Acorns of the Holm oak (Quercus ilex) have been part of the landscape of Mediterranean forests and traditional agroforestry systems like the Spanish dehesa and Portuguese montado for centuries. Although they are an indispensable food for wildlife and Iberian pigs, in human nutrition they most often remain merely a curiosity or a memory of war and post-war years. Modern science now shows in detail that this is a nutritionally very valuable and functional ingredient that fits perfectly into the trend of healthy and sustainable nutrition.

Recent research by a team from the University of Córdoba and the IFAPA Alameda del Obispo center, published in the journal Future Foods on September 6, 2025, brings the most comprehensive cross-section of the chemical, nutritional, and metabolomic profile of Holm oak acorns to date. Analyzing fruits from 14 individual trees, researchers compared the composition of starch, fatty acids, minerals, and a series of bioactive molecules, while also tracking differences in metabolomic signatures linked to the genetics of the tree and the microbial community on the surface and in the interior of the seed.

Such an approach shifts the focus from the stereotypical image of the "bitter acorn for pigs" toward a sophisticated understanding of the acorn as a potential raw material for gluten-free flour products, nutritionally rich snacks, or designed functional foods. The research not only confirms that the acorn can be nutritious but also suggests how to recognize the best specimens for human consumption, which is a key prerequisite for the future selection of trees and the development of a sustainable market for acorn products.

The Holm oak as a symbol of the Mediterranean forest and forgotten food

The Holm oak is one of the most recognizable evergreen oaks in the Mediterranean belt. Its dense canopies provide shade for livestock, contribute to soil protection against erosion and moisture retention, and shoots regenerate the forest relatively quickly after logging or fire. In systems like the dehesa, where forestry, grazing, and agricultural use are combined, the Holm oak acorn is the key energy base for raising pigs famous for hams with a designation of origin.

Despite such ecological and economic importance, human consumption of acorns has almost disappeared in the greater part of the Mediterranean. Historical records and ethnographic studies testify that acorn flour was traditionally used for bread, porridges, or as an addition to cereals in years of scarcity. Today, apart from a few local specialties, the acorn is mainly considered animal feed, although global trends in the search for sustainable, local, and gluten-free carbohydrate sources are reopening space for its return to menus.

In recent years, projects dedicated to the valorization of acorns have been lining up in the European research space – from mapping the main oak species and their acorns, through the development of pilot plants for acorn flour production, to testing new gluten-free products and analyzing health effects. In this context, the new research from Córdoba connects as a fundamental link: it describes in detail exactly what is found in the Holm oak acorn and how much the composition can vary from tree to tree.

Starch, healthy fats, and minerals: what hides in the Holm oak acorn

One of the key findings of the study is the confirmation that the Holm oak acorn belongs to the group of fruits rich in starch, with a share that makes it an interesting alternative to classic cereals in flour production. Unlike many nuts, the acorn contains a relatively high proportion of complex carbohydrates, thanks to which it can give dough a structure comparable to that of wheat or corn, but without the presence of gluten.

The fatty acid profile of Holm oak acorn oil is particularly attractive: unsaturated fatty acids predominate, with a high proportion of oleic acid, similar to that in olive oil. This places the acorn in the group of foods that contribute to a favorable lipid profile of the diet, associated with a lower risk of cardiovascular diseases when replacing sources of saturated fats.

Along with starch and healthy fats, the acorn also contains significant amounts of dietary fiber which contribute to satiety and proper function of the digestive system. Fiber in combination with complex carbohydrates slows down the rise of glucose in the blood, which is interesting in the context of nutrition for persons monitoring glycemic response, although additional clinical research is needed for concrete health recommendations.

Researchers also recorded the presence of minerals such as iron, calcium, and sodium in the analyzed acorns. Iron is important for the creation of red blood cells and oxygen transport, while calcium contributes to bone and teeth health. The combination of carbohydrates, fats, fibers, and micronutrients makes the acorn an interesting candidate for so-called "forest food" – food from the forest that can supplement the diet for persons with limited access to conventional crops, especially in arid and marginal areas.

Phenolic compounds and secondary metabolites: antioxidant and anti-inflammatory potential

Besides basic nutrients, the Holm oak acorn is rich in diverse phenolic compounds and other secondary metabolites. In numerous papers on different oak species, phenolic acids, flavonoids, and tannins are linked to antioxidant, anti-inflammatory, and antimicrobial activity. In the seed itself, these compounds participate in defense against pathogens, UV radiation, and herbivores, while for humans they represent potential functional food ingredients.

