Grapes

Infobox on Grapes
Example of Grapes
Freshness facts
Optimum carrying temperature	-0,5°C to 0°C
Highest freezing point	-2,1°C
Acceptable product temp. at loading into containers	Max. 2°C above carrying temperature
Optimum humidity	90% to 95%
Ventilation setting for containers	10 m³/hr
Storage life	2-6 months, dependent upon variety
Climacteric / non-climacteric	Non-Climacteric
Ethylene production	Very Low
Ethylene sensitivity	Moderate
Modified / controlled atmosphere	1%-5% CO2; 2%-5% O2
Potential benefits	Increased CO2 - slight; reduced O2 - moderate
Availability
Australia/New Zealand South Africa South America	April - June December - June December - June

Grapes

Harvesting and handling

Grapes do not continue to ripen after harvest; they should be harvested at optimal maturity based on soluble solids content.

Grapes awaiting transport should be sound, firm, exhibit no foreign odours or flavours, be free of abnormal moisture, have no pressure marks or bruises, should not include any burst or shrivelled grapes or any which have fallen from the bunch, nor exhibit any fungal growth or dried out stems. Grapes to be eaten fresh must have reached their full ripeness and colour and must not be withered.

Grapes are mostly packaged in cartons, each containing 5-10 kg. bunches, often in retail carrying bags or punnets. Since grapes are highly sensitive to impact due to their relatively thin skins, they must be handled with appropriate care. High consumer acceptance is attained for fruit with high SSC or SSC/TA ratio¹. Berry firmness is also an important factor for consumer acceptance as are lack of defects such as decay, cracked berries, stem browning, shrivelling, sunburned or dried berries, and insect damage.

The required refrigeration temperature must always be maintained, even during cargo handling.

Cooling and storage

Since the storage or transport temperature may vary depending upon the variety, exact refrigeration instructions should be obtained from the consignor. Where the grapes have a high sugar content, temperatures may be maintained slightly below freezing. Grapes with a low sugar content, on the other hand, require temperatures slightly above freezing conditions.

In order to prevent or limit the extent of postharvest water loss and consequent reduction in quality, the cargo must be refrigerated as rapidly as possible. The temperature of the cargo on loading should not be > 5°C.

A temperature of approx. 0°C is generally recommended for storage of grapes, at which they have a storage potential of several months. The storage life is dependent on maturity at harvest, pre-cooling, an effective fumigation programme and low constant storage temperature. Keeping quality is dependent upon variety, e.g. Thompson Seedless will keep for 2-3 months. Ribier keeps for 2-4 months, Alphonse Lavallée, Waltham Cross 3 months and Emperor 4-6 months.The shelf-life of grapes reduces by approx. 40% at +4°C and by approx. 70% at +8°C. Stem respiration rate is approximately 15 times higher than berry respiration.

Sweet berries may not freeze until -3°C, but their stems (containing much less sugar) can freeze at -2°C, leading to wilting and berry drop. To avoid the risk of freezing damage, the recommended carriage temperature is preferably to be around the freezing point rather than well below.

High relative humidity is necessary to minimise moisture loss and maintain stems in good condition. If shrivelling of the grapes is to be avoided, relative humidity should be approx. 90 - 95%. Shrivelling and drying of the grapes may be avoided by packaging them in perforated polyethylene film inside the fruit cartons. The normal weight loss due to a reduction in the moisture content of the product is 2 - 3%.

One of the most common and obvious symptoms of deterioration results from the activity of bacteria and fungi. Attack by most organisms follows physical injury or physiological breakdown of the commodity. In a few cases, pathogens can infect apparently healthy tissues and become the primary cause of deterioration. In general, fruits and vegetables exhibit considerable resistance to potential pathogens during most of their postharvest life.

The onset of ripening in fruit and senescence in all commodities renders them susceptible to infection by pathogens.

Botrytis cinerea is the most common cause of spoilage. Grapes harvested after rain are much more prone to decay than those harvested after a dry period. The mould grows best at 24°C, but is still capable of growing and causing rot even at a temperature of 0°C. Botrytis infection can be reduced by removing desiccated, infected grapes of the previous season from vines, leaf-removal canopy management, pre-harvest fungicides, trimming visibly infected, split, cracked, or otherwise damaged grapes before packing, prompt cooling and fumigation with sulphur dioxide and/or use of continuous release SO₂ pads in combination with box liners.

The cargo must be protected from seawater, rain and condensation water as moistening of the cargo and packaging materials increases the risk of spoilage. Grapes deteriorate in storage through decay or natural ageing. When approaching the end of their storage life, the berries are losing their brightness and acquire a soft and flaccid texture.

Mixed loads

If grapes have SO₂ packets, they should not be mixed with other produce. The SO₂ can damage many kinds of produce. Otherwise grapes can be shipped in mixed loads. However, they should not be shipped with leaks, garlic, or onions to avoid odour transfer.

Cautions

O₂ below 1% can cause off-flavours. CO₂ greater than 15% may be used as a substitute for SO₂ treatment to control grey mould but after two weeks, it can cause browning of berries and stems. SO₂ is incompatible with CA so they should not be used together.

Storage disorders

Alternaria rot, Anthracnose, Aspergillus rot, Berry drop, Black mould, Blue mould, Botrytis cenerea, Cracking, Downy mildew, Grey mould rot, Powdery mildew, Rhizopus rot, Sour rot, Splitting, Sulphur dioxide injury.

¹ SSC = Soluble Solids Content; TA = Titratable Acidity