English oak / pedunculate oak, Quercus robur (Fagaceae)

Quercus robur, commonly known as English oak or pedunculate oak, is one of the most emblematic temperate forest trees in Europe and holds exceptional historical, ecological and cultural significance. Belonging to the family Fagaceae, it is a majestic, long-lived tree, capable of exceeding 30–40 metres in height and surviving for several centuries, in some cases even close to a millennium. Its powerful trunk and wide, irregular crown make it a true structural pillar of many lowland and riparian forests in central and northern Europe.

Morphologically, Quercus robur develops a massive bole, often deeply furrowed in mature individuals, with a thick, dark grey bark that becomes increasingly fissured with age. The leaves are deciduous and relatively short-stalked, with a blade that is characteristically deeply lobed, the lobes broad and rounded, giving the species a very recognisable foliar outline. During the growing season, the leaves are a deep green, turning to yellowish or brownish tones in autumn before abscission. The male flowers are grouped in long, pendulous catkins that appear in spring, while the female flowers, more discreet and solitary or in small clusters, give rise to the typical acorns, the fruits that are central to the species’ ecological cycle.

From an ecological standpoint, Quercus robur is one of the characteristic trees of lowland oak forests and riparian woodlands. It favours fresh, deep, well-drained soils with good moisture availability, although it can adapt to a fairly wide range of edaphic and climatic conditions. English oak acts as a foundation species in many forest ecosystems, supporting an exceptionally rich biodiversity. Numerous ecological surveys have shown that a single mature oak can host hundreds of associated species, including insects, fungi, lichens, mosses and epiphytic plants, effectively functioning as a complex microhabitat platform. The acorns are a key food resource for many animals, such as birds, small mammals and ungulates, and play a major role in zoochorous seed dispersal and in the natural regeneration of oak-dominated stands.

Historically and culturally, Quercus robur has long been associated with ideas of strength, endurance and longevity, and is deeply rooted in European symbolism, from Norse mythology to medieval and modern literature. Its wood, notable for being hard, heavy and highly durable, has been extensively used for shipbuilding, construction, high-quality furniture and, in particular, for casks used in the ageing of wines and spirits. In cooperage, oak wood contributes a range of aromatic and phenolic compounds, notably tannins and ellagitannins, which influence the flavour, structure and ageing potential of beverages stored in oak barrels.

Chemically and from a phytotherapeutic point of view, Quercus robur is characterised by a high content of tannins, especially ellagitannins, concentrated predominantly in the bark, but also present to a lesser extent in the leaves and outer layers of the acorn. Oak bark has a long tradition of use in herbal medicine for its astringent, anti-inflammatory and mildly antimicrobial properties. Topical preparations based on bark extracts have been employed in cases of minor skin irritations, small wounds, dermatitis and mucosal disorders, where the tannins help precipitate proteins, reduce exudation and support the restoration of irritated or inflamed surfaces.

The acorns, once properly processed to remove excess bitterness due to tannins, have been used in various traditional cultures as an emergency food or as a flour ingredient for bread and porridges. In recent years there has been renewed interest in acorn flours from a nutritional and gastronomic perspective, due to their content of complex carbohydrates, dietary fibre and selected bioactive molecules, although this remains a relatively niche use.

Botanical classification (APG IV system)

Indicative nutritional values per 100 g (oak acorns, including Quercus robur)
Values refer to raw acorns and are indicative for the genus Quercus; composition can vary considerably with species, maturity and environment.

In typical traditional uses (for example 20–30 g of acorn flour mixed with cereal flours, after proper processing), the caloric contribution is high relative to the serving, with energy coming mainly from starch and lipids.

Plant Characteristics

Quercus robur can grow up to 40 meters (130 feet) tall and has a wide, spreading canopy. The leaves are lobed with rounded edges and turn yellow-brown in the autumn. It produces small, greenish flowers that develop into acorns with a distinctive cup-shaped base. The tree's bark is deeply ridged and fissured, providing a rough texture.

Chemical Composition and Structure

The oak tree, particularly its bark and acorns, contains various beneficial compounds:

• Tannins: Found in the bark and acorns, tannins have astringent properties and are used in traditional medicine and tanning processes.
• Triterpenoids: Found in the bark and acorns
• Flavonoids: Includes compounds such as quercetin, which have antioxidant properties.
• Saponins: Present in the acorns, these compounds have potential antimicrobial and anti-inflammatory effects.
• Phenolic Acids: Contribute to the tree's antioxidant properties and are found in the bark and leaves.

Cultivation

Quercus robur thrives in a range of soil types, from clay to sandy soils, and prefers well-drained conditions. It is suitable for a variety of climatic conditions but does best in temperate zones. The tree is propagated from acorns and requires ample space for growth. It benefits from occasional pruning to maintain a strong structure and to manage its large size. It is relatively low-maintenance once established but can be susceptible to pests and diseases.

Uses and Benefits

• Wood: The wood of Quercus robur is highly durable and used in construction, furniture making, and for making barrels, especially for aging wine and spirits.

• Medicinal: Oak bark and acorns have been used in traditional medicine for their astringent, anti-inflammatory, and antimicrobial properties. They are often used in herbal remedies for skin conditions and digestive issues.

Cosmetics:

INCI Functions:

Skin conditioning agent. It is the mainstay of topical skin treatment as it has the function of restoring, increasing or improving skin tolerance to external factors, including melanocyte tolerance. The most important function of the conditioning agent is to prevent skin dehydration, but the subject is rather complex and involves emollients and humectants that can be added in the formulation.

Cosmetic astringent. This ingredient exerts a direct effect on the skin by tightening dilated pores by contracting stratum corneum cells and removing superfluous oil.

