Joan Montserrat
www.balanotrees.org
info@balanotrees.org

Published in May 2021 in International Oaks No. 32: 60-74

ABSTRACT

BalanoTrees, a nursery created by Joan Montserrat and Francesc Ribera specialized in the selection and production of oaks that bear sweet acorns, has been locating and cataloging oaks that bear sweet acorns across the Iberian Peninsula and on the island of Mallorca for five years.

The idea that acorns were an important food source in human history and proto-history is generally accepted today but explanations as to why they were abandoned vary; some stressing the presumed difficulties in their cultivation and others the ideological component represented by cereal agriculture in developing central states.

Reviving interest in acorns-as-food has gained considerable momentum in the recent past and, in this context, the Iberian Peninsula’s historical dehesa model has attracted much attention.

The present paper explores different aspects of the acorn-as-food, starting with a not-so-simple question: what is a truly “sweet acorn”?

Introduction

Back in 1929 J. Russel Smith wrote about sweet acorns and their potential in his visionary book Tree Crops: A permanent agriculture. Since then, very few efforts have been made to find the best acorns for human consumption, not to mention carrying out formal breeding. For some of us, passionate about recovering forgotten food plants and their use as future sustainable crops, the elusive sweet acorn has almost become a mythological being. Do really sweet acorns exist? Can they be eaten directly without processing? Are they really as good as almonds, walnuts or chestnuts? In this article I try to shed some light on this matter. Although there are many things we don’t know about them yet, I can say with confidence that the answers to the previous three questions are: yes, yes and yes.

Interest in bringing the acorn-as-food back from oblivion has been on and off for decades. The nut of the oak tree is highly nutritious and is readily available for harvest by anyone willing to compete with boars, squirrels and jays. Around the world today, there are very exciting initiatives focused on promoting traditional acorn foods (like those of California’s native Indians or the Korean acorn jelly dotorimuk) as well as others devoted to developing new uses for the culinary acorn (from cookies and pasta to drinks and fermented foods). Two of the most long-lived and inspiring endeavors in this area are OAKMEAL in Greece and Freixo do Meio in Portugal. Most of these initiatives use bitter acorns so leaching or some other kind of processing is needed to reduce the tannin concentrations to make the acorns edible.

Processed food products based on bitter acorns on one hand, and whole kernel sweet acorns on the other, are synergistic pathways to restoring the glory of the oak nut. The advantage of using sweet acorns to elaborate processed foods is the simplification of the production process, since leaching is not necessary. Equally, their use is a definite plus in terms of flavor.

To be or not to be sweet

First of all we need to clarify what a “sweet acorn” is because the term is used around the world in different ways. Most of the time, when people talk about sweet acorns they are referring to acorns that are less bitter or astringent than those usually found growing on their local oaks. These “not too bitter” acorns would be easier to process or could, if needed in difficult times, be eaten directly. But when I use the term “sweet acorn” I am referring to acorns that our 21st century, spoiled, taste buds can not only tolerate but can really enjoy. True “sweet acorns” in this sense require no processing other than minimal drying – and they taste as good as any other commercially produced nut.

BalanoTrees has been locating and cataloguing oaks that bear sweet acorns across the Iberian Peninsula and on the island of Mallorca for five years. We have found truly sweet acorns on very few individual trees, all of them holm oak* , and most of them located in the dehesa “agrosylvopastoral” system of southern Spain and Portugal, with some also to be found in Mallorca’s agricultural fields. In both cases, the trees have been tended by humans for millennia and this, in turn, implies a certain degree of domestication.

Holm oak produces one of the least bitter acorns of all oaks, but the variation in tannin content from tree to tree is quite significant and most acorns are not truly sweet. They are astringent to varying degrees and still need leaching especially if they are to be eaten in large quantities.

