)riginalarbeiten 9 Original! )riginalarbeiten 9 Original! I Holz als Roh- und Werkstoff 52 (t994) zn-zl4 9 Springer-Verlag 1994 Yield and quality in the production of cork stoppers H. Pereira, B. Melo, R. Pinto Bottle stoppers of natural cork are produced from cork planks as cylinders which are bored in the axial direction. The process was followed for the production of 38 mm x 24 mm (length x diameter) stoppers, using cork planks of different quality grades. The mass yield is on average 24% of the initial cork with net productions of 98 stoppers/kg and 747 stoppers/m 2. The net yields are higher when using the best quality raw-material due to an increasing number of refused stoppers with the decrease in quality of the cork planks. The quality profiles of the stoppers are determined by the quality of the raw-material: i.e. xst grade cork planks produce 82% of high quality stoppers (superior and 1st class) and 5th grade planks only 22%. The results obtained allowed to establish correla- tions between the number of stoppers produced and raw-material parameters, namely the area and dry weight, which can be used to estimate production from a given lot of cork planks. Ausbeute und Oualit~t bei der Herstellung von Korkstopfen Korkstopfen werden aus Korkplatten als Zylinder in axialer Richtung ausgebohrt. Dieser Herstellungsprozefl for Stopfen mit den Abmessungen 38 mm x 24 mm (L~inge x Durchmesser) wurde unter Verwendung yon Korkplatten unterschiedlicher Qualit~t aberprtift. Die Massenausbeute betragt im Schnitt 24% des Ausgangsmaterials mit einer Nettoproduktion yon 98 Stopfen/kg bzw. 747 Stopfen/mL Bei Verwendung hochwertigen Ausgangsmaterials liegen bei Ausbeuten h6her als bei Korkplatten minderer Qualit~it, weft der Anteil am Ausschufl mit fallender Qualit~it der Korkplatten ansteigt. So erh~ilt man z.B. aus erstklassigen Korkplatten 82% hochwertige Stopfen, w~ihren aus Korkplatten der Gtiteklasse 5 nur 22% hochwertige Stopfen hergestellt werden kOnnen. Aus den Untersuchungsergebnissen konnten Korrelationen zwischen der Anzahl der m6glichen Stopfen und den Qualit~itskriterien des Rohmaterials abgeleitet werden. Vor allem die Fl~iche und das Trockengewicht der Korkplatten k6nnen zum Absch~itzen der Endproduktion herangezogen werden. 1 Introduction Stoppers of natural cork are used worldwide for the bottling of quality wines. Most of the world production is concentrated in Portugal, where approximately z5 thousand tons per year of cork stoppers are produced and exported to the wine rich countries, mainly of the EC. The cork stoppers are the most valuable prod- uct of the cork industry: they represent ca. 17% in quantity of all Portuguese cork exports and 53% in value. Quality is a major parameter for establishing prices and a good quality stopper is 4-5 times more expensive than a low grade cork (at present good quality stoppers may cost ca. 80 Ecu/looo stoppers). Departamento de Engenharia Florestal, Instituto Superior de Agronomia, P-1399 Lisboa Codex, Portugal Dedicated to Prof. Dr. D. Fengel on occasion of his retirement The stoppers are bored as cylinders from the cork planks so that the cylinder axis is in the axial direction of the plank. For that purpose, the planks are previously cut transversely into strips with an height corresponding to the length of the stopper with ca. 2 mm in excess. Obviously, the thickness of the cork planks must be larger than the diameter of the stopper. Different dimensions are in use, but most wine stoppers are produced with 38 mm x 24 mm and 45 mm x 24 mm (length x diameter), and are bored from planks with an average thickness of4o mm. The overall process of cork stoppers production is schemati- cally represented in Fig. 1, although some deviations to the gener- al procedure may occur following mill objectives and organiza- tion or client specifications. The cork planks are immersed in boiling water during 1-1.5 h or ca. 3o min, respectively if this is the first boiling treatment or if they had already been previously treated, stacked in piles and allowed to rest for some days. This operation will flatten the planks, induce some thickness increase and facilitate the subsequent cutting steps (Rosa et al. 199o). If the planks are boiled for the first time, they will be trimmed, cut to dimensions and separated into quality classes. The planks are then cut transversely into strips and the stoppers are bored as cylinders as explained. A first classification will sort out broken and damaged stoppers after which the stoppers are dried and dimensionnaly rectified in length by sanding. There follows a washing/bleaching step with the subsequent drying, and a quality classification, usually into 7 classes and refuse. A surface treat- ment with waxes, silicone or other products will constitute the finishing operations. Large quantities of residues are obtained in this process, rep- resenting approx. 75% of the initial cork weight, which will be granulated and further processed into agglomerates. It is known from the industrial practice that the quality of the cork plank used for stoppers' production is important for establishing yields and quality profiles. However there have been no systematic studies on the influence of the quality of the raw-material in yields and quality distribution of the stoppers. In this paper we give experimental results on the production of cork stoppers from cork planks of different qualities, in relation to mass yields for each process step and to the corresponding quality profiles. 2 Material and methods The material used was made up of commercially obtained cork planks with a thickness of 34-4o mm (trade type "marca"), bought from a cork preparation industry, as five lots of different qualities (1st to 5th quality) with 4o planks each. The total weight of cork processed was 213 kg. Each plank was weighted and the area and thickness (average of the four sides) were measured. The process of stoppers' production was carried out by an important cork factory, using automatic boring of 38 mm x 24 mm stoppers and following the usual mill procedures for the production of unbleached stoppers. The following steps were covered: water boiling for o.5 h; piling and rest for 5 days; cutting into strips; boring of stoppers; drying at 6o-7o~ during 2o h; 211