INTRODUCTION

Gentler handling of grapes, particularly upon receiving at the winery, is becoming crucial as winemakers strive to produce higher quality wines. The consensus of opinion is that by minimizing grape skin abrasion, premature crushing and maceration, and eventually abrasion of seed coats, softer, more drinkable wines can be presented to the consumers within a shorter period of time. The primary reason being that tannin extraction should be minimized to prevent undue incorporation of astringent and bitter constituents into the must.

Further, many believe, and rightfully so, that gentler handling of grapes at the receiving end helps prevent incipient fermentation by wild yeasts and, in many instances, premature oxidation of the must. This is particularly important for white wines, where one must retain fruity and floral characters without harshness and, in addition, prevent discoloration of the must. The latter has become increasingly important as winemakers have also tried to minimize addition of sulfur dioxide at crush. Thus, preventing premature and excessive release of polyphenol oxidases prior to fermentation has become crucial in preserving quality attributes.

This line of thought has been extended to the point that winemakers wishing to produce ultra premium wines have foregone the use of crusher-stemmers altogether and have resorted to whole cluster press instead. Thus, gone are devices such as drag screens that were devised mainly to maximize must yield without regard for its quality.

Yet, regardless of how direct one wishes to handle grapes prior to inoculation, the problem remains that grapes must be unloaded from gondolas or bins at receiving and then conveyed in some manner to the appropriate device, whether this be a crusher-stemmer, destemmer, or press, or as in the case of carbonic maceration, directly into a tank or a rotary fermentor.

Traditionally, this transfer of grapes has been accomplished by dumping the grapes from a gondola when machine-picked, or from bins when hand-picked, into a receiving trough that is provided with an auger. The grapes are then conveyed by the rotational action of the auger into a crusher stemmer.

The classic design, still used by many wineries, is of a shallow, V-shaped trough, usually with a 15 to 30 degree angle from the horizontal, onto which the auger rides. In this system, grapes are pushed along by the auger through a bulkhead onto either an auger extension, or onto another inclined auger, for delivery to a crusher stemmer. This system, although highly efficient, has the inherent drawback that since the auger does not ride on an enclosed, or at least partially surrounding enclosure, the grapes and clusters are not conveyed as a "packet" between the screw pitches; instead, some of the grapes and clusters ride back, and back again alongside the revolving screw at each rotation.

This riding back and back again provides for increasing maceration of the grapes, including stems and seeds, and definitely for longer permanency of some of the grapes in the trough. What this means is that some of the grapes in a continuously operated receiving system might have an unduly long permanency that will allow for excessive tannin extraction and for the indigenous yeasts and liberated enzymes, i.e., polyphenol oxidases, to act on the must extemporaneously.

In order to minimize this problem, the industry has resorted  to conveying the grapes into presses, crusher stemmers, or tanks by means of belt conveyors. Belt conveyors certainly provide the ultimate in gentleness as the clusters are delivered whole, without maceration, and definitely at speeds consistent with minimal permanency in the conveying system. Further, when the belt conveyor is operated without exceeding its inherent designed capacity, no tumbling back of clusters and grapes is experienced. Also, because the grapes and clusters are conveyed with minimal contact with hard surfaces, except perhaps the side walls of the channel onto which the belt rides, abrasion is minimized. This leads to minimal juicing during the conveying operation. Usually any free juice present at the belt is derived not from the belt-transport operation itself, but from previous operations such as harvest and gondola transport.

However, not all wineries that have tried substituting screw conveyors for belt conveyors have been completely satisfied with the latter.  Their contention being that belt conveyors do not last long, particularly with increased sun exposure as under central San Joaquin Valley conditions.

Thus, it occurred to us that perhaps a modification of the time honored design of the screw conveyor might provide for its inherent advantages of durability and conveying efficiency without its main drawback: that of allowing grapes and clusters to ride back and back again along the screw as the screw traverses.

To this end, it was decided to allow the screw to ride within a semi-circular channel located at the bottom of a modified V-shaped trough. The expectation being that by riding on a channel, the riding back and back again of clusters and grapes would be averted.

In order to test our hypothesis, we designed, and had Fine Arc Manufacturing (Fresno, CA) construct for us a modified V-shaped trough, or grape dump, that has its screw riding onto a semicircular channel. In addition, the sides of the trough were designed with much larger, and different  angles from the horizontal than the traditional 15° to 30°. In our device the angles are 45° for the proximal (unloading) and 60° for the distal side. This was done to ensure that as the grapes are unloaded, grapes and clusters would fill the screw pitches, and also to assure that those pitches would continue to remain filled by the weight of the grapes unloaded thereafter. Otherwise, empty or partially filled "pockets" could form in some screw pitches making the unit inefficient. Figures 1, and 2 show actual dimensions of the device and its support structure. Figures 3 and 4 are pictorial representations of the trough and the position of the screw within.

