Preparation of Molasses Wort

1 Dipl. Distil. Module 1B: Preparation of Molasses Wort

Institute of Brewing and Distilling Diploma in Distilling (Dipl. Distil.)

Module 1B: Preparation of Molasses Wort - Summary

1. Sources and Types of Molasses

1.1. Molasses, inherent variability and world sugar prices

Molasses (or treacle) is a thick syrup by-product from the processing of sugarcane (or sugar beet) into sugar; sometimes “molasses” also refers to sorghum syrup. However, this Elective section will be restricted to distillation products from the use of sugarcane molasses only. In this context, “Molasses” is defined as the sugar-containing syrup derived from sugar cane, from which no more sugar can be removed economically. It is also called ‘final molasses’ or ‘blackstrap’.

It must be emphasized that the objective of a sugar refiner is to make a profit from making and selling raw sugar. Any money made from molasses is a welcome but small contribution. When the world sugar price is very high, it becomes economic to recover more sugar from molasses. This leads to the molasses having a different composition, lower in sugar and higher in ash (dissolved salts).

In contrast when the world sugar price is very low it might be necessary for the refiner to make a product called ‘high-test cane’ or ‘high-test molasses’. This is cane juice concentrated to syrup containing about 80% by weight of sugar. It has a low content of yeast nutrients including dissolved salts (sometimes called ‘ash’). The dissolved salts can be deposited in the still during distillation, a process called ‘scaling’. Scale is not easy to remove, so high-test cane molasses is excellent for distillers to use.

A distiller must always regard molasses as being what it is, the co-product of another industry. There is little or no incentive for the producer of sugar to ensure that the molasses is always the same. On the contrary, it is safer for the distiller to anticipate that molasses quality will be variable. Variations in molasses are caused by: • variation in the sugar cane crop (soil, climate, water etc) and • variations in the refining process (different factories, different process details, world sugar prices etc.)

These variations may be minimised by the distillery being located near to the refinery. Then there could be close communication concerning the quality of the molasses. Often, this is done by molasses traders who will select and ship

© The Institute of Brewing and Distilling, February 2009 (Version 2009) 2 Dipl. Distil. Module 1B: Preparation of Molasses Wort molasses from a molasses-producing region to a molasses-using region. The traders business depends on the molasses being of suitable quality for the distiller.

1.2. Types of Molasses

Sulphured molasses is made from young green sugar cane and is treated with sulphur dioxide, which acts as a preservative during the sugar extraction process. Unsulphured molasses is made from mature sugar cane and does not require treatment with SO 2 during the extraction process. There are three grades of molasses: • Mild or first molasses, • Dark or second molasses, and • Blackstrap. These grades may be sulphured or unsulphured.

Molasses is made from pure sugar cane juice, extracted from the harvested cane plants, stripped of their leaves, by crushing or mashing. The juice is boiled to concentrate and promote the crystallization of the sugar. The result of the first boiling and removal of crystal sugar is first molasses , which has the highest sugar content, because comparatively little sugar has been extracted from the juice. Second molasses is created from a second boiling and sugar extraction, and has a slight bitter tinge to its taste. The third boiling of the sugar syrup gives blackstrap molasses . The majority of the sucrose from the original juice has been crystallized, but blackstrap molasses is still mostly sugar by calories; however, unlike refined sugars, it contains significant amounts of vitamins and minerals (such as calcium, magnesium, potassium and iron).

1.3. Molasses as a raw material for distillation Only blackstrap molasses is used for distillation and all subsequent references to molasses imply blackstrap . Molasses is a dark coloured syrup containing a significant amount of sugar, often in the region of 50% by weight, giving it a specific gravity of about 1.45.

The sugar content makes the syrup viscous (thick and sticky). The viscosity is important because a batch of molasses that is more viscous than usual might be difficult to handle in the distillery. Viscosity is much greater at low temperatures so it is necessary to insulate or even heat molasses pipelines in climates where the minimum temperature falls below about 5 oC. Distillers use molasses of various types worldwide. This is because molasses has three useful characteristics: • it is a relatively cheap source of fermentable sugar and • it contains at least some of the yeast nutrients needed for the growth of the yeast at the start of fermentation plus • it is stable, easy to transport, to store and is relatively easy to handle at the distillery.

