From Garbage to Gas: Contour Plots Show How to Create Optimal Ethanol using Pineapple Waste

The last time you cut a pineapple, chances are you tossed the peel in the trash. After all, what else would you do with it?

Researchers in Malaysia have discovered you can turn it into bio-fuel.

The team of scientists used Minitab Statistical Software to study ethanol fermentation with pineapple trimmings. Their findings suggest that pineapple waste could provide an alternative to traditional bio-fuel production methods.

Ethanol is produced during the fermentation process when yeast converts sugar into energy. It can be used as fuel, and is commonly found in beer, wine, and bread.

The research team worked with Pineapple Estate—a plantation in Johor, Malaysia with over 1.1 million acres of land planted with pineapples—to study ethanol production using the plantation’s discarded pineapple peels.

To accomplish this, the researchers needed to devise an experiment to assess the influence of several variables on ethanol production, while using the lowest possible number of experimental trials.

That’s where Minitab Statistical Software comes in.

Using Minitab, the team applied a statistical technique called response surface methodology (RSM) to assess the interactions between multiple variables—pineapple peel concentration, pH level, sugar concentration, temperature, and fermentation time—and their impact on the response variable, ethanol concentration.

The researchers set low and high values to create a range for each of the five input variables. Within each range, the variables were assigned five different levels to use for their central composite design—a commonly used experimental design in RSM.

This designed experiment allowed the team to measure the effect of each factor on the ethanol concentration level and to examine the interactions between the factors with the fewest experimental runs—saving time, glassware, chemicals, and labor.

The calculations involved in RSM can be complicated, but Minitab creates contour plots that make it easy to visualize input settings for multiple factors simultaneously and consider how they affect the response.

For example, in the contour plot below, the third darkest green region shows where sugar concentration and temperature can be set to optimize ethanol production during fermentation, given pineapple peel concentration and pH level are fixed at the highest levels of their defined ranges—14% and 6, respectively—and fermentation time is fixed at 30 minutes, its lowest level.

Ethanol Contour Plot

The research team’s analysis using Minitab’s response optimizer showed the optimal ethanol concentration of 8.637% is produced when the sugar concentration is 22°Brix, the temperature is 26°C, Peel% is 14%, pH is 6, and the fermentation is 30 hours. These settings, combined with the naturally high content of fermentable sugars in pineapple peels, produced ethanol with an alcohol content of 0.21%, compared to ethanol produced under similar conditions from banana waste, which had an alcohol content of 0.035%.

Minitab Statistical Software made it easy for the researchers to design their experiment, analyze their data, and understand and visualize their results.

The results of this research suggest there is a benefit to salvaging the more than 550,000 tons of fruit peels produced at Pineapple Estate, and could have implications for producing fuel from other fruit wastes.

This story was adapted from an article published by the World Academy of Science, Engineering and Technology in December 2012.

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