Controlled carbon dioxide enrichment using beer fermentation off-gas results in 89% spinach yield increase

1. Introduction

Vertical and indoor farming has become an attractive alternative to traditional industrial agricultural practices as producers seek to maximize yields per unit of land area.¹ Intense population increases in urban centers, the need for improved water-use efficiency, and the existence of a finite amount of arable land for agricultural use will continue to drive interest and investment in the indoor farming sector,^2–4 and it is estimated that the global indoor farming industry will be valued at $91.1B by 2030.⁵ Many leafy-green and table vegetable crops benefit from growth in a controlled environment and do not require the footprint of common mass-produced cereal crops.^6,7

While indoor agriculture thrives in many niche industries, transitioning a major portion of the current agricultural industry indoors will require innovation in all aspects of production. A common strategy implemented with indoor farming to improve yields is carbon dioxide (CO₂) enrichment,⁸ whereby the local concentration of CO₂ within the growth environment is artificially elevated. The elevated CO₂ concentrations lead to improved photosynthetic efficiency, which has positive downstream impacts on both yield and flavor profile.⁹ CO₂ enrichment, however, must be implemented for as long as possible to achieve full effectiveness, which can create a financial burden when using compressed CO₂ or propane burners for enrichment considering the need for indoor air circulation and temperature control.¹⁰ Therefore, a strategy of implementing CO₂ enrichment only during peak photosynthetic light-reaction output, rather than at all hours of the day, has been implemented to maximize the return on investment of CO₂ enrichment for producers.¹¹ This results in a loss of yield potential, as the carbon-capturing Calvin Cycle of C3 photosynthetic plants is active at all hours of the day and for a limited period in the dark.¹² A more affordable source of CO₂, however, is readily available.

The beer industry produces roughly 1.5 to 2.1 kg of CO₂ per barrel of beer produced.¹³ In 2023, the United States alone brewed 165 million barrels of beer,¹⁴ resulting in the production of roughly 300 million kg of CO₂ as a waste byproduct. Many large-scale breweries have implemented direct CO₂ capture from industrial beer fermentation facilities for re-use in beer carbonation,^15,16 although this requires a large capital expenditure that may not be readily available to all breweries and is an energy-intensive process, calling into question its actual impacts on industry sustainability.

The circular bioeconomy has been proposed as a means to help navigate the risks posed to global and local economies by a changing climate, where the value of bioproducts is stretched as far as possible to extract the highest possible gain with the lowest possible impact.^17,18 As the agricultural industry continues to transition away from traditional practices and towards optimized practices that emphasize efficiency and sustainability, innovation in all aspects of production, from planting to packaging and delivery, will be required. To this end, this study evaluates the feasibility of the direct-use of alcohol fermentation CO₂ off-gas for use in controlled indoor agriculture CO₂ enrichment.

2. Methods

2.3 Treatment-chamber CO₂ enrichment

Low-pressure CO₂ fermentation off-gas flow from the fermentation vessel (5-gallon glass carboy) was used to maintain CO₂ levels within the treatment growth chamber at 1200 parts-per-million (ppm). A feedback-controlled flow control device consisting of a CO₂ sensor wirelessly communicating with a 3-way solenoid valve piped into the growth chamber and fermentation vessel was constructed and programmed according to the schematic in Figure 1. Low-pressure CO₂ flow produced by the fermenting substrate was directed to the treatment chamber when chamber levels were below 1200 ppm and directed to outdoor ventilation when chamber levels were above 1200 ppm, with sensor/valve communication occurring every 6 seconds. The control chamber was maintained under ambient conditions throughout the trial. Separate dataloggers within each growth chamber recorded CO₂ levels for trending and comparison throughout the trial.

Figure 1. Schematic representation of the control system designed to utilize beer fermentation CO₂ off-gas to maintain CO₂ levels within an enclosed growth chamber at 1200 ppm.

3. Results

3.1 Fermentation off-gas CO₂ enrichment and control

CO₂ enrichment was performed on a treatment growth chamber to compare spinach growth to a control growth chamber. To provide a constant flow of CO₂ for enrichment, a beer fermentation vessel was piped into the treatment growth chamber with a wireless control device capable of directing low-pressure off-gas flow from the fermentation vessel to either the treatment growth chamber or to an outdoor exhaust via a 3-way solenoid valve ( Figure 1). The control system was programmed to maintain CO₂ levels at 1200 ppm within the chamber. Throughout the trial, fermentation substrate (pilsner beer wort) was replenished within the fermentation vessel to ensure a constant state of active fermentation. The control system achieved a strong oscillation around 1200 ppm, with early, peak, and late-stage fermentation levels on a per-beer-batch basis evident on the control trend ( Figure 2A). As fermentation rates ramped up, the increased off-gas flow forced exaggerated spikes in CO₂ levels within the treatment chamber, while during early and late-stage fermentation a much finer oscillation was maintained ( Figure 2B). To ensure that the chamber did not experience extended periods of drastically elevated CO₂ concentrations, the control device was programmed to communicate a valve state-change every 6 seconds. Active CO₂ enrichment at 1200 ppm was achieved for >95% of the growth trial.

Figure 2. A. The control growth chamber was allowed to vent to ambient air, while the treatment growth chamber was CO₂ enriched using off-gas flow from an active beer fermentation vessel. The control trends show CO₂ values over the entire trial. Data points were logged using a datalogger independent from the flow control system every 5 minutes. B. A zoomed in view of an arbitrary 3-hour window during the trial to eliminate the noise associated with the large, compressed dataset.