Sugar cane is a plant, scientific name Saccharum officinarum . It has stiff stems that can reach 4m in height. The sap inside the stems is called the juice. It contains a high concentration of sugar of the type known as sucrose (cane sugar). More than half of the world supply of sugar is from sugar cane that is grown worldwide in tropical and sub-tropical climates.

Many other plants contain relatively high concentrations of sugar. Their juice or molasses made from their juice is used in distilleries worldwide. For instance, the sugar palm, Arenga saccharifera , from South-East Asia has a high concentration of sucrose in its sap. Another example is citrus molasses, a co-product of the citrus juice industry (orange juice). Sugar beet, Beta vulgaris is a plant that grows well in temperate climates. Like sugar cane it is grown in large quantities and provides much of the world sugar supply. Only cane molasses is covered in this course of study.

Large-scale sugar production from sugar cane has three products: • pure sucrose • plant residues such as bagasse and filter mud, and • molasses.

Molasses provides much of the sugar needed for the distilling industry worldwide and of course provides a useful source of income for the sugar refiners whose molasses can be sold to the distillers.

2.1. Sugar cane growth and juice extraction

Sugar cane is planted on suitable soils from cuttings. Each plant could have a useful life of between 3 and 7 years, after which it is replaced by new cuttings. The plants usually receive fertilisers and irrigation to ensure a high yield of ripe canes. The canes are ripe when the sugar concentration in the juice reaches its maximum value. This varies greatly with soil, climate and water supply: the maximum could be in the region of 9 to 17% sucrose by weight, commonly about 13%.

When ripe, the canes are cut as close to the ground as possible because the sugar concentration is high at the bottom of the cane. As soon as possible, the canes are taken to the cane milling plant in the sugar mill. It is crushed to extract the juice. Water is added to help the extraction. The co-product, bagasse, plays no further direct role in the refining process, although it may be burned to produce steam. At this stage, the objective for the refiner is to collect as much sugar as possible from the cane.

2.2. Cane sugar refining and the production of molasses

The next objective of the sugar refiner is to collect as much sugar from the juice as is economic.

Sugar mills differ in the way that they achieve this objective. However the basic principles are much the same, first the removal of suspended and soluble plant material by clarification and filtration, then the concentration of the liquor until crystals of sugar are formed and then the collection of the crystals by centrifuge.

For example the basic steps involved could be: • the addition of lime, (calcium hydroxide) to precipitate plant debris • heating to bring about the reactions leading to precipitation • settling of solids as a sludge in the clarifier • removal of the sludge to the filter and return of the liquid to the product stream • evaporating the liquid using vacuum pans • allowing crystals to form the crystalliser • separation of the sugar crystals from the molasses in centrifuges and • further crystallizations of sugar from the molasses until it is not economic to crystallise more.

The main product of this process is raw sugar and the co-product is molasses. As a guide to the yield obtainable: • 100 tonnes of cane crushed could give 11 tonnes of raw sugar plus 3 tonnes of molasses.

Molasses contains so much sugar that microorganisms are unable to grow in it although many bacteria and yeast cells can survive in it. The latter must be taken into account when the distiller prepares the molasses for fermentation in the distillery. The stability of molasses is excellent. It can be transported in road or ship tankers to storage at the distillery site. It will remain usable for many months if not years.

3. The Composition of Molasses

3.1. The chemical analysis of molasses

The sugar in cane molasses is mainly sucrose with some glucose and fructose. Glucose and fructose are formed by the breakdown of sucrose during the sugar extraction process described above. It is not possible to give a typical chemical analysis of cane molasses because of the variability referred to above.

