Concept: ethanol fermentation kinetics

In batch fermentation, Saccharomyces cerevisiae first grows using available sugar, causing a gradual increase in biomass that follows a lag, exponential and stationary phase pattern. As cells grow and ferment, sugar is steadily consumed, and ethanol accumulates as a growth‑associated product until the substrate is depleted or ethanol inhibition stops further production. Typically, maximum ethanol concentration is reached close to the time when sugar is almost exhausted, while biomass has just reached its maximum and then remains roughly constant.​

Why option (4) is correct

In option (4), the sugar curve starts high and declines continuously toward zero, reflecting consumption of glucose or sucrose during yeast fermentation. The biomass curve shows a sigmoidal rise that levels off, consistent with yeast entering stationary phase after nutrients are limiting or ethanol becomes inhibitory. The ethanol curve begins near zero and increases over time until it reaches a plateau, matching observations that ethanol accumulates during active fermentation and then stabilizes once sugar is depleted and growth largely stops.​

Why option (1) is incorrect

In option (1), sugar decreases as expected, but the biomass curve remains very low for most of the process and rises abruptly late, which does not fit the typical exponential growth pattern of yeast in batch culture. Ethanol is shown to increase quite early and then plateau while biomass is still low, whereas in reality significant ethanol production usually coincides with active cell growth and reaches maximum when biomass is already relatively high. Thus, the timing of biomass growth relative to ethanol formation is unrealistic in this curve.​

Why option (2) is incorrect

In option (2), sugar again declines as expected, but the biomass curve rises very sharply and then levels off early, suggesting almost instantaneous attainment of maximum cell mass, which is not compatible with the typical multi‑hour growth phases of yeast. Ethanol is drawn as starting to accumulate only after biomass has already reached its plateau, implying a purely non‑growth‑associated product, whereas many kinetic studies show ethanol formation in yeast is largely growth‑associated during the main fermentation phase. Therefore the sequential separation of growth and ethanol production in this option does not mirror usual batch ethanol fermentation behavior.​

Why option (3) is incorrect

In option (3), the sugar curve forms a downward‑opening arc that reaches zero roughly when biomass peaks, but the ethanol curve starts high and then declines, which is opposite to real fermentation where ethanol begins near zero and increases. Additionally, biomass is drawn to decrease strongly after reaching a maximum, whereas in standard batch fermentations cell count generally stays relatively constant or declines only slightly due to lysis or stress once nutrients are exhausted. Because ethanol should accumulate, not fall, with time, this curve cannot correctly represent ethanol production by yeast.​

SEO‑oriented introduction (for article use)

Ethanol production by yeast in batch fermentation follows a characteristic pattern where sugar is consumed, yeast biomass grows and then stabilizes, and ethanol accumulates until the process stops. Understanding the correct ethanol production by yeast curve helps in interpreting fermentation data, optimizing industrial bioethanol processes, and answering exam questions that compare sugar, biomass and ethanol profiles over time.​