Factors that affect the formation of crystals on stored raisins

The formation of crystalline substances on or within raisins is an undesirable post-harvest phenomenon commonly referred to as "sugaring".  The crystals are formed from natural grape sugars and acids that are a normal and essential component of all fresh and dried grapes, including non-sugared raisins.  Sugaring is undesirable because the crystals impart an unattractive appearance and disagreeable texture to the raisins; however, sugared raisins have the same nutritional value and are just as safe to eat as non-sugared raisins. 

Normal raisins on the left, and "sugared" raisins on the right.

Because sugaring is undesirable, raisin packers are interested in understanding the factors that contribute to sugaring so that it may be prevented.  The exact cause of crystal formation remains unknown, but a series of studies by L. Peter Christensen, former UC Davis viticulture specialist emeritus, suggest that processing and post-harvest storage conditions have a pronounced influence on the likelihood of sugaring.  Christensen showed that one of the most important factors affecting sugaring is raisin moisture content during storage.  Raisins stored at 10% to 12% moisture showed little sign of sugaring after one year, but sugaring increased dramatically as moisture content increased beyond 12% such that after one year of storage raisins having 14%, 16%, or 18% moisture, developed crystals on 11%, 28%, or 36% of their surface, respectively.  The handling and cleaning of raisins that occurs during processing also seems to increase sugaring problems, but the relative contributions of surface abrasions that may result from destemming, and the increased moisture content that results from washing, is uncertain.  The storage of processed raisins under conditions of fluctuating temperature and humidity also favors sugaring.  Thus, to reduce the likelihood of sugaring, unprocessed raisins should be dried 14% moisture or less, not processed until necessary, and the gentlest cleaning and processing methods possible should be used. 

Vine Covers for Table Grapes

Table grapes are a labor and material intensive crop; annual operating expenses may exceed $7,000 per acre. Thus, it is critically important for growers to protect late season grapes from rain, as exposure to precipitation within six weeks of harvest can stimulate the development of rots and molds that may render them unmarketable. Rain damage may be avoided by harvesting the fruit before the fall rainy season, but the grapes of some late maturing varieties may not ripen by then, and the price paid for grapes often increases towards the end of the season, thus providing an incentive to harvest fruit as late as possible.
Thus, many growers with late-season table grapes cover their vines with sheets of plastic film, beginning in late August or early September. At least two different colors of film, green and white, are
available, but data to distinguish the potential differences the two films might have on vine physiology or fruit quality are not available.  The cost to procure, install, and remove the films is very expensive, so growers need objective information on film performance.  To provide such information, we have just begun a study to determine how the two most common film covers may affect canopy microclimate, vine physiology, and fruit quality, to help optimize the productivity and quality of late-season table grapes in California.  Follow the blog and we'll update you periodically on our findings.

 

Different vine covers being evaluated in a commercial vineyard near Fresno, CA.

Soon after installing the covers, noticeable changes in canopy microclimate were observed, including higher temperatures, and condensation was evident on the inside of the covers.

Handling Raisins in the Event of Rain

By all accounts, grapes are maturing later than usual this year.  As a result, many growers have delayed harvest to enable the berries to accumulate more sugar.  The late season has also prompted some labor crews to seek other work opportunities and thus some other growers are harvesting later than they would like due to difficulty in securing labor.  In any case, a late harvest increases the risk that rain could occur while the raisins are drying. 
The potential for rain to damage raisins on trays depends on the amount of rain, and the weather conditions following a storm.  A light rain storm, dropping 1/4 inch or less of precipitation, will probably cause little if any damage, especially if followed by dry weather.  A moderate storm, producing 1/4 inch to 3/4 inch of rain could cause minor damage, if followed by ideal drying weather, or substantial damage, if followed by cloudy weather.  Rainfall over 3/4 of an inch will almost certainly cause extensive damage, regardless of the weather that follows. 

Severe storms may flood vineyards, leading to complete crop loss.

Fungal infections led to complete crop loss following severe rains.

If a manageable amount of rain occurs during drying, growers should keep in mind certain handling methods and finish-drying options that will help minimize crop damage. 