Reports of sweet acorns from other oak species from around the world abound. I haven’t tasted many of them, but I tend to be skeptical since they probably refer to acorns that have little to no astringency/ bitterness but that are not sweet (as defined above) and are therefore far from having the gastronomic value of other nuts commonly consumed unprocessed. There are though two cases that seem quite convincing. One of them is about the acorns from the Q. suber forest of Maâmora, near Rabat in Morocco. There, seasonal acorn consumption remains quite popular and acorns can be found in food stands on the streets of towns and cities close by. The other refers to acorns from a Q. alba, the Lint White Oak, from the state of Pennsylvania in the USA. This tree was the winner of a nationwide White Oak contest organized by the Tennessee Valley Authority (TVA) in the 1930s, with the goal of identifying oak selections from across the United States for human and livestock use. Many low-tannin “sweet-acorn” oak specimens were submitted, including a promising, high-yielding Q. montana, but the winner of the White Oak group was the Lint White, whose acorn is reported to be as sweet and tannin-free as a chestnut. Unfortunately, the project was short lived due to the reallocation of funds for World War II. Many of the trees from the TVA contest were subsequently lost in the landscape or cut down. Only recently, the original Lint White Oak was tracked down by The Fruit Explorers, Eliza Greenman and Taylor Malone, after uncovering a map of the tree’s location in an archive. It is expected that grafted trees of the Lint White Oak will be available soon.

Theoretically any oak species might include individuals that bear sweet acorns, but logically the chances are greater with species that naturally produce low-tannin acorns. In addition to those mentioned above, these include: Q. mongolica, Q. emoryi, Q. muehlenbergii, Q. macrocarpa and Q. virgiliana**.

* The common name, holm oak, that designates all of the forms in the “ilex complex”, will be used throughout this article for the following reasons: 1) due to the high degree of polymorphism, it is often difficult to determine whether the sweet-acorn-bearing trees in Portugal and Spain are Q. ilex, Q. rotundifolia or hybrids between the two; 2) Q. ilex subsp. ballota, is considered by many, but not all, authorities to be a non-valid synonym of Q. rotundifolia; 3) Q. ilex subsp. ballota is the name that BalanoTrees is required to use when commercializing plants. For a detailed analysis of the Q. ilex complex, see, for example, Lebreton et al. 2013; Michaud et al. 1996; Rafii et al. 1991a, 1991b; Solla et al. 2016.

** Quercus virgiliana is a synonym of Q. pubescens.

What makes an acorn sweet?

Little research has been done on this matter, but it seems that there is one main character involved, tannins, and two secondary roles, sugars and lipids. Tannins, that biochemically belong to the polyphenol family, are responsible for the astringency in acorns and many other plant foods. They are a diverse group of complex substances whose biological function is to ward off predators. Traditionally they were used for tanning leather, hence the name. Tannins have antioxidant properties beneficial for human health, but, in high concentrations, such as in most acorns, they act as antinutrients that block the absorption of minerals and therefore can be harmful. Due to their diversity, and the fact that they evolve with time and interact with other nutrients, their organoleptic properties are difficult to assess. Lab analyses on tannin content are complex and they are not included in most acorn nutritional data sheets. In the end, correlation between tannin content and astringency or bitterness is not as direct and simple as one would think.

There are acorns that are so low in tannins that when eaten fresh, directly from the tree, they show no astringency or bitterness at all, but at the same time they are as dull as a piece of cork and therefore not interesting to eat as a nut. Fernando Pulido – a Forestry Professor at the University of Extremadura and a long-time promoter of using acorns as human food – calls them neutral acorns (pers. comm.). These neutral acorns demonstrate two interesting things: first, a low tannin content is a necessary but not sufficient condition to make a sweet acorn, a high sugar content and a certain lipid composition is also needed for a pleasant taste; and second, tannins contribute to an intense and complex taste in acorns, especially after a light drying of the nuts. David Morcuende, a food science researcher and a colleague of Fernando’s at the University of Extremadura, compares the contrast of sweetness and bitterness found in the best tasting acorns to that of coffee or chocolate with added sugar (pers. comm.). It is probably not a coincidence that acorns have been used historically as both coffee and chocolate substitutes.

In southern Spain, avellanamiento is a widely used term that would translate to something like “hazelnutment”, in the sense of becoming like a hazelnut, and refers specifically to the drying process that occurs when acorns fall on a bush such that they are protected from soil moisture thus drying slowly in a natural way to become as tasty as a hazelnut. The process can be easily imitated by just letting sweet acorns dry indoors till they start losing some of their crispiness, this usually takes two to five weeks depending on acorn size and chemical composition. At this point sweet acorns are at their flavor peak to be eaten as whole nuts, and they can be stored in the refrigerator for up to five months. If left to dry for longer, they will become rancid or, depending on the conditions, dry completely and become hard as a rock, thus requiring rehydration or processing into flour.

It is important to note that tannins act as a natural preservative, so the shelf life of sweet acorns, whether fresh or dried, tends to be shorter than non-sweet acorns.