The study from Córdoba combined targeted analyses of well-known phenolic compounds with non-targeted metabolomic approaches, whereby a series of molecules were discovered that have only recently been described in scientific literature or their occurrence in the Holm oak acorn is now reported for the first time. Although the mere presence of these compounds does not automatically mean a health benefit, their antioxidant properties open space for further research into the nutraceutical potential of the acorn.

Part of these secondary metabolites can have a dual character: on one hand, they contribute to the bitterness and astringency of the acorn, which limits its immediate acceptability to consumers, while on the other hand, exactly the same compounds can, after appropriate processing and dosing, become desirable functional ingredients of food or dietary supplements. Understanding the balance between sensory properties and biological activity is therefore crucial for the development of acorn-based products.

It is important to highlight that traditional preparation techniques – soaking, rinsing, and thermal processing – significantly reduce the share of tannins and other compounds responsible for bitterness, which newer reviews on the safety and utility of acorns in the diet also confirm. This opens the possibility that a part of the phenolic compounds is preserved in amounts that contribute to the antioxidant effect, while simultaneously reducing the risk of negative effects of excessive intake.

Why every acorn is different: lack of domestication and genetic diversity

One of the starting points of the research is the fact that the Holm oak is not a classically bred or "domesticated" crop. Unlike wheat, corn, or grapevines, where humans have selected relatively uniform varieties over millennia, in natural populations of the Holm oak almost every tree carries a unique combination of genes and environmental influences.

The consequence of this is extremely high variability among acorns: some are large and almost without bitterness, others tiny and very bitter; the composition of starch, fats, and phenolic compounds also varies. For a consumer gathering acorns in the forest or for a producer thinking about commercial flour, that unevenness means uncertain raw material quality and makes product standardization difficult.

The research team, using a combination of spectroscopic and chromatographic techniques, showed that despite the relatively small number of analyzed trees, clear groups of acorns can be distinguished. Two main groups were observed: large acorns without pronounced bitterness and medium-large but very bitter acorns. Both groups share a general picture of high starch content and dominant unsaturated fatty acids, but differ in the detailed metabolomic profile.

Metabolomic "fingerprint" maps of acorns proved to be so specific that for most samples it could be reliably determined from which tree they originated. Such a level of distinction resembles a chemical "fingerprint" and opens the possibility that in the future acorns will be sorted and classified not only by size and taste, but also by molecular indicators of nutritional and functional quality.

Acorn microbiome: hidden allies in the seed

Yet another intriguing aspect of the study is the identification of a large number of metabolites of microbial origin. The analysis showed that the surface and interior of the acorn are not sterile, but inhabited by a diverse community of microorganisms that leave a recognizable chemical trace. These microbes can participate in the degradation of certain compounds, the synthesis of new metabolites, or influence the defense of the seed against pathogens.

Researchers suggest that the microbiome could also have a role in the formation of bitterness, that is, that certain microbial groups could encourage the creation or degradation of tannins and related compounds. If this link is confirmed in future papers, the possibility opens that the acceptability of the acorn for human consumption is improved not only by classical tree selection but also by targeted management of the microbiome during storage and processing.

The concept of "food with a microbiome" is already well known through fermented products like yogurt, sauerkraut, or kombucha. The Holm oak acorn, with its own microbial community, could in the future be viewed in a similar light – as a raw material whose nutritional and sensory properties are shaped by the interaction of plant and microbial metabolites.

Methods of analysis: from NIRS to mass spectrometry

To encompass such compositional complexity, researchers applied a combination of "holistic" and targeted analytical techniques. Near-infrared spectroscopy (NIRS) enabled rapid, non-destructive assessment of basic components like moisture, starch, and fats on a large number of samples. Colorimetric reactions served for the quantification of certain groups of compounds, for example, total phenols.

For detailed insight into individual molecules, high-performance liquid chromatography (HPLC) in combination with mass spectrometry (LC-MS/MS) was used, which is today the standard in metabolomic food research. This approach enables simultaneous monitoring of hundreds of compounds, from simple organic acids and amino acids to complex polyphenols and microbial metabolites.