First, "slip" the trays, to prevent them from sticking to the soil and wicking moisture from the soil into the tray.  This can be achieved by sliding the trays just a few inches--enough to break the seal between the soil surface and the paper trays.

Next, invert the fruit on the trays as soon as possible to expose "bottom" fruit to sun and air.

A raisin tray, immediately after inverting the partially dried fruit.  The green-colored berries were not previously exposed to the sun, which is why they have not yet browned.

Do not try to save any fruit that is stuck to the bottom of the trays, nor allow such fruit to be mixed in with the other fruit.  If necessary, replace the trays with new ones, or invert the trays (if the bottom side is clean), leaving stuck fruit on the underside of trays, and placing unstuck fruit on the "clean" (formerly bottom) side of the trays.

Pick moldy or rotten fruit from the tray before rolling, and remove rolled fruit from the field as soon as possible.  If moisture content is >18 percent, do not fill bins more than half-way.  Begin on-farm or commercial drying as soon as possible--immediately if moisture is >22 percent.

Having thinned the clusters, it's time to fatten the berries

A few weeks ago, during bloom, we took steps to manage fruit set of our 'Crimson Seedless' table grapes, hoping to reduce set just enough to provide ample room for the berries to achieve their full size potential without becoming too crowded. 

 Whoops!  There will not enough berries on this cluster.

  This cluster is well thinned, with plenty of berries, and room for them to grow.

Now that our crop has been set, we are focusing our efforts on practices that maximize berry size.  The application of certain plant growth regulators including gibberellic acid (GA3), and forchlorfenuron, a synthetic cytokinin, promote the division and expansion of cells in the berries, and thereby improve berry size.  Girdling vine trunks, that is removing a thin ring of tissue from around the trunk, can also improve fruit size.  Girdling temporarily disrupts the phloem, the vascular tissue in which sugars are transported from leaves, making more sugar available to the developing fruits. 

A fresh girdle, made just below the one from last year. Girdling temporarily disrupts the phloem, causing sugars made by the leaves to accumulate in vine parts above the girdle. The extra sugar promotes the growth of young berries, the purpose of girdling, but the trunk also responds, as evidenced by its wider girth above last year's girdling scar. Ants are also capitalizing on the opportunity.

Making a Stink About Bugs

The introduction of new insect pests damaging to grapes is an ongoing threat for California’s grape growers.  Light brown apple (LBAM) and European grapevine (EGVM) moths are two recent examples of introduced insects the presence of which has been very costly to growers.  Although safe and effective conventional and organic control methods are available to manage moths, these particular insects are considered ‘quarantine’ pests, so extra special measures must be taken in the vineyard and throughout the fruit distribution and processing network.  Quarantine pests are a particular concern for the trade of fresh fruits, such as table grapes, due to import restrictions in some domestic and international markets.  Exactly how these insects made it to California is not presently known, but past insect introductions have come from contaminated plants and plant products.  Thus, it is critically important that everyone recognize the potential danger of violating plant protection and quarantine regulations.

Just as grape growers learn they have made excellent progress toward eradicating EGVM, officials are expressing concern over a new exotic insect pest, the brown marmorated stink bug (BMSB).  The BMSB is now found in 33 States.  Although not established in California, it has been identified in Los Angeles and Solano Counties.  BMBS can fly, but they primarily move into new areas by hitchhiking on vehicles and equipment.  Native to Asia, its thought that BMSB arrived in packing crates shipped to the Eastern US.  It has a large host range that includes grapes and many of the fruits and vegetables grown in California.  Damage can be substantial when BMSB populations are not identified early and managed appropriately. Apple growers in the Mid-Atlantic States have reported losses of $37 million representing 18% of their fresh apple market.  Growers and wineries are also concerned that the “stink” from any bugs accidentally crushed in wine or juice grapes could taint the product with off flavors. This insect should concern homeowners as well, since people in the Mid-Atlantic States have reported large populations of BMBS overwintering in their homes and becoming a nuisance.  

BMSBs resemble some other California stinkbugs such as the rough stink bug, a beneficial predator of other insects.  If you think you’ve found a BMSB, or any other odd or unique looking insect pest, you should collect it and bring it to your local university advisor, ag commissioner or state ag department entomologist for proper identification.  Early identification of invasive pests is critical for protecting California’s billion dollar agricultural industries.

