Grape maturity indicators
In California, harvest date is determined by Soluble Solids Concentration (SSC) of 14 to 17.5% depending on cultivar and production area. In some situations, the SSC/titratable acidity (TA) ratio of 20 or higher is used to determine maturity for early maturing cultivars from early production areas. For red and black colored cultivars, there is also a minimum color requirement.
Grape Quality Indices
High consumer acceptance is attained for fruit with high SSC or SSC/TA ratio. Berry firmness is also an important factor for consumer acceptance as are lack of defects such as decay, cracked berries, stem browning, shriveling, sunburned or dried berries, and insect damage.
Berry storage at -1.0 to 0° C (30-32° F) is recommended.
The highest freezing point for berries is -2.1° C (28.1° F), but freezing point varies depending on SSC. A -2.0° C (28° F) stem freezing point has been reported.
Optimum Relative Humidity
90-95% RH and an air velocity of approximately 20-40 feet per minute (FPM) is suggested during storage.
Rates of Respiration (of grape clusters, i.e. berries + stems)
ml CO2 /kg·hr*
0° C (32° F)
5° C (41° F)
10° C (50° F)
20° C (68° F)
Stem respiration rate is approximately 15 times higher than berry respiration.
* To calculate heat production, multiply ml CO2 /kg·hr by 440 to get BTU/ton/day or by 122 to get kcal/metric ton/day.
Rates of Ethylene Production
<0.1 m l/kg·hr at 20° C (68° F)
Responses to Ethylene
Table grapes are not very sensitive to ethylene. However, exposure to ethylene (>10 ppm) may be a secondary factor in shatter.
Responses to Controlled Atmospheres (CA)
CA (2-5% O2 + 1-5% CO2 ) during storage/shipment is not currently recommended for table grapes because its benefit is slight and SO2 used for decay control.