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Helena Coelho: A scientific journey with cork


As part of the celebrations of Corticeira Amorim’s 150th anniversary and, as such, the celebration of the global history of the cork industry, I would like to leave my personal contribution about the path of scientific knowledge related to cork, since I have been an active participant in scientific research projects and have witnessed the technological innovation that has occurred in the industry.

From antiquity to the 1990s

People have appreciated cork’s unique properties for millennia and have used it for different applications: in military helmets and buildings, due to its thermal insulation capacity; in fishing articles and buoys, due to the buoyancy properties resulting from its low density and low water absorption; in plugs and stoppers, due to its impermeability and compressibility.

Cork has aroused major scientific curiosity and played an important role in the history of science. In the 17th century, in a period of great scientific curiosity, cork was one of the materials observed by the researcher Robert Hooke (1635-1703), using the microscope he was developing, which allowed him to establish that cork has a continuous structure of small alveoli or boxes (he was the first person to use the word “cell”), with different designs when viewed from the two planes of observation - tangential and transversal - of a cork board. This structure was widely disseminated at the time, through his book Micrographia: or some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon, published in London in 1665, which stirred up tremendous interest, and continues to be reproduced in books on plant biology. As interesting as the microscopic description of the structure itself is Hooke’s explanation of the properties of cork - lightness, flotation, compressibility and dimensional recovery, based on the microscopic characteristics he observed. He also describes the formation of cork bark in the tree: “… Having researched the History of Cork, I found its consideration as the outer bark of a certain tree, which is distinct from the two inner layers of bark, which are common to other trees. It takes some time before the cork layer, that covers the young and tender layers, become discernible. It easily tears, weakens and breaks producing very large cracks, leaving the inner bark underneath. It can be separated and removed from the Tree without, however, in any way injuring the two layers of inner bark... ”

Cork continued to arouse scientific curiosity, and this extended to the field of chemistry. Knowledge about this material grew during the 18th and early 19th centuries. The book Subericultura, by Joaquim Vieira Natividade (1899-1968), published in 1950, in Lisbon, reviews all the knowledge produced about cork up until that time. However, there were still important flaws in the knowledge about this material, in particular its chemical composition and the chemical structure of its specific component - suberin - and the relationship between its chemistry, structure and properties

My research journey related to cork

Forty years ago I was a young doctorate student, coming from Germany. I wanted to pursue scientific research in the areas where I had gained skills - chemistry of natural products and full use of resources - in an area that was relevant for Portugal. Economic data had shown me the importance of cork… Since those days I have been discovering this fascinating, complex material, that only slowly unveils its secrets.

I started with an overview of the cork manufacturing process, studying the extraction of the components of cork, for example the boiling operation of cork planks, and the chemical characteristics of virgin and amadia cork. The first publications that appeared in the Portuguese magazine “A Cortiça”, published by the Instituto dos Produtos Florestais, between 1979 and 1988, dealt with these topics.

Cork’s chemistry, structure and biology were subsequently the guiding themes of my research programme, including relevant aspects of the use of cork as an industrial raw material.
In the early 1980s, a structured plan promoted by the Minister of Industry, Ricardo Bayão Horta, that aimed to increase knowledge about the sector, made it possible to develop a modern period of research that initially involved a group from the Instituto Superior Técnico, led by Manuel Amaral Fortes, and my own group, from the Instituto Superior de Agronomia. This later expanded to other research groups in different universities and institutions. From this collaboration, a review book, A cortiça (Cork), was published in 2004, in Lisbon, edited by the authors Manuel Amaral Fortes, Emília Rosa and Helena Pereira, who provided an integrated synthesis of the existing knowledge, from the perspective of materials science, i.e. integrating the structure, chemistry and biology to support material properties and industrial operations of processing and applications.

Meanwhile, research into cork extended to multiple groups, in Portugal and other countries, mainly in Spain, and spanned different areas, ranging from fundamental studies of genetics and biology, and mainly of chemistry, to more in-depth understanding of properties such as the material dimension and the characteristics of cork products. The publication of scientific articles expanded exponentially during this period and the frontiers of knowledge have substantially broadened.

My research group has actively contributed to increasing knowledge in several areas. For example, significant progress has been made in the following areas:

  • cork’s structure, revisiting the pioneering work by Hooke and later by Natividade, determining the relevant parameters of this cellular material, its relationship with the biological formation process and its behaviour during application in products;
  • cork’s chemistry, in particular in relation to suberin, establishing the basic chemical structure of the polymer as a glycerol polyester with long chain aliphatic acids and alcohols, as well as in relation to lignin, determining, for the first time, its monomeric composition, and polymeric characteristics;
  • the importance of the chemical composition, namely of the relationship between suberin and lignin, and of the cellular structure, for the definition of cork’s main properties, also encompassing aspects of the natural variability that occurs;
  • cork’s technological quality as a raw material for industry, including studies on porosity and evaluation methods, as well as the performance of products, in particular in terms of the transfer of oxygen from cork stoppers.

In 2007, at the invitation of the international publisher Elsevier, I published a review book, “Cork: biology, production and uses” that summarises existing knowledge and lists all the publications to date.

Industrial innovation

During this period, I witnessed the industrial path taken by the sector and the amazing technological evolution and industrial innovation. In the first visits I made to our industrial units, including small factories with a few workers and large complexes, such as Mundet or Robinson, which no longer exist, many of the operations had characteristics that resembled the early years of the industrial revolution, with predominant use of manual labour, poorly lit environments, and a mentality governed by supposed trade secrets and empirical truths that could not be shared. I remember not being allowed to visit the room used for storing and maturing the cork planks after they had been boiled…

What is happening today, and for several years now, is quite different, with a profound change in practices and with full adoption of quality and control criteria. The flows of the industrial process have been optimized and rationalised, automated production methods are applied in several operations, new equipment. Above all, there is a climate of transparency and valuation of knowledge. I would like to mention two emblematic examples:

  • the boiling of cork planks, in automated autoclaves with controlled temperature and timing, with water circulation and volatile removal circuits, and without the maturation of boiled planks in a closed chamber, all carried out in a clean environment, ventilated and without any microbial contamination;
  • the classification of stoppers and discs using artificial intelligence, based on information from the surface obtained by image analysis, with algorithms adapted to the client and with automated processes.
  • The industrial innovation that has been adopted followed the increased knowledge about cork and has gained its own momentum with an almost continuous process. My visits to cork factories now always show me something new that didn’t exist on the previous visit. But the great driver has been the existence of dialogue and interaction between the cork industry and researchers, creating a climate of valorisation of scientific and technological knowledge.