Acorn-based coffee boosts antioxidants with minimal health risk

Researchers reveal how oak acorns turned into coffee-like drinks, pack potent antioxidants while staying safely below toxin limits, making them a surprising new superfood for health-conscious coffee drinkers.

Study: Oak Acorns as Functional Foods: Antioxidant Potential and Safety Assessment. Image credit: New Africa/Shutterstock.com

Thermally processed oak acorns (Quercus robur) in their native flour form are rich in antioxidants and are safe for consumption. A recent study in Foods investigated whether different processing methods impact bioactive compounds in acorn-based products.

Why are oak acorns a rich source of food and beverage?

Acorns are the nuts of the oak tree (Quercus spp.), which have been traditionally used in many cultures for centuries. The oak tree belongs to the Fagaceae family and is widely distributed across Asia, Europe, and North America. Numerous studies have demonstrated that Quercus species are rich in bioactive compounds, including phenolic acids, flavonoids, and tannins, which possess antioxidant, anticancer, and antimicrobial properties.

Over the years, the consumption of oak acorns has declined due to their high tannin content, which makes them bitter and impairs nutrient absorption by binding to proteins and minerals. However, a recent surge in acorn consumption has been noted due to its sustainability and health-promoting properties. For example, acorn flour has been used as a gluten-free and fiber-rich alternative to wheat flour, enhancing the nutritional and sensory properties of many baked products, such as bread, cakes, and cookies.

Recently, many health-conscious individuals with dietary restrictions have substituted conventional coffee with alternatives such as chicory, barley, spelt, dandelion, and acorns. These alternatives are generally rich in vitamins, fiber, minerals, and phenolic acids (e.g., gallic, caffeic, and chlorogenic acid). Phytochemical analysis has revealed that acorns contain a high concentration of carbohydrates, lipids, dietary fiber, proteins, and unsaturated fatty acids.

There is a lack of research regarding the antioxidant activity of Q. robur fruits, particularly in coffee-like beverages made from roasted acorns. Acorn roasting is a standard pre-treatment process performed to enhance flavor, which alters the chemical profile of the seeds. A previous study has indicated that when combined with ginseng, acorn-based beverages have a superior antioxidant potential compared to commercial coffee alternatives.

Thermal processing at high temperature, typically above 200°C, leads to the formation of polycyclic aromatic hydrocarbons (PAHs) that possess potential mutagenic and carcinogenic properties. Although coffee and coffee substitutes are exempt from European Union PAH limits, some studies have identified these harmful chemicals, particularly benzo[a]anthracene and anthracene. Therefore, analyzing the benefit-to-risk profile of roasted acorns used as coffee substitutes is important. However, the study confirmed that benzo[a]pyrene, the most concerning PAH carcinogen, was not detected in any acorn product, further supporting their safety

About the study

The current study investigated the PAH content and the antioxidant potential of coffee-like beverages prepared from roasted Q. robur acorns. Oak acorns were processed using different methods, and each sample was individually assessed.

Oak acorns were collected from the Kosmaj mountain, air-dried at room temperature, and powdered to produce “acorn flour”. In addition, 100 grams of dry acorns were roasted at 200°C for 30 minutes, and were powdered as “coffee”. For another sample type, 100 grams of dry-shelled acorns were coarsely chopped and soaked in water (60 to 70 °C) for 30 minutes. After repeating this soaking process a couple of times, the oak acorns were dried at 40°C for around three hours. Subsequently, these acorns were roasted at 200 °C for 30 minutes and powdered to make “super coffee”. Methanol, acetone, and hexane extracts of acorn coffee and flour were prepared.

Study findings

Highly polar solvents, such as methanol and aqueous acetone, readily extracted phenols and flavonoids from Quercus robur acorns. Total phenol content (TPC) in methanol extracts was estimated to be 66.53 mg gallic acid equivalent (GAE) per gram dry weight (dw), while acetone extracts yielded 24.21 mg GAE/g dw. This difference in phenolic content is because acetone is less polar than methanol.

Both thermally processed coffee and super coffee indicated a 15% and 9% increase in TPC in methanol and acetone extracts, respectively, relative to raw flour. This increase in phenolic concentration could be attributed to the formation of low-molecular-weight phenolics during roasting.

Similarly, the highest total flavonoid content (TFC) was found in the methanolic extracts of the acorn flour sample, followed by “coffee” and “super coffee”. Flavonoid levels declined from about 181.5 mg RE/g dw in flour to 161.7 mg RE/g dw in “coffee” and 150.7 mg RE/g dw in “super coffee”, confirming thermal degradation of these compounds. Regardless of solvent polarity, the native acorn flour consistently exhibited the highest flavonoid levels compared to its roasted counterparts. The decrease in TFC post-roasting confirms the susceptibility of flavonoids to thermal degradation.

The antioxidant activity of acorn extracts was evaluated using ABTS, DPPH, CUPRAC radical scavenging assays, and FRAP and TRP electron transfer assays, which indicated high antioxidant properties. Compared to the hexane extract, methanol extracts of all samples exhibited the highest antioxidant activity, reflecting the minimal extraction of hydrophilic phenolics, as indicated by trolox equivalents (TE)/g dw estimates. However, acetone extracts exhibited a marginally higher DPPH scavenging activity, implying that certain mid-polarity antioxidants are better extracted by acetone.

Methanol CUPRAC values reached up to 584 mg TE/g dw, ABTS values were about 127 mg TE/g dw, and FRAP values approached 200 mg Fe/g dw, all of which compare favorably to typical roasted coffee beans.

The FRAP assay revealed that thermal processing increased antioxidant power in nonpolar extracts. FRAP values rose dramatically post-roasting, by approximately 100% in methanol, 124% in acetone, and 375% in hexane extracts. This highlights how Maillard reaction products formed during roasting significantly enhance antioxidant capacity, particularly in lipophilic fractions.

The bioactive estimates of “super coffee” decreased with increasing lipophilicity of the solvent. The sums of TPC for acorn flour, “coffee”, and “super coffee” were 89.2, 92.9, and 87 mg GAE/g dw, respectively, while the sums of total reduction potential (TRP) were 6.86, 7.0, and 6.99 mg AAE/g dw, respectively. The studied samples’ antioxidant activity was associated with the total content of phenols and flavonoids.

In the current study, “coffee” and “super coffee” samples contained PAHs that were 2 to 3 times higher than those in native acorn flour. “Super coffee” was found to contain both low-molecular-weight (LMW) and high-molecular-weight (HMW) PAHs. Since none of the PAHs were present in native acorn flour (aside from indeno[1,2,3-cd] pyrene at ~8.8 ng/g dw, likely from environmental contamination), it indicates that roasting was the source. The researchers note that only about 30% of PAHs transfer into brewed acorn infusions, reducing actual dietary exposure.

The total PAH level reached 222 ng/g dw in “coffee” and 290 ng/g dw in “super coffee”, comparable to the ranges reported for other coffee substitutes. The primary PAH4 marker was benzo[a]anthracene (34 – 39.8 ng/g) while chrysene, benzo[b]fluoranthene, and benzo[a]pyrene were not detected.

Conclusions

The current study demonstrated that native and heat-treated acorns contain a high amount of phenol and flavonoid compounds. Methanol extracts exhibited the highest antioxidant activity. The absence of carcinogenic PAH4 marker in the native flour indicated that unprocessed acorn powder is safe for dietary use. 

Despite measurable PAHs in roasted samples, all levels remained well below EU regulatory limits for cocoa-based products, suggesting minimal health risk when consumed as coffee substitutes.

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