The device was also designed to allow for delivery  of grapes and clusters onto various types of crusher-stemmers, destemmers, and presses alternatively placed under the unloading chute. Speed of the screw is controlled by a Vari-Drive. Its maximum delivery speed is approximately 16 tons per hour; that is slightly higher than the maximum capacity per hour of our largest crusher stemmer.

The primary intention for the construction of this modified V-shaped trough then, was to try and provide a device that could approximate the gentleness of handling grapes and clusters that is epitomized by belt conveyors; realizing of course that inherently a screw conveyor might still be somewhat inferior in terms of gentleness in delivery.  Screw transport, because of the often loose tolerances between the screw and the bottom of the channel (or "V") in a trough or grape dump, allows for maceration of some skins, stems, and seeds. Maceration to some extent is almost inevitable in screw operated conveying devices.

Since the industry standard for gentleness in grape and cluster delivery is indeed the belt conveyor, experiments were carried out as described below to compare our screw-operated device with a belt conveyor. To this end, WESTEC-IRAPP of Healdsburg, CA, constructed and donated to the Enology Program at Fresno State, an all-stainless steel belt conveyor designed and built to our specifications. The conveyor features a 24 inch wide endless food-grade belt driven by a frequency-controlled variable drive capable of delivery of grapes from an almost imperceptible crawl to speeds up to 60 tons an hour. The belt has semi-rigid transverse slats that are slightly concave to allow for carrying a "quantum" of grapes between slats without allowing for the slats to flex back with subsequent falling back of clusters and grapes onto the next slat. In this manner, a continuum of "paquets" or "quanta" of grapes and clusters are efficiently carried and delivered to whatever device is placed under its delivery chute. The belt conveyor is built to such tight tolerances, that it can pump water.

EXPERIMENTAL

Approximately 18 tons of French colombard grapes grown at Fresno State vineyards were hand harvested in 4' x 4' x 2' Macrobins(tm)  over a two-day period (Aug 25 and 26, 1997). The first nine tons (Lot I) were conveyed to a Demoisey crusher-stemmer via the auger delivery system at a rate of approximately four tons per hour.  The second nine tons (Lot II) were delivered to the same crusher-stemmer using the Westec belt conveyor.

After crushing, each lot was independently pressed in a Bucher Model RPL 18 membrane press to a maximum of 1.5 bars pressure. Free run and press fractions were combined for each lot and the juice cold clarified without the addition of sulfur dioxide or bentonite for  72 hrs. at 45°F (7°C). Upon completion of prefermentation clarification, juice and lees volumes were determined. Samples were also collected for analysis of Brix, pH., T.A., and suspended solids.

The clarified juice was then warmed to 55°F (13°C) and inoculated with recently rehydrated and acclimated active dry yeast (Saccharomyces cereviseae, Prise de Mousse). The heavy solids were discarded. Fermentation was then carried out at 55°F (13°C) in stainless steel fermentors. Upon completion of fermentation, each tank was cooled and post-fermentation clarification allowed to occur over a three-week period. At the end of the clarification period, sulfur dioxide (35 mg/L) and bentonite (360 mg/L) was added to each lot. Both lots were stored at 30°F (-1.1°C) for 60 days at which time they were filtered through a 0.45µ membrane and bottled.

RESULTS AND DISCUSSION

As indicated in the Table 1, the only significant difference between the two systems can be found in the prefermentation volume of lees (a difference of 3.2%). Indicating that the belt delivery system is indeed gentler (not as abrasive) as expected.

However, the post-fermentation data indicates that there is no significant difference in the flavonoid and non-flavonoid phenol content of both systems. Indicating that phenol extraction is not increased by the use of the auger delivery system.

ACKNOWLEDGEMENTS

We thank the California Agricultural Technology Institute (CATI) for generous financial support. Our gratitude to Candandaigua Wine Company, Canandaigua, N. Y., and Westec-IRAPP Healdsburg, CA for their donation of equipment used in this study.

The use of brand or company names in this report does not imply endorsement by California State University, the California Agricultural Institute (CATI),  the Viticulture and Enology Research Center (VERC), or the Enology Program at Fresno State.

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CALIFORNIA AGRICULTURAL TECHNOLOGY INSTITUTE - CATI
College of Agricultural Sciences and Technology
California State University, Fresno