You can learn more about the BMSB and current research here.

Breeding New Pest Resistant Rootstocks for Grapes

With bloom well underway, grapevine breeders and geneticists, such as Peter Cousins, with the USDA’s Grape Genetics Research Unit, in Geneva, NY, are busy making crosses between pairs of vines having favorable traits.  Cousins is a rootstock breeder, so traits that interest him include resistance to root-knot nematodes, soil-borne pests whose feeding activities can cause extensive damage to grapevine roots, thereby weakening the vines and reducing yields.  Though Cousins, is based in NY, he is cultivating some advanced selections from his rootstock breeding program in a vineyard at the Kearney Agricultural Research and Extension Center in Parlier, California.  By growing the vines at Kearney, Cousins can assess their performance in the San Joaquin Valley, one of the most important grape-growing regions in the world.  Vines that maintain their nematode resistance, grow well, and are easily propagated, may eventually be released as commercial rootstocks, and serve as a genetic resource for future crosses aimed at further strengthening their nematode resistance by incorporating as many resistance mechanisms for resistance as possible.

Some wild grapevine species have unique nematode resistance mechanisms, and are thus used as parents in rootstock breeding.  Cousin’s goal is to incorporate as many different forms of resistance as possible into new rootstocks, so some advanced selections are complex hybrids of several species.  Wild grapevines, and their hybrids, are dioecious; invididual vines bear clusters of either male or female flowers.  Dioecious vines are relatively easy to cross as the flowers do not have to be emasculated; to prevent unwanted crosses, he simply encloses female flower clusters in paper bags prior to bloom. 

As bloom approaches, he periodically he taps on the bags, listening for the rattle of detached petals that signals the blossoms are ready to be pollinated.  Properly prepared, pollen, which Cousins prepares by grinding flowers in a coffee mill, can be stored for a relatively long time, facilitating crosses between vines that might not flower at the same time.  To pollinate, the bags are removed, the appropriate pollen is slathered on with paint brush, and the cluster is labeled, and recovered with the bag.  Cousins will return in the summer, to extract the seeds from the fruit.  Seedlings will then be screened for nematode resistance, and the best selections may eventually end up returning to Parlier, to be evaluated alongside their parents.  To learn more about Dr. Cousin’s research program, and see his rootstock selection vineyard, plan to attend Grape Day at Kearney, August 18, 2011.

Managing fruit set; an important early step in producing quality table grapes

Grapes in the San Joaquin Valley are blooming.  The flowers are not particularly impressive to look at—they bear a superficial resemblance to brocolli—but the have a fresh, clean, invigorating scent.  That’s good, because there’s a lot of work to be done in the vineyard around bloom.  Table grape growers are among the busiest of viticulturists at bloom, as they must take steps to limit the amount of fruit set, so that the remaining berries will have ample room to grow without becoming excessively crowded.  If too many berries are set, the cluster will become excessively compact, full of misshapen and cracked berries that are highly susceptible to rot.  Home growers can thin their grapes by brushing the flower clusters with a scalp brush until about half the flowers are removed, but this approach is impractical for most commercial growers.

Instead, most growers apply a natural plant growth regulator gibberellic acid (GA₃) during bloom to reduce fruit set.  Generally, between 0.5 to 20 grams/acre GA₃ is applied to the vines when 30% to 80% of the flowers have opened, and multiple applications may be needed in some cases.  The proper application timing and dose depends on a variety of factors including the cultivar, environmental conditions at bloom, and cultural practices such as type of trellis and spray equipment used.  The use of GA₃ can reduce return fruitfulness of certain seedless cultivars, and many recently released cultivars require less GA₃ than traditional cultivars to achieve adequate thinning.  Seeded grapes, including wine grapes, may also respond to GA₃  thinning sprays, but seeds are a natural source of GA₃, and seeded varieties are more apt to respond to such sprays with unwanted side effects including reduced shoot and cluster counts the following spring, and increased proportions of tiny “shot” berries that detract from the appearance of table grape clusters. Before applying GA₃, or any other agricultural chemical, be sure to review the label and ensure the instructions are followed.