A few of the sweet acorns we have found have no astringency or bitterness whatsoever, even when not completely ripe, while others, although sweet, are slightly to moderately astringent/bitter when eaten directly from the tree (after the drying process the astringency/ bitterness disappears completely). So my advice to any sweet-acorn explorer would be: when approaching an oak tree that is reported to bear sweet acorns, if the first bite is disappointing, harvest a few more nuts and give them another opportunity by drying them a few weeks indoors.

Haines and Wescot (2014), in one of the few existing documents on sweet acorns claim that: “There are acorns that do not taste bitter, but there are no acorns that actually taste sweet.” They also argue that in these non-bitter acorns the astringency and bitterness are somehow concealed but the acorns nevertheless contain enough tannins and phytic acid to be harmful for humans and therefore need leaching.

As commented above, indeed some acorns are non-bitter and dull tasting, but, contrary to the opinion of Haines and Wescott, sweet and pleasant tasting acorns do exist. Of course they aren’t as sweet as a ripe persimmon for instance, but they are comparable to chestnuts and objective tasters find them delicious.

Haines’ second affirmation is very interesting, and seems to be in line with Morcuende’s coffee and chocolate comparison: could a relatively high sugar content, or other substances in acorns, mask the warning flavors of potentially harmful concentrations of tannins and phytic acid in sweet acorns? There is no research that I know about on this issue. Taking a look at the traditional consumption of sweet acorns might help. In the south of the Iberian Peninsula and also in Mallorca, still today many people eat raw sweet acorns or, most often, roast them without any other processing, and they have been doing it for a long time (García-Gómez et al. 2013). Cutural evidence is not scarce: in chapter XI of Don Quixote some shepherds share sweet acorns with the knight-errant and his squire, specifically “hazelnutted” acorns. There is no information on whether the acorns were raw or roasted.

Nevertheless, sweet acorns are rare and perishable, so it is likely that their consumption as unprocessed whole nuts has mostly been seasonal and in small amounts suggesting that the antinutrient adverse effects would have been insignificant. Further research is needed on this issue.

Of botany and ideology

The tale of the sweet acorn is the tale of oak tree domestication. Like other stories about domestication, it involves biology, archaeology and even ideology, all wrapped up in the mist of a distant past which often only lets us ramble around in difficult-to-prove theories.

It is well known that agricultural food production has led to an intense simplification of our diet and our food sources. The list of forgotten food plants is endless, but the neglect on acorns by modern agriculture is especially striking for different reasons: first, acorns are very nutritive; they are a rich source of carbohydrates and fats (as well as other nutrients), which makes them a staple-food candidate (Toernsmeier 2012); second, the genus Quercus has an extraordinarily wide distribution with species adapted to very different environments; and third, acorns were an important food for many civilizations before the birth of agriculture, including most of the ones that “invented” agriculture. The Fertile Crescent in the Middle East, regions of Central America and areas along the Yangtze and Yellow River Valleys in China are where most of the current agricultural crops originated. In all of these regions oaks grow abundantly and there is also archaeological evidence from all of them of acorn use as human food. So, the question of why humans are not cultivating oaks for food in modern times is intriguing. This complex and fascinating issue goes beyond the scope of this article and exceeds the range of my knowledge and capabilities. At a high risk of oversimplification, some of the explanations focus on certain botanical and genetic characteristics of oaks (e.g., slow growth, years to fruiting, challenging vegetative reproduction, masting behavior, and low heritability of the “sweet-acorn trait”) that would have hindered breeding possibilities and use as an agricultural food crop (Diamond 1997). Others, for example Scott (2018) put the emphasis on social and ideological factors such as the need for social control in developing central states, the difficulties in controlling (and taxing) “forest activities”, the possibilities afforded by agriculture for capital accumulation, and the need for a subservient workforce to make this possible. All of these factors would have contributed to stigmatizing acorn consumption.

My opinion is that biological limitations probably played a role historically, especially in the beginnings of plant domestication, but later on, ideological limitations grew in importance until they became the reason why most of us are eating cereals instead of acorns for breakfast. Botanical hurdles to oak breeding and its use as a tree crop are actually shared with other species that have indeed been domesticated. Considering current breeding and agronomic techniques, it seems plausible that we could overcome these hurdles easily. 

Perhaps I have gotten a little ahead of myself. Humans haven’t domesticated oaks as nut trees, right…or not?