Although the number of trees in the study is limited, results show that already on such a small sample, the great internal diversity of the species is clearly outlined. This is an important signal for future projects: to fully utilize the potential of the acorn as a foodstuff, it is necessary to systematically map the morphological, chemical, and genetic features of a large number of trees in different regions of the Mediterranean.

The acorn as a functional food: between tradition and new products

Parallel to fundamental metabolomic research, the number of papers examining the practical use of acorns in food products is growing. Flour from Holm oak acorns and related species has already been tested in gluten-free bread, biscuits, and pasta, where it contributes to structure and aroma, while simultaneously increasing the share of fiber and phenolic compounds in the final product.

Review papers on the acorn as a "modern sustainable resource" highlight that traditional use from the past – when the acorn served as a substitute for cereals in hard times – can today be upgraded into completely new concepts of functional food. This includes acorn-based beverages, plant-based caffeine-free "coffees", energy bars, or fermented products in which microorganisms additionally modify the composition and bioavailability of phenolic compounds.

However, the acorn is not without challenges. Tannins and other phenolic compounds responsible for bitterness require proper processing – soaking, multiple rinsing, or controlled thermal processing – in order to reduce astringency and remove potentially harmful excesses. Because of this, the technological development of acorn processing is just as important as the agricultural and genetic selection of trees itself.

Safety aspects also require attention. Although the acorn is traditionally consumed in many cultures, modern research reminds us that tannins in raw acorns in larger quantities are potentially harmful and that the acorn belongs to the group of tree nuts, which means it can cause allergic reactions in sensitive persons. Therefore, it is important for the food industry to establish clear guidelines for processing, labeling, and quality control of acorn products.

Acorn domestication: how to choose the "best" trees

One of the most interesting conclusions of the study is the message that the future "domestication" of the Holm oak should not be conducted according to the model of intensive agriculture that reduces genetic diversity to a few highly productive varieties. Instead, the creation of a catalog of trees with desirable properties is proposed – for example, low bitterness, stable high starch content, favorable fatty acid profile, or elevated content of certain bioactive compounds.

Such a catalog could also encompass data on the region, soil type, climate, and present microbiome, thereby enabling selection to be guided not only by genotype, but also by the interaction of the tree with the environment. Based on this information, it would be possible to design breeding programs that combine natural diversity with targeted planting of trees for the production of acorns intended for human consumption.

At the same time, researchers emphasize that it is important to avoid a scenario in which intensive selection and plantation establishment would lead to the loss of genetic richness of natural populations. Instead of "replacing" wild forests with monocultures, the idea is to develop a network of selected trees and forest stands that are used as seed sources and supplements to traditional agroforestry systems.

In that sense, the study of the Holm oak fits into the broader picture of European projects that strive to revalorize forest fruits and include them in sustainable nutrition models. Systematic data collection on chemical composition, health effects, and consumer preferences creates foundations for the acorn to cross from the niche of "forgotten food" into the category of a recognizable Mediterranean ingredient.

What follows: from the laboratory to wider application

The authors of the study highlight several open questions that will mark the next phase of Holm oak acorn research. First, it is necessary to examine in detail the biological activity of identified compounds – especially those newly discovered or rarely described – through in vitro and in vivo models, and long-term through clinical research on humans. Only such studies will enable individual acorn ingredients to be justifiably called nutraceuticals.

Second, it is important to expand the number of analyzed trees and include different bioclimatic zones of the Mediterranean, from southwest Iberia to the eastern Adriatic and North Africa. By doing so, hotspots of particularly high-quality populations could be mapped, but also how climate, soil, and forest management influence acorn composition could be better understood.

Third, results of metabolomic and microbiome analyses should be linked with technological tests of flour, oil, and other acorn products – from dough rheology and sensory evaluations to the impact on shelf life and microbiological stability. Only such integration of laboratory and applied data can lead to concrete innovations on the market, such as new gluten-free bakery products or acorn-based snacks.

Finally, Holm oak acorn research fits into the broader endeavor to recognize forest ecosystems not only as sources of timber mass, but also as a source of diverse food of high added value. In a time when food supply security, waste reduction, and adaptation to climate change are at the center of public debates, the rediscovery of the acorn's potential offers a concrete example of how traditional knowledge and modern analytical technology can together shape a more sustainable future of nutrition.