An example of the chemical analysis by weight of one sample of blackstrap (cane molasses) is given below: 1. sucrose ……………………………………..35% 2. glucose plus fructose……………………...15% 3. water………………………………………...20% 4. other plant materials……………………….20% 5. inorganic salts (ash)………………………..10% 6. specific gravity………………………………1.45 7. degrees Brix (ºBrix)………………………….85

Analyses such as this are important to a distiller because the content of fermentable sugars determines how much alcohol can be produced. The fermentable sugars are sucrose and its breakdown products, glucose and fructose (the latter two sugars are sometimes called the ‘reducing sugars’). Most strains of yeast can quickly ferment all three of these sugars.

The gravity of the molasses gives an indication of the total solid content. As can be seen from the example analysis above, most of the solids content, in this case 50%, is dissolved sugar. However, 30% of this molasses is non-sugar material: so specific gravity measurements on molasses cannot give the exact sugar content.

Despite this fact, it is a useful to measure specific gravity. This is because molasses must receive some treatment before it is used in the distillery. It is necessary to keep track of the specific gravity to show that pre-treatment is proceeding to plan.

The specific gravity can be obtained quickly by using a hydrometer. This is a glass instrument with a floatation bulb connected to stem that has a scale of marks on it. The hydrometer is allowed to float in the molasses and a reading is taken on the scale at the level at which it settles in the molasses.

There are various types of hydrometer, one of which gives a reading of specific gravity directly. In the sugar industries another hydrometer is used, the ‘Brix hydrometer’. In a pure sugar solution, the degrees Brix figure gives a direct reading of sugar concentration. For instance, a hydrometer reading of 49 degrees Brix (49 ºBrix) in a pure sucrose solution indicates that there is 49% by weight of sugar in the solution. In molasses there is much more dissolved and suspended matter than simply sugar, so the Brix hydrometer can not be used to find the exact sugar concentration of molasses: a laboratory analysis for sugar content is also needed. The Balling scale for specific gravity is used by some distillers. It is very similar to the Brix and Plato scales.

Specific gravity varies with temperature so the temperature of the molasses must be taken at the time of the hydrometer reading. A correction to the reading is added or subtracted as indicated by the Brix tables. The standard temperature for the Brix hydrometer readings is 20 ºC.

The water content of the molasses received is of no direct importance to the distiller, apart from the fact that purchased molasses should not contain too much water and too little sugar.

The plant materials are a complex mix of compounds originating from the cane. Within this mix are: • a small amount of nitrogen-containing compounds that can be used by the yeast to grow at the start of fermentation, • the vitamins biotin and pantothenate for yeast growth and • the vitamin thiamine for yeast fermentation.

The mineral salts are also important for yeast growth and fermentation.

3.2. The biological analysis of molasses

Molasses is not a sterile product; it usually contains live microorganisms, yeasts, bacteria and fungi. It is not usually necessary for the distiller to carry out a biological analysis either to find out what kinds of microorganism are present nor to find out how many there are. This is because: • the cells present can not grow in undiluted molasses, they just remain alive for months or years and • the molasses pre-treatments kill some of them and • a rapid start to fermentation provides too much competition for these molasses based microorganisms to have any significant effect.

The reason why microorganisms cannot grow in raw (undiluted) molasses is that the osmotic pressure is too high; there is too little water available to the yeast or bacteria cells to support growth or fermentation.

Two particular types of bacteria can occur in molasses if the sugar refinery has problems such as long delays during production or has very poor hygiene: • Leuconostoc mesenteroides infection can cause loss of yield because it converts sucrose into non-fermentable dextran and • Zymomonas mobilis produces unwanted aroma compounds that can carry over to distilled spirit.

4. Molasses pre-treatment (‘Mashing’)

Molasses must be prepared in suitable form just before fermentation. This process is sometimes called ‘mashing’, or it might be called ‘molasses pre- treatment’. When the molasses has been prepared it is sometimes called ‘the mash’. Alternative names are ‘unfermented molasses’ or ‘molasses wort’.