Well, the concepts of domestication and agriculture have blurry limits (Chassé 2016). There is no “eureka” moment that announces the domestication of a species. Furthermore, at what point is human intervention in an ecosystem intense enough to be considered agriculture? In addition, these terms are part of the (today) controversial mindset that considers humanity and nature two separate things thus creating a simplistic dichotomy between human-controlled ecosystems and natural ones. An intermediate category, sometimes referred to as “semi-domestication”, is where the history of humans and Holm oak (and probably other oaks) falls.

The magnificent dehesa

This fascinating oak agroforestry system extends across most of the southwestern quarter of the Iberian Peninsula. In the dehesa (montado in Portuguese; devesa in Catalan), the spacing between trees is generous, so they can grow unrestrained with their lower branches being cleared by livestock. This allows for a panoramic prairie-like landscape while, at the same time, the majestic oak trees convey the shelter and protection of a forest. The evergreen, darker colored foliage of the oaks contrasts with the evolving lighter color tones of the grasses and other herbaceous plants underneath. Many domestic and wild animals call the dehesa home, the most iconic of them being the acorn-fed Iberian pig. The production of the world-renowned ham made from them is limited by acorn availability and the mysterious masting ways of oaks.

The savannah is where primates came down from the trees and started walking erectly. It is the landscape we first evolved in. Maybe that’s why it has a soothing effect on many of us, giving us an ancestral feeling of home. And, on top of that, when the trees growing on the gently undulating hills are oaks, with some of them bearing sweet acorns… well, what else can one ask for?

Holm oak populations in the dehesa tend to produce bigger acorns with lower tannin concentrations and sweet-acorn bearing trees are more abundant. This is most notable in some of the oldest dehesas.

In the past, timber was a more important crop in the dehesa ecosystem than it is today. At some point in history the demand for timber put great pressure on the Iberian forests and on the dehesas. It is thought that oaks producing sweet acorns as well as the highly productive ones, would have been treated more gently than their bitter sisters. Additionally, I have spoken to people who still remember how shepherds, when passing by a sweet-acorn tree, would pick up a few of the nuts, not only to eat, but also to plant along the way by pushing them down into the soil with their walking sticks.

Today, the dehesa is not free from threats. The relentless pressure for economic profitability and subsequent attempts “to fit” the dehesa into the European regulations and agricultural subsidies designed for monoculture farming have led both to an intensification and a simplification of the system. This in turn has led to a loss in biodiversity and to overgrazing, compromising the successful recruitment of oak seedlings and ultimately the long-term regeneration of the tree canopy.

In order to amend this problem, many oak seedlings have been planted during the last decades in dehesas, usually through governmental programs and subsidies. However, too often little or no attention has been paid to acorn quality or provenance. Under existing regulations, only acorns from the few certified seed sources can be used for reforestation purposes and this has introduced non-local genotypes which of course has jeopardized the valuable semi-domesticated gene pool of the dehesas.

Another big concern is the pathogen Phytophthora cinnamomi that causes root rot and is often fatal to trees. This disease keeps spreading through the dehesas leaving a phantasmagorical landscape of dead trees in its path. Research on possible treatments and on resistant genotypes is being carried out, but without much success until now.

Maria Dolores Carbonero, a researcher of the dehesa system at IFAPA (Institute of Agricultural and Fishing Research and Education in Andalusia, Spain) explains (personal communication) that in past times when she visited farmers they would mainly talk proudly of their Iberian hogs, but since the oaks are dying and their main income source is threatened they are paying more attention to the trees. The mentality is changing little by little.

With the slow decay of the dehesa, many sweet-acorn genotypes are at risk of disappearing. Due to the vast extension of this fascinating ecosystem, the task of locating and cataloguing sweet-acorn bearing trees appears to be herculean. At the same time it is becoming increasingly urgent if we want to preserve these unique trees that are the result of centuries of oak-human interaction.

Sweet-acorn cultivars in Mallorca

Behind the touristic frenzy and the relentless real estate pressure, the biggest of the Balearic islands hides a rich agricultural tradition that refuses to disappear. The skill of its tree-grafting experts was once renowned across Spain and these traditional techniques for grafting are, I believe, unique.

When travelling inland, away from the busy seaside, the landscape is dominated by orchards of olive, almond, and carob, as well as vineyards, delineated by endless dry-stone walls. From time to time, an isolated oak, or a small group of them, stands out over the rest of rather small trees. When looking closely at their sturdy trunks, a protuberance or a perpendicular line at breast height shows that a long time ago many of them were grafted. Still today, a handful of skilled grafters keep propagating the numerous cultivars using old techniques and tools.

