The Sugar Engineers

Alcohol from Cane Molasses

This article sets out a way of calculating how much alcohol you can can make from the molasses your sugar factory produces.

How much molasses?

The fist step is to calculate how much molasses you will produce. In the Southern African Industry it is usual to express the amount of molasses made per tonne of cane crushed at a standard molasses brix of 85°

The South African Industry average figures for the past five years are shown below

Molasses at a standard 85°Bx
Year	Molasses%Cane
2000/01	3.70
2001/02	3.93
2002/03	4.03
2003/04	3.73
2004/05	4.16

So the amount of molasses produced is

M = C · M₈₅ · 0.85 / B_m

where

= tonnes molasses produced

= tonnes cane crushed

M₈₅

= Molasses at 85° brix as a percentage on cane crushed

B_m

= Actual brix of molasses produced

Fermentable sugars

The next step is to calculate the amount of fermentable sugars (FS) in the molasses. The fermetable sugars in molasses are sucrose, glucose and fructose; there are other sugars present in molasses, they are either unfermentable or are in small enough quatities that they can be ignored.

There are a number of ways of measuring fermentable sugars in molasses; the most accurate is High Performance Liquid Chromatography (HPLC). This method is described in Naidoo, Schoonees and Schorn, SASTA Laboratory Manual Including the Official Methods, South African Sugar Technologists' Association, Durban, 2005, ISBN 1-874903-32-8

The Lane and Eynon method also described in the SASTA Lab Manual is a two step process, which measures reducing sugars by titration. Reducing sugars are those sugars which reduce Fehlings reagents. Glucose and fructose reduce Fehlings reagents, sucrose does not, so the sucrose is inverted using hydrochloric acid and the reduction titration is repeated and the total reducing sugars can be calculated.

The problems with this method are

there are other substances in the molasses which are also reducing agents, but are not fermentable sugars, so this methed overestimates the amount of fermentable sugars, and
the titration is complex and requires a degree of skill to ensure repeatbility, that may not always be present in a sugar factory laboratory.

South African Industry data on molasses quality are given as a guideline

South African Industry Average Molasses Quality
Year	Refractometer brix	Sucrose/refractometer brix Purity	Fructose%	Glucose%	FS%brix in molasses
2000/01	84.26%	37.21%	7.55%	5.41%	52.59%
2001/02	84.44%	37.03%	7.58%	5.47%	52.48%
2002/03	85.09%	37.24%	7.14%	5.13%	51.66%
2003/04	84.79%	37.92%	7.08%	5.22%	52.43%
2004/05	83.97%	36.94%	7.93%	5.20%	52.58%

So, it is clear that about 52.5% of the brix in molasses are fermetable sugars. To calculate the tonnes of fermentable sugars in molasses we use the following formula

FS = M · B_m · FS%B

where

= tonnes fermentable sugars in molasses

= tonnes molasses produced

B_m

= Actual brix of molasses produced

FS%B

= fermetable sugars as a percentage on brix in molasses

Alcohol Yield

The amount of alcohol produced is given by

A = FS · Y_f · E_de

where

= litres of alcohol produced

Y_f

= Fermentation yield

E_de

= Alcohol recovery (or distillation and evaporation efficiency)

On-line Calculation

Calculate the amount of alcohol you can produce from your molasses