (De Villiers, 1987). Bunch and berry mass in the same cultivar can therefore differ considerably from season to season and from one locality to the next. Berry mass is also an indication of berry size.
Bunch and berry mass may be used indirectly to predict yields. The best time to gather bunch and berry samples for yield prediction is at véraison (approximately 14°B). At this stage the mass of the berries is approximately the same as at the full ripe stage. The calculation of these masses is simple and can easily be done by the producer himself. A representative sample of the bunches and berries in a vineyard block is obtained by gathering a random number of bunches in a vineyard block. The size of the block determines the size of the bunch sample to be taken. In most instances one full picking crate per hectare is sufficient to calculate the average bunch mass. The bunches are counted and weighed to determine the average bunch mass.

Fig. 1 Use small scissors to sever the berries as closely as possible to the berry.

Fig. 2 Freshly cut berries; note that a small part of the bunch stem remains intact.

Fig. 3 Small tuft that remains when the berry is rubbed off instead of cut off.

Fig. 4 Determine berry mass using an electronic scale.

Fig. 5 Berries are rubbed off.

Fig. 6 Very small portions of the tufts remain on the berry stems and the berries remain almost entirely intact once they have been rubbed off.

Fig. 7 One hundred berries are thrown into a measuring cylinder to determine the berry volume.

Two methods may be used to determine the berry mass:
1. Determining fresh berry mass
When the berry mass of fresh grapes is determined, the following procedure must be followed:

Collect a random bunch sample from a vineyard block (e.g. one full picking crate).
Select 5 - 7 bunches randomly from the bigger bunch sample collected from the block and removed all the berries from the bunches.
Remove the berries from the stems using a pair of scissors, cutting as closely as possible between the skin and the stem thickening (Fig. 1). Clean cutting is not always possible, therefore a small section of the berry stem will remain intact (Fig. 2).
The berries must not be rubbed off as a large part of the berries (tuft) remain on the berry stem. The size of the tuft may differ considerably among berries and cultivars and result in inaccurate calculation of the berry mass (Fig. 3).

Berry mass is usually indicated per 100 berries and not per bunch. One of the following two procedures must be followed when determining berry mass:

One method is to count the total number of berries that have been cut off, then to determine the mass thereof using an electronic scale and in the light thereof to calculate the mass for 100 berries.
An alternative method is to count 100 berries randomly once the berries have been properly mixed, then to determine the mass thereof (Fig. 4).

2. Determining frozen berry mass
With the multiple actions that have to be conducted under experimental conditions during the harvest, it is not always possible to determine the fresh berry mass directly after the harvest of the grapes. In this instance the grapes may be frozen at approximately -20°C so that the berry mass can be calculated later and other analyses conducted on the grapes. The mass of the frozen berries must be determined before being thawed, because at that stage there is very little difference between fresh and frozen berry mass.

Select 5 - 7 frozen bunches randomly from the representative bunch sample, but instead of cutting the berries, either rub them off or pick them (Fig. 5).

When frozen berries are rubbed off, a very small part of the tuft remains on the berry stem (Fig. 6).
3. Determining berry volume
There is a correlation of approximately 96 % between berry mass and volume, and berry mass may therefore be used as a good measure to calculate berry volume (De Villiers, 1987).
The determination of berry volume is simple.

Use the same 100 berries that were used to determine the berry mass.
Fill a 1 000 ml measuring cylinder with enough water to cover 100 berries and take a reading (e.g. 200 ml). Now add the 100 berries to the water in the measuring cylinder and take a reading (e.g. 350 ml).
Subtract the first reading (e.g. 200 ml) from the second reading (e.g. 350 ml), so as to arrive at the volume of the 100 berries (e.g. 150 ml/100 berries) (Fig. 7).
Top the water up to the initial volume each time the berries are removed from the measuring cylinder (e.g. 200 ml).

4. Practical application of bunch and berry mass yield estimate
When making yield estimates, a number of representative bunches are randomly gathered across the entire vineyard block (full picking crate). The bunches that have been gathered are counted and the mass of the grapes in the picking crate determined to calculate the average bunch mass. Also count the number of bunches of a random number of vines across the block and calculate the average number of bunches/vine. The expected yield for the block can then be calculated as follows:
Mechanically harvested grapes
With mechanical harvesting bunch stems remain in the vineyard and it is important for the producer to know the percentage of these bunch stems so that the actual production per hectare may be calculated, considering that this has implications for remuneration. Bunch stem mass is also important to the winemaker to calculate expected juice recovery. Bunch stem mass may be calculated by deducting the berry mass from the bunch mass. Bunch stem mass is especially influenced by the cultivar, length of the bunch stem, thickness and differences in density and have substantial economic implications for mechanical harvesting and vinification. Depending on the cultivar and cultivation conditions, the bunch stem mass may amount to between 3.0 - 12.7 % of the bunch mass (De Villiers & Theron, 1988).
Juice recovery

Expected production/block (t/ha)

Production/vine = Average number of bunches/vine x average bunch mass (g)
= (production/vine (g) ÷ 1 000) kg
= a kg

Production/block = (a kg x number of vines/block) ÷ 1 000
= y tons

Production/ha = production/block (t) ÷ block size (ha)
= z t/ha

Bunch, berry mass and berry volume may be used by the winemaker to calculate the expected juice recovery. Under normal conditions approximately 600 - 650 l of juice are recovered from white cultivars and 750 - 800 l from red cultivars from 1 000 kg of grapes, with a view to vinifying good wine.
Mathematically the calculated skin surface of 100 berries with a mass of 216 g and a volume of 216 mm3 are equal to 216 mm2. This is the equivalent of a skin surface to volume ratio of 1:1. Smaller and lighter berries will therefore have a bigger skin surface to berry/volume ratio, while bigger and heavier berries have a smaller skin surface to berry mass/volume. A high berry mass therefore means that more juice may be recovered seeing that there is a bigger pulp to skin ratio, while less juice may be recovered from lighter berries due to the smaller pulp to skin ratio.
References
DE VILLIERS, F.S., 1987. 'n Vergelykende ampelografiese en ampelometriese studie van die tros van verskillende wyndruifkultivars. MSc. Thesis, Stellenbosch University.
DE VILLIERS, F.S. & THERON, J.C.D., 1988. Die troseienskappe en fisiese toestand van druiwe tydens die oes van verskeie wyndruifkultivars. Boerdery in Suid-Afrika. NIWW 220/1998.
For more information contact Danie van Schalkwyk at ARC Infruitec-Nietvoorbij, tel (021) 809-3156, fax (021) 809-3002 or email vschalkwykd@arc.agric.za.

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