In 2019, the Associació de Varietats Locals de Mallorca – a non-profit organization dedicated to the conservation of local heirloom varieties and the traditional knowledge of Balearic farmers – started the work of cataloging sweet-acorn cultivars to promote their conservation. Around 15 cultivars have been identified and the count keeps growing. Some of them have a very local distribution while others are present all over the island. There are sweet-acorn cultivars for human consumption and also highly productive cultivars traditionally used for raising livestock in a dehesa-like fashion. Today, these once cherished trees are largely ignored but fresh sweet acorns are still sold in a few local markets and people eat them roasted like chestnuts.

Perhaps the three most popular cultivars are:

• Castanyenca (Chestnutty) – This name appears to include different clones that bear relatively big and sweet acorns.
• De la campaneta (Small bell) – Small round acorns that are probably the best tasting ones in Mallorca. Low productivity.
• D’en Pere Andreu (Petter Andrew) – This name probably refers to the owner of the original tree. Not sweet, but low-tannin acorns. Very productive cultivar for animal fodder.

Unlike in Southern Spain and Portugal, most of the trees in wild stands in Mallorca produce relatively bitter acorns. Old folks on the island say that if you plant a sweet acorn, it will express itself bitterly (i.e., the resulting tree will produce bitter acorns). Due to the chance nature of the inheritance of sweetness this can be said for all sweet acorns, but even more for Mallorcan ones, because the majority of the surrounding pollinators produce “bitter” acorns. On the other hand, the dehesa populations in Spain and Portugal, on the whole, bear acorns with relatively little tannins, so the chances of getting sweet nuts from seedlings, or at least non-bitter ones, are a little higher.

Unfortunately, since 2016 the Balearic islands are under plant quarantine due to Xylella fastidiosa. This pathogen has been devastating for olive trees in southern Italy and, in Mallorca, almond trees are also dying at worrisome rates. Xylella fastidiosa infects a wide array of woody plants including Quercus spp., hence the prohibition to export plant material for propagation. So, for now, the Mallorcan sweet-acorn cultivars will need to stay there.

Meanwhile, the Associació de Varietats Locals continue cataloguing (this year with the support of the local government) as well as organizing events and training courses on sweet acorns and traditional Mallorcan methods of grafting. It is comforting to see how the Mallorcan cultivars, together with the traditional practices associated with them, are being brought back to light and now safe from disappearing.

Why don’t you just plant almond trees?

About two years ago I happened tear my Achilles tendon, it is a relatively serious injury that requires surgery and a long rehabilitation process. The hours spent on painful physical therapy have a good side though: one gets quite a lot of time to speak about anything he wants to a listener, the physiotherapist, who cannot run away. Obviously, I took the opportunity and, on one of the first sessions, I spoke at length about sweet acorns. After my enthusiastic speech, the physiotherapist and patient listener asked me: why don’t you just plant almond trees?

At that time, such a direct and basic question caught me off guard. Why would we want to domesticate oaks as nut trees anyway? Wouldn’t it be better to leave them in peace in their wild majestic beauty? Is it just because of the capitalistic insatiable appetite for new products? Or maybe the human drive to conquest the wilderness?

Holm oak forests, in their different variants, are the climax community in vast areas of the western Mediterranean basin. Wherever agricultural land is abandoned, seedlings start colonizing, sometimes rather immediately and sometimes after shrubs and pioneer trees have paved the way for them on the rougher terrains. It is the last stage in ecological succession, the direction towards which the land moves. Therefore, in our Mediterranean context, if we are to practice an agriculture that works together with nature and its processes instead of trying to subdue it, then few things make more sense than reintroducing the acorn and the oak in our food-production systems. Not only because they would provide valuable ecosystem services but also because it can contribute to creating an agricultural system truly adapted to local conditions, resilient to climate change and much less dependent on irrigation, chemicals and oil.

A look at the future of sweet acorns

The challenges facing sweet acorn cultivation abound. For instance, important ups and downs in production due to masting are a big headache for any aspiring sweet-acorn farmer. In addition we have found that variations in acorn taste can be significant from year to year. We need to further understand the effects of factors such as climate, water availability, soil type, rootstock and predatory pressure on acorn composition, particularly the effects on tannin concentrations.

