TECHNIQUES TO IMPROVE GRAPE QUALITY

GIRDLING

is a common practice for table grape growers

is the removal of a complete ring of bark 1/8 to 1/4 inches wide from the trunk

the carbohydrate materials produced in the leaves accumulate in the parts above the wound

the farmer's objective is to increase the size of and sometimes the earliness of maturity of the grape berry

specially designed knives are used to perform the girdling technique

occasionally, farmers will double girdle

THINNING

purpose is to reduce the grape crop to a normal load so that high-quality fruit is produced

Flower-Cluster Thinning or Berry Thinning

used in table grape production

is the removal of individual flowers from the inflorescence

the purpose is to give individual berries enough space to fully develop and still have a fruit cluster that is not too compact

is common in Perlette table grapes

removal of the tip of the flower cluster is a normal practice in Ruby Seedless table grapes

saves time and expense at fruit packing time, plus the berries are larger

Cluster Thinning

removal of the entire clusters after the berries have set following bloom

purpose is to reduce the overall crop load

PLANT REGULATORS (Growth Hormones)

Gibberellic Acid (GA₃)

used extensively on Thompson Seedless Grapes

used on Flame Seedless Grapes

can reduce color on red varieties of table grapes

thin individual clusters of small berries (immediately before fruit set; 50% or full bloom) kills the embryo (seed) inside the small berry (table grapes)

increases the berry size of grapes after the individual berries are set and begin to size; increases the size of the individual fruit cells inside each berry (table grapes)

changes the berry shape of Thompson Seedless grapes

reduces cluster compactness by lengthening the rachis and the individual pedicels (raisins)

Ethephon can increase anthocyanin pigment in Flame seedless grapes

HARVEST AND POSTHARVEST OPERATIONS

TABLE GRAPES

Proper Picking Time

veraison is the stage of fruit development when the berries begin to soften and color

ripening consists mainly of an increase in sugar, a decrease in acidity, development of flavor and color consistent with the varietal characteristics

rate of ripening is temperature dependent

acid and sugar content and in some cases RATIO are the best measurements to use for determining when the grapes should be harvested

at the moment of picking, the grapes are at their maximum quality for eating

at the moment of picking, the quality of the grapes begins to decrease

Sugar Test

measured with a refractometer

calibrated in Brix and read directly in percent sugar by weight

always use a representative berry sample to determine the degrees Brix or sugar

for some varieties of table grapes, the sugar is tested by an official of the Agricultural Commissions office

Acid Test

acid titration test

10 ml of grape juice

50 - 100 ml of distilled water

2 to 3 drops of phenolphthalein for pH indication

solution of NaOH (0.133 Normal)

is equivalent to 0.01 grams of tartaric acid/ml

Degree Brix Acid Ratio

the degree Brix divided by the acidity in gram per 100 ml gives the degree Brix acid ratio (sugar:acid ratio)

20 : 1 ratio or 17% sugar is required for Thompson seedless grapes

other varieties vary with some varieties having no standards.

cool weather can keep the sugar acid ratio too high.

Cooling and Storage

cool grapes as soon as possible

store at 32⁰F and at 95% relative humidity

varieties vary greatly as to their storage life

storage boxes should be vented for air exchange

Sulfur Dioxide Fumigation (gas)

reduces bunch rot of the grapes while in storage (Botrytis)

before fumigated grapes are shipped the SO₂ must be below 10 ppm

RAISIN GRAPES

the higher the degree Brix at harvest time, the better will be the quality of the raisins produced

the best berries are at least 22⁰ Brix

berries with low degree Brix reduce yield, increase drying time, make flat, meatless raisins that grade low and bring low prices

farmers must decide between harvesting grapes with high sugar,or harvesting and drying the fruit before the Fall rains

at 21 to 22 degrees Brix the drying ratio of fresh berries to raisins is about 4 to 1

fruit picked by hand and placed on special paper for drying

the drying grapes are on a slope that should be at right angle with the sun

in case of rain, the slope allows for drainage

the raisins are graded for mold or rot, sand, capstems and air-stream sorter