Pre-treatment includes some or all of the following: • removal of materials to avoid contamination of the distillation equipment with solids and scale • diluting with water to the desired ºBrix for the start of fermentation • addition of yeast nutrients to ensure a satisfactory fermentation • adjusting the acidity level to the correct value for the start of fermentation and • pasteurising or sterilising the molasses just before fermentation.

4.1. Removal of solids and scale-forming compounds (molasses clarification)

There are two groups of molasses constituents that could cause handling problems in a distillery: • various plant materials, about 20% by weight of the molasses and • dissolved salts (ash), about 10% by weight of the molasses.

The first group could settle out or be deposited in various parts of the plant, particularly in holding tanks, fermentation vessels and the still. Such deposits have to be removed and this leads to extra cleaning and increased plant downtime.

The second group, the mineral salts, can form a hard scale in the still. The compound responsible for most of the scale is calcium sulphate (gypsum). If this is deposited on the sieve plates of a column still it reduces the size of the sieve plate holes. This alters the amount of contact between the vapour and the liquid on the plate that is a critically important design factor. The result is reduced efficiency of distillation.

There are various methods for the reduction of the amount of these two types of material in the molasses. In the first example below, temperature alone is used and the effect is to remove some of the plant material but not much of the calcium that leads to scaling: • molasses is diluted to below 50 ºBrix and held at about 75 ºC for some hours and then either centrifuged to remove the solids or the solids are allowed to settle out and the partially clarified molasses is decanted from the vessel. In the second treatment, the chemical addition (sulphuric acid) causes the formation of insoluble calcium sulphate that is removed by centrifugation: • molasses at about 45 ºBrix is acidified using sulphuric acid, held at 90 ºC for one hour and then passed through a centrifuge.

The second treatment also removes much of the plant material.

4.2. Dilution of molasses to the start gravity for fermentation

Yeast cannot ferment concentrated sugar so it cannot ferment molasses at its natural strength; the osmotic pressure is too high and there is too little water available to the yeast cells to support growth or fermentation.

For most yeast strains, the molasses must be diluted with water to below 25 ºBrix for fermentation to start. Fermentations work better at 22 ºBrix and better still at lower gravities such as 17 ºBrix for example.

4.3. Addition of yeast nutrients

It is often necessary to add yeast nutrients to molasses in order to obtain a rapid and complete fermentation. The yeast is most likely to need nitrogen and might also need some phosphate. These can both be supplied by diammonium phosphate. If only nitrogen is required then ammonium sulphate can be used.

Other forms of available nitrogen and phosphate can be used with the exception of urea. Urea should never be used for beverage alcohol production as it can be converted into ethyl carbamate, which is a carcinogen (a cancer-producing chemical).

4.4. Adjusting acidity

Fermentations have an initial acidity level at which they have been found by experience to work best. This pH value is called the ‘optimum pH’, and usually lies between pH 5 and pH 6. Any adjustment would be made by adding an acid (e.g. sulphuric acid) to reduce the pH value or an alkali (e.g. sodium carbonate) to raise the pH value.

5. Reduction of Microbial Count.

5.1. Pasteurising or sterilising the molasses

If there is a strong chance that the molasses contains infecting micro-organisms, further treatment may be necessary. The diluted and nutrient supplemented molasses can be given a heat treatment to reduce the content of live bacterial cells before adding the yeast. Sterilisation is the killing of all bacteria and yeast cells together with their spores. This is seldom necessary. The word pasteurisation is used to describe a heat treatment that produces a significant reduction in the number of bacteria but does not kill them all. For instance holding the diluted molasses at 75 ºC for 30 minutes would severely reduce the level of any Lactobacilli bacteria that might be present.

Antibiotics (e.g. Penicillin) can also be used to control microbial contamination.

Many distilleries do not need to use any further treatment because: • the other pre-treatments have reduced sufficiently the number of bacterial cells present and • the most useful way of preventing the growth of infections is for the fermentation conditions to be optimum and for the yeast to start fermenting without delay.