We believe that many of the strengths, and even some of the challenges, of oaks as a tree crop make them better suited for agroforestry models, where they grow in combination with other crops and/or livestock, as they have been traditionally cultivated in the dehesa and in Mallorcan agriculture

Holm oak’s deep root system is compatible with alley cropping and, considering its relative slow growth, the income from crops growing between tree rows could sustain the operation during the first years till the trees start delivering a substantial nut crop. In addition, this model greatly facilitates implementation, because tree rows can just be added to already productive farmland as an addition to existing farming practices.

As do all trees, Holm oaks benefit from relationships with mycorrhizal fungi. These include high market value Tuber spp. that are already extensively cultivated in Holm oak orchards. The combined production of truffle and sweet acorns seems like a natural step to take, and we are focusing our efforts there as well.

The combination of livestock and sweet-acorn production for human consumption is very interesting as a biological pest control. The main concern in this context is Curculio spp. (weevils), whose larvae feeds on acorns. With the livestock feeding on the acorns that are not harvested for other uses the biological cycle of these beetles is broken and this greatly controls their numbers.

Other possible models for sweet-acorn production range from traditional fruit-tree orchards to food forests in which staple foods are probably the most challenging element to integrate.

Currently, trial orchards of super-intensive grafted Quercus spp. cultivars for acorn production already exist (Peñuelas 2017), where oaks are planted in very high densities and kept small as hedge-like formations. Pruning and harvesting are mechanized with similar machines as the ones used for grape harvesting. For now, the goal of these plantations is to overcome the problems due to masting in the Iberian hog industry and to take the intensification of the dehesa one step further. However, this model could be easily adapted to sweet-acorn production for human consumption.

The cultivation of sweet-acorn-producing oaks can be done in many ways. I believe that depending on the direction we choose, the sweet acorn can either become a tool in transforming food production towards more sustainable models or it can turn into the new superfood that will be grown in conventional questionable ways, ultimately, once again, transforming a potential solution into yet another problem.

Works cited

Chassé, B. 2016. Eating Acorns: What Story Do the Distant, Far, and Near Past Tell Us, and Why? International Oaks 27: 107-135.

Diamond, J.M. 1997. Guns, germs, and steel: the fates of human societies. New York: W.W Norton & Co. 

García-Gómez, E., R. Pérez-Badia, J. Pereira, and  K.P. Rajindra, K.P. 2013. The Consumption of acorns (from Quercus spp.) in de Central West of the Iberian Peninsula in the 20th Century. Economic Botany. DOI 10.1007/s12231-017-9391-1.

Haines, A., and D. Wescot. 2014. Do sweet acorns still need to be leached? The Digital Archeological Record.

Lebreton, P., M. Barbéro, and P. Quézal. Contribution morphométrique et bocihimique à) la structuration et à la systématique du complexe spécifique Chêne vert Quercus ilex L. 2013. Acta Botanica Gallica 148(4): 289-317.

Michaud, H., L. Toumi, R. Lumaret, T.X. Li, F. Romane, and F. Di Giusto. 1996. Effect of geographical discontinuity on genetic variation in Quercus ilex L. (holm oak). Evidence from enzyme polymorphism. Heredity 74: 590-606.

Peñuelas Rubira, J. L. 2017. ¿Son posibles las plantaciones de Quercus mediterráneos para la producción intesiva de bellota y son una oportunidad de desarrollo rural? 7° Congreso Forestal Español. 26-30 junio 2017.

Rafii, Z.A., E. Zavarin, and Y. Pelleau. 1991a. Chemosystematic Differentiation of Quercus ilex and Q. rotundifolia Based on Acorn Fatty Acids. Biochemical Systematics and Ecology 19(2): 163-166.

Rafii, Z.A., E. Zavarin, and Y. Pelleau. 1991a. Chemosystematic Differentiation of Quercus ilex and Q. rotundifolia Based on Acorn Steroids. Biochemical Systematics and Ecology 19(2): 249-252.

Scott, J.C. 2018. Against the Grain: A Deep History of the Earliest States (Reprint ed.). New Haven: Yale University Press.

Smith, J.R. 1929. Tree Crops: A permanent agriculture. New York: Harcourt, Brace & Company.

Solla, A., S. Milanović, A. Gallardo, A. Bueno1, T. Corcobado, Y. Cáceres, D. Morcuende, A. Quesada, G. Moreno, and F. Pulido. 2016. Genetic determination of tannins and herbivore resistance in Quercus ilex. Tree Genetics & Genomes 12 : 117-129.

Toernsmeier, E. 2012. Perennial Staple Crops of the World. Permaculture News.