This article provides both an experiment and a framework for discussion that students can use to compare the efficiency of producing ethanol by using corn versus sugarcane as a raw material.
The current controversy over the use of food crops to produce fuel provides a new way to engage students in learning some important biological concepts. Biofuels, such as ethanol made from corn or sugarcane and biodiesel made from soybeans, may represent a means for the United States to reduce dependence on imported oil and cut greenhouse gas emissions. In his 2007 State of the Union Address, President George W. Bush announced a national goal to increase U.S. consumption of biofuels from the current 5 billion gallons per year to as much as 35 billion gallons per year by 2017 (Baker and Abramowitz, 2007). This amount of biofuel could substitute for as much as 10%% of U.S. gasoline supplies. President Obama pledged support for the corn-based ethanol biofuel industry during the 2008 campaign, and his administration has moved to increase ethanol production and use through a series of coordinated policy measures (Power, 2009). However, serious concerns about ethanol as biofuel have been raised by scientists, environmentalists, and legislators (Pimentel and Patzek, 2005; Bourne, 2007). This article presents a laboratory experiment that involves students in a hands-on investigation of the biology of ethanol production and the related economic and environmental policy issues.
We developed this exercise for first-year, mostly non-science majors in an interdisciplinary college course, Solving Environmental Problems. The course takes an integrative approach to examine the science, economics, public policy, and business aspects of environmental issues, is discussion-oriented and writing-intensive, and has a laboratory component. We used this laboratory in a unit of the course that focused on climate change and alternative energy. We introduced the experiment covering the basics of climate change and discussing a case study about automobiles of the future that illustrates broader policy issues regarding the automobile industry (Letovsky, 2002). The relevance of this experiment and its use of inexpensive equipment and readily available materials make it suitable for use in high school environmental science and biology courses, as well as in college-level courses.
In the experiment, students compare the biological efficiency of the first step of the ethanol fuel production process, fermentation, for two important raw materials, corn and sugarcane, as well as for some raw materials of their choosing. After analyzing the data on fermentation rates, students are asked to consider government policy regarding ethanol in light of their own results, as well as with respect to the potential economic and environmental impacts of ethanol. The learning objectives of the exercise are described in Figure 1.
The Biological Process of Ethanol Production from Fermentation of Corn & Sugarcane
Historically, humans have used yeast fermentation to produce ethyl alcohol (ethanol). During eras when food safety and availability were problematic, such as the Middle Ages in Europe, fermentation provided beverages that were nutritious (carbohydrate-rich and sometimes protein-rich) and safe (free of pathogenic bacteria) (Engs, 2001). During more recent times, the process of fermentation by yeast has been co-opted to produce ethanol fuel. In fact, Henry Ford's first car ran on pure ethanol (Bourne, 2007). Today, ethanol produced for fuel is 200 proof and contains an additive that allows biofuel producers to avoid paying the ethanol-as-beverage tax.
For food or for fuel, the starting point in the ethanol production process is to provide the yeast (Saccharomyces cereviseae) with a source of carbohydrates (e.g., corn, sugarcane, grapes, barley, etc.) and allow it to use the carbohydrates in the metabolic process called fermentation. The chemical equation for the fermentation process is
where C6H12O6 is glucose (a simple carbohydrate) and 2C2H5OH is two molecules of ethanol.
During the fermentation reaction, the yeast breaks down the carbohydrates to release energy that it can use for its metabolic processes. Complex carbohydrates (e.g., starch) can be used in the process, but if they are, the bonds that hold their units together must be broken to allow those units —— simple sugars like glucose —— to enter the reaction. Students should be asked to consider this information when thinking about how corn and sugarcane will fare as raw materials for fermentation. They should be prompted to think about which raw material (corn or sugarcane) contains a greater proportion of simple sugars and which contains a greater proportion of complex carbohydrates.
Ethanol is a byproduct of the process that is toxic to the yeast cells once the alcohol concentration reaches 10——14%%, depending on the strain of yeast used; this is why alcoholic beverages produced solely from fermentation (e.g., beer and wine) do not usually reach alcohol concentrations exceeding approximately 14%% without supplementation. Therefore, fermentation of raw materials such as sugarcane or corn must be followed by distillation to remove excess water in the mixture to reach the desired 200 proof (100%%) concentration. After noting that carbon dioxide is another byproduct of the fermentation process, students should consider whether or not this presents any problems for the argument that ethanol is an environmentally friendly fuel. Students should also consider the argument that biofuels are carbon-neutral because the plants they are derived from were recently alive and photosynthesizing; this argument should be considered along with the information on economic and environmental impacts of ethanol as fuel.