Antioxidant agent. Ingredient that counteracts oxidative stress and prevents cell damage. Free radicals, pathological inflammatory processes, reactive nitrogen species and reactive oxygen species are responsible for the ageing process and many diseases caused by oxidation.

• Ecological: The tree supports a wide range of wildlife, including birds, insects, and fungi, making it an important component of forest ecosystems.

Applications

• Woodworking: Used in the production of high-quality furniture, flooring, and construction materials due to its strength and durability.

• Medicinal: Incorporated into herbal remedies and traditional medicine for its therapeutic properties.

• Ecological: Planted in reforestation projects and conservation efforts to support biodiversity.

Environmental and Safety Considerations

Quercus robur is generally safe for use in woodworking and traditional medicine. However, care should be taken when using oak bark and acorns, as their high tannin content can cause irritation if not used properly. Sustainable harvesting and management practices should be employed to ensure the health of oak populations and their ecosystems.

References__________________________________________________________________________

Šukele R, Skadiņš I, Koka R, Bandere D. Antibacterial effects of oak bark (Quercus robur) and heather herb (Calluna vulgaris L.) extracts against the causative bacteria of bovine mastitis. Vet World. 2022 Sep;15(9):2315-2322. doi: 10.14202/vetworld.2022.2315-2322. Epub 2022 Sep 28. PMID: 36341072; PMCID: PMC9631361.

Kebert M, Vuksanović V, Stefels J, Bojović M, Horák R, Kostić S, Kovačević B, Orlović S, Neri L, Magli M, Rapparini F. Species-Level Differences in Osmoprotectants and Antioxidants Contribute to Stress Tolerance of Quercus robur L., and Q. cerris L. Seedlings under Water Deficit and High Temperatures. Plants (Basel). 2022 Jun 30;11(13):1744. doi: 10.3390/plants11131744. 

Abstract. The general aim of this work was to compare the leaf-level responses of different protective components to water deficit and high temperatures in Quercus cerris L. and Quercus robur L. Several biochemical components of the osmotic adjustment and antioxidant system were investigated together with changes in hormones. Q. cerris and Q. robur seedlings responded to water deficit and high temperatures by: (1) activating a different pattern of osmoregulation and antioxidant mechanisms depending on the species and on the nature of the stress; (2) upregulating the synthesis of a newly-explored osmoprotectant, dimethylsulphoniopropionate (DMSP); (3) trading-off between metabolites; and (4) modulating hormone levels. Under water deficit, Q. cerris had a higher antioxidant capacity compared to Q. robur, which showed a lower investment in the antioxidant system. In both species, exposure to high temperatures induced a strong osmoregulation capacity that appeared largely conferred by DMSP in Q. cerris and by glycine betaine in Q. robur. Collectively, the more stress-responsive compounds in each species were those present at a significant basal level in non-stress conditions. Our results were discussed in terms of pre-adaptation and stress-induced metabolic patterns as related to species-specific stress tolerance features.

Pérez AJ, Pecio Ł, Kowalczyk M, Kontek R, Gajek G, Stopinsek L, Mirt I, Oleszek W, Stochmal A. Triterpenoid Components from Oak Heartwood (Quercus robur) and Their Potential Health Benefits. J Agric Food Chem. 2017 Jun 14;65(23):4611-4623. doi: 10.1021/acs.jafc.7b01396. 

Abstract. For centuries oak wood (Quercus robur) has been used in aging of wines and spirits, which is based on pleasant flavors given to beverages by phenolics transferred to the liquid during the maturation process. Other metabolites, such as triterpenoids, can also be released. Searching for extractable triterpenoids in oak heartwood, 12 new, 1-12, and five known, 13-17, oleanane types were isolated and characterized. Their cytotoxicities were tested against cancer cells (PC3 and MCF-7) and lymphocytes. Breast cancer cells (MCF-7) were the most affected by triterpenoids, with roburgenic acid, 4, being the most active compound (IC50 = 19.7 μM). Selectivity was observed for compounds 1-3, 8, 9, and 16, exhibiting an IC50 > 200 μM against lymphocytes, while active against cancer cells. A galloyl unit attached to the triterpenoid moiety was established as the key feature for such effect. These results highlight the occurrence of triterpenoids in oak heartwood and their relevance for chemoprevention of breast cancer.

Gammacurta M, Waffo-Teguo P, Winstel D, Dubourdieu D, Marchal A. Isolation of Taste-Active Triterpenoids from Quercus robur: Sensory Assessment and Identification in Wines and Spirit. J Nat Prod. 2020 May 22;83(5):1611-1622. doi: 10.1021/acs.jnatprod.0c00106. 

Abstract. Six new triterpenoids (1-6), two known genins (7 and 8), and five known functionalized triterpenoids (9-13) were isolated from a Quercus robur heartwood extract. The purification protocol was guided by LC-HRMS by searching for structural analogues of bartogenic acid on the basis of their putative empirical formula. The structures of the new compounds were unequivocally elucidated using HRESIMS and 1D/2D NMR experiments. Sensory analyses were performed in water and in a non-oaked white wine on the pure compounds 1-13 at 5 mg/L. All molecules were perceived as bitter in water and wine, but they were mostly reported as modifying the wine taste balance. Using LC-HRMS, compounds 1-13 were observed in oaked red wine and cognac and were semiquantified in oak wood extracts. The influence of two cooperage parameters, oak species and toasting process, on compounds 1-13 content was studied. All compounds were found in quantities significantly higher in pedunculate than in sessile oak wood. Toasting is a key step in barrel manufacture and modulates the concentration of the discussed compounds. Significantly higher quantities were observed in untoasted wood compared to medium or highly toasted wood. These findings provide new insights into the molecular origin of taste changes due to oak aging.