3.2 Spinach growth

Spinach plants were harvested on day 50 of the growth trial. The 16 plants (8 from each chamber) were cut from the root base at the soil surface and weighed. The plants from the treatment growth chamber achieved a statistically significant 89% increase in above-ground fresh biomass ( Figure 3A). Due to the wide range of stem lengths on leaves within each plant due to the regular emergence of new leaves, only the longest primary leaf was used to determine leaf length. For the majority of plants, this was the first primary leaf to emerge, but this was not the case for every plant. After fresh weights were determined, the longest leaf from each plant was measured from stem base to leaf tip. The leaves from the treatment growth chamber achieved a statistically significant increase in length ( Figure 3B). Finally, after fresh weight and leaf length measurements were determined, the total above-soil biomass from each plant was placed in a drying oven to remove all moisture content. Plants were dried at 65^oC and weighed, with dried above-ground biomass from the treatment chamber achieving a statistically significant increase over those from the control chamber ( Figure 3C). The dry weight comparison of the two chambers is a direct measurement of the increased carbon sequestration capacity in the treatment chamber and an indirect indication of improved photosynthetic efficiency. As all other treatments and conditions (watering, fertilizing, temperature, light intensity/exposure) were controlled for across both chambers, these increases in fresh weight, leaf length, and dry weight are directly attributable to the CO₂ enrichment treatment.

Figure 3. Fresh weight, dry weight, and longest-leaf length (A, B, and C, respectively).

Asterisks represent statistical significance at an alpha of 0.05, calculated with a two-sample t-test.

4. Discussion

The results of this study show that alcohol fermentation off-gas can be used for CO₂ enrichment in an adjacently located indoor farm. CO₂ enrichment is a common strategy implemented in indoor farming to improve crop photosynthetic efficiency and increase total yields. This occurs due to the increased carboxylation activity of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) enzyme at elevated CO₂ levels.^20–22 This enzyme is responsible for the initial fixation of CO₂ to 3-phosphoglycerate, which then proceeds through the Calvin Cycle to ultimately produce glucose. This glucose can then be used to drive glycolysis or incorporated into cellulose or other biomass components.²³ Under ambient conditions, the carboxylase activity of the Rubisco enzyme is in constant competition with it’s oxygenase activity, the latter of which ultimately leads to a net loss of energy.²⁴ C3 crops, which include most of the common table vegetables associated with the modern diet, are particularly susceptible to this energy drain compared to their cereal crop C4 relatives.^25,26

The CO₂ off-gas flow controller used in this study was able to maintain a heavy oscillation around 1200 ppm throughout the entire growth trial. 1200 ppm was selected as the setpoint because it has previously been used with spinach.²⁷ However, it will be important to further investigate a wide range of setpoints to further optimize growth potential. The CO₂ setpoints will also require tailoring to specific crop species, as there has been reported a wide range of effects across different species.²⁸ The controller was also designed to function in a binary “on/off” manner, in which CO₂ flow was not finely controlled, but allowed to flow unrestricted until a valve state-change was called upon. This manner of control resulted in large oscillations and momentary swings in CO₂ concentration within the treatment growth chamber. Although the treatment growth chamber clearly did not experience CO₂ concentrations high enough to become overall detrimental to plant growth, a more-fine-tuned control system (proportional/integral/derivative, “PID”) may be capable of delivering improved results.

While the use of alcohol fermentation off-gas in this experiment for CO₂ enrichment is novel, the results are not necessarily surprising. CO₂ enrichment has been shown previously to increase photosynthetic rates in spinach leaf discs²⁹ and to increase starch accumulation in above-ground spinach biomass.³⁰ This proof-of-concept experiment has validated that the complex mixture of CO₂, water vapor, and volatile organic compounds that make up a beer fermentation off-gas stream is capable of reproducing (and improving on) data produced using standard industrial and academic practices. From a plant science perspective, this study is useful in that it adds support to the scientific literature for studies showing improved photosynthetic efficiencies and growth under increased CO₂ conditions across many different crop species.^31–33 However, from an agricultural perspective, this study is important because it presents a novel means to tap into a common waste stream for use in significantly increasing yields across a rapidly growing indoor farming industry while simultaneously mitigating the Scope 1 greenhouse gas emissions of the alcoholic beverage industry.

A large-scale transition to a circular bioeconomy will require innovative technologies that incentivize the switch. Waste stream re-use across many different industries will play an important role in this transition, and this proof-of-concept study presents a unique opportunity to improve indoor farming yields while cutting carbon emissions. An 89% increase in yield is a promising starting point for this technology that can still be further optimized and improved, but further work will be required for implementation across multiple crops with complex and undefined fermentation substrates.

Conclusion

These results show a beneficial impact on spinach yield of CO₂ enrichment using a beer fermentation off-gas waste stream in an enclosed and controlled growth environment. Spinach plants were grown in a control growth chamber and a treatment growth chamber, in which CO₂ levels were maintained at 1200 ppm by a control device utilizing free-flowing CO₂ from a beer fermentation vessel. After 50 days, the spinach plants grown in the treatment chamber produced 89% more fresh above-ground biomass than those grown in the control chamber. This study validates the feasibility of using fermentation waste off-gas for indoor farm CO₂ enrichment.

Data availability