Economic & Environmental Impacts of Ethanol Production from Corn & Sugarcane
Ethanol blended from corn is already widely used in the United States, but not as a stand-alone fuel for cars. Instead, it is blended into conventional gasoline because it is an oxygenated fuel and octane enhancer that reduces smog by promoting more complete combustion to carbon dioxide. In New England, for example, it is quite common to find gasoline sold with up to 10%% ethanol. Only about 800 service stations in the country presently sell ““E 85,”” a blend that contains 85%% ethanol and 15%% crude-oil-based gasoline, and these are concentrated in the Midwest. This is less than one-half of one percent of the more than 161,000 service stations in the United States (Campoy, 2008). However, in response to tax breaks offered by the U.S. Congress to encourage biofuel production, there are several hundred new ethanol production factories under construction, again mainly in the Midwest.
There are a number of potential problems with using ethanol as fuel. The most fundamental issue is how much ““new”” energy is produced by either corn- or sugarcane-based ethanol, after subtracting the energy used to produce the crop itself. The answer to this question is intimately tied to the yield/conversion rate that relates an acre of corn or sugarcane to how much ethanol each acre can produce. Using existing technologies, an acre of sugarcane produces almost twice as much ethanol as an acre of corn. The relatively low yield of corn has important policy implications for the United States in terms of biofuel production. Some experts say that even if all 70 million acres in the country that are presently used for corn production were used exclusively to grow corn for ethanol, this would meet only 2.5%% of total energy needs (Tilman & Hill, 2007). This argument, however, is slightly misleading in that ethanol's backers have never argued that it could entirely replace traditional fossil fuels in the nation's energy mix. Kline et al. (2009) contend that there is adequate cropland both in the U.S. and worldwide to support a dramatic increase in biofuel crop production without radical changes to the composition of croplands.
In terms of greenhouse gas emissions, corn plants absorb carbon dioxide from the atmosphere as they grow, so burning corn ethanol, unlike burning fossil fuels, only releases the same carbon into the atmosphere that was recently absorbed and fixed by the corn plant during photosynthesis. However, when tallying carbon dioxide emissions, one also has to consider the fossil fuel used to produce and harvest the corn and then convert it into ethanol. Fossil fuels currently power corn planting and harvesting machinery as well as the manufacture of inorganic fertilizers. These processes result in considerable amounts of carbon dioxide emissions, to the point where driving a car on corn-based ethanol reduces greenhouse gas emissions only 15——22%% less than driving the same car on traditional gasoline (Tilman & Hill, 2007; Bourne, 2007).
One alternative to using corn as the basis for ethanol production is to rely on sugarcane. Brazil, the world leader in sugarcane-based ethanol production, has successfully translated a massive investment in the process into independence from oil imports. Unlike the water- and fertilizer-intensive processes used by American corn farms, Brazilian sugarcane plantations rely on rain and use relatively low amounts of fertilizer and agrochemicals (Kline et al., 2009). Not only does land devoted to sugarcane produce much more ethanol than land devoted to corn, but Brazilian ethanol refineries derive most of their energy from burning sugarcane residue, thus avoiding burning of fossil fuel and further cutting the greenhouse gas emissions associated with the production process. Some estimates claim that sugarcane ethanol produced on established plantations offers an 80%% reduction of greenhouse gas emissions compared to traditional gasoline.
However, a serious potential problem with Brazilian sugarcane-based ethanol production is that it may contribute to deforestation of the country's rainforests. In the global carbon cycle, plants and soil contain three times more carbon than exists in the atmosphere. When Brazilian rainforests are cleared to make room for sugarcane production, about 25%% of the carbon dioxide previously stored in the forests is released into the atmosphere through cutting and burning of trees and decay of roots. Even more carbon dioxide is released in the first 20——50 years of farming on former rainforest lands as the carbon-rich soil decomposes. Overall, clearing tropical rainforests to produce sugarcane for ethanol emits almost 50%% more greenhouse gases than producing and burning the same amount of traditional gasoline (Tilman & Hill, 2007). Moreover, clearing Brazil's rainforest to produce more sugarcane for the ethanol industry can be expected to have a serious impact on biodiversity in the country as countless habitats are destroyed. However, ethanol supporters offer an alternative narrative: Kline et al. (2009) challenge the notion that ethanol production is the key driver in the deforestation of Brazil, noting that increases in sugarcane production have mostly occurred on lands that were previously used as pasture or already cultivated for less profitable crops.
Another criticism of ethanol production is that both sugarcane- and corn-based production of ethanol have the potential to drive food prices up and decrease global food security. A leading source of animal feed, corn is a key input for the U.S. dairy, poultry, and beef industries. Corn is the most widely used agricultural product in processed food, as well. According to Pollan (2006), more than 25%% of the 45,000 items found in an average American supermarket contain corn. The uses of corn range from high-fructose corn syrup as a sweetener in processed foods to filler in disposable diapers and charcoal briquettes (Pollan, 2006). As demand for corn to supply the rising number of ethanol refineries has soared, so have corn prices, resulting in higher prices for consumers on a wide range of food products. A dramatic illustration of this food-versus-energy struggle occurred in 2007 in Mexico, when thousands of peasants took to the streets of the capital city to protest the rising prices for corn tortillas, a food staple of the country's poor. Yet ethanol defenders cite extensive data to challenge the notion that increased ethanol production contributes significantly to higher food prices. Kline et al. (2009) contend that although ethanol production has contributed to higher corn prices, it has been, at most, a minor factor in escalating food prices in world markets, ranking well behind rising prices for fuel and fertilizer.
Production of corn-based ethanol is also water-intensive. A 50-million-gallon ethanol refinery can be expected to use some 150 million gallons of water in the refining process. This is equivalent to the water demand of a small town (Barrett, 2007). Already, some midwestern U.S. states have introduced reductions in water allotments for farm irrigation as concerns mount about drawing down aquifers in the face of rising water demand from the ethanol industry. Because sugarcane fields in Brazil rely on rain to meet crop water demands, similar issues do not come into play there.
To promote the domestic ethanol industry and in response to extensive lobbying by the nation's corn farmers, the U.S. government has introduced a number of tax and financial incentives for firms in the business of biofuel production. The federal government presently gives refiners of ethanol a tax break of 45 cents per gallon to encourage more domestic production. Meanwhile, the United States maintains a tariff of 54 cents per gallon, as well as a percentage (““ad valorem””) tariff of between 4 and 7 cents per gallon, on imported Brazilian sugarcane-based ethanol. Despite interest in producing sugarcane for ethanol in regions of the United States where sugarcane is grown, including Florida, Louisiana, Hawaii, and Texas (U.S. Department of Agriculture, 2006), government policy will have to encourage such production or it will not be economically favorable while the price of refined sugar (for food) remains high (Spinner, 2006). Legislators from Florida are trying to incorporate a mandate for sugarcane use for some percentage of U.S. ethanol production into future alternative-energy legislation. Up to the present, the larger and more powerful corn lobby has held greater sway in the development of legislation.
In light of the conflicting claims and studies regarding the impacts of biofuels derived from different sources, the key question for policy makers remains unresolved: can either sugarcane- or corn-based ethanol appropriately address the competing needs of generating energy economically and minimizing environmental harm? While pondering this question, students should use the background information provided here or information they find by doing their own research to fill out the worksheet in Table 1. Teachers may also wish to direct students to use the interactive website provided by National Geographic magazine to accompany the article by Bourne (2007) (http://ngm.nationalgeographic.com/2007/10/biofuels/biofuels-interactive) to find information for filling out the worksheet. After completing the worksheet, students should proceed to their scientific investigation, which will provide some answers about the efficiency of the biological process that produces ethanol from corn and sugarcane.
We present suggestions for the timetable of the entire exercise in Table 2. After contemplating the issues surrounding ethanol and filling out the worksheet (Table 1), the students work in small groups to develop a plan for an experiment comparing fermentation rates of corn and sugarcane. The teacher should ask the students to devise the research hypothesis for their planned experiment. An example hypothesis is that different materials will vary in their rate of fermentation by the yeast. The students may predict that sugarcane will be fermented more rapidly than corn because of the presence of abundant simple sugars in sugarcane but more complex carbohydrates (starch) in corn. They may wish to test other materials that are readily available outside the classroom or brought in by the instructor, such as grass, moss, cereals (processed or whole), berries, and so on. Allowing each student group to test a material of their choosing enhances student engagement in the project.
To assess the rate of fermentation, the students measure the rate of carbon dioxide production by the yeast in graduated fermentation tubes (available from scientific glassware suppliers such as Fisher Scientific). These tubes have a blind end that traps gases and allows students to measure the production of carbon dioxide in milliliters on a scale on the tube (Figure 2). The students test a standard yeast solution (Fleischmann's Rapid-Rise®® yeast, water, NaCl) and added raw materials of interest: ground feed corn, ground raw sugarcane (available fresh at produce markets or canned at Asian food markets), other materials they are interested in testing, and some controls.
Before the students develop their plan for what solutions to test, the instructor should introduce the concept of testing controls in an experiment and discuss the need for a positive control to show that the standard yeast solution will rapidly ferment a material known to be an excellent raw material for fermentation (e.g., glucose or table sugar [sucrose]). The students should also be prompted to think about an appropriate negative control to show that given no raw material for fermentation, the standard yeast solution will produce little carbon dioxide (e.g., add nothing to the standard yeast solution or add water equivalent to the amount of the other materials to be tested). The experimental plan that students develop should involve replication of each solution they choose to test; the amount of replication achieved will depend on the number of student groups in the class and the number of fermentation tubes, but, regardless, the concept and purpose of replication should be explained to the students as necessary to ensure generalizability and repeatability of any results obtained.
The standard recipe for the yeast solution, the type of yeast used, and other specifics of running the experiment are detailed in Figure 3. Within a 2-hour lab period, students will be able to set up the experiment and collect sufficient data for analysis, with sugarcane and any positive controls (e.g., glucose) fermenting rapidly (results within 20 minutes). To save time in lab, we ground the corn and sugarcane shortly beforehand and stored them in the refrigerator, but if time allows, students may be asked to grind their own materials using blenders or mortars and pestles, depending on the materials. If the sugarcane to be used in the experiment is wet, the ground corn should also be wetted to achieve approximately the same moisture level. We found that it was best to wet the corn after it was ground. While collecting data in lab, students will wonder if the corn solutions ever ““catch up”” to the level of carbon dioxide produced in the other solutions tested, so, if possible, tubes should remain set up and examined again at 24 to 48 hours past the starting time. This follow-up data collection can be made fairly quickly, so it need not take place during another scheduled class period.
Data Analysis & Report
Each group of students can analyze their data by entering them into Excel and plotting the volume of carbon dioxide produced over time for each raw material and control. Replicate treatments should be averaged before plotting. Once the students produce a graph depicting carbon dioxide production over time (example in Figure 4), the instructor can lead a discussion about the trends in the data and how to calculate the rate of carbon dioxide production for each treatment.
The students should be prompted to remember that the rate of carbon dioxide production is a measure of the fermentation rate of each raw material and therefore reflects the efficiency of the given raw material for ethanol production. The rate is simply the slope of the plot of carbon dioxide versus time. It can be calculated as volume of carbon dioxide divided by time for a given portion of the experiment. We asked the students to produce a summary bar graph depicting the mean rate of carbon dioxide production for each material and control tested (example in Figure 5). If students test materials other than corn and sugarcane and run both positive and negative controls, their graphs will feature more x-axis categories.
We required each student to write an individual report on this experiment in scientific format. We gave them the questions in Figure 6 to talk over in class and address in the discussion section of their reports, where they were asked to consider their group's results not only in relation to their research hypothesis, but also in the broader context of the future of ethanol as fuel.
This laboratory exercise prompts students to consider the science related to a major controversy regarding alternative energy. Both our college class and a high school group enjoyed working hands-on to measure a process that forms the basis of ethanol production. In fact, the students were eager to test a variety of raw food materials. We encouraged them to test a variety of substances they obtained outdoors (e.g., berries from shrubs) and from the college dining hall (cereals, raw and processed). Making choices about controls and test materials allows non-science majors to experience some of the excitement of science, as does the clear relevance of this experiment to a topic that is currently a subject of fierce debate. This is a classic biology laboratory that has a renewed luster thanks to the tie-in to environmental issues. The integration of scientific, policy, and economic aspects of environmental issues occurs when students draw conclusions regarding their own data and then relate their conclusions to the policies that have been put in place by governments and the economic and environmental impacts of those policies.
Thanks to Professor Will Marquess, coordinator of the First-Year Seminar program at Saint Michael's College, for giving us the opportunity to collaborate in the classroom. Also, we are grateful to Edward Griffin for his work in testing some aspects of the laboratory exercise. Finally, we thank Professor Donna Bozzone and the anonymous reviewers for their comments on the manuscript.