3.4 Fermentation Products
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The group of products under consideration have in common that they are produced by microbial conversion with date sugar as the main nutrient. In most cases the target product is the metabolic by-product of this microbial conversion like alcohol from sugar or acetic acid from alcohol. However, in a few instances the target product is the microbial biomass itself such as baker's or fodder yeast. Another distinction can be made between products in which the other components of the date also play a role in the final aroma, colour and general quality of the target product like date wine or vinegar, whilst others are required in a pure form as in the case of distilled alcohol or citric acid.
Glucose is considered a fundamental organic building block in nature from which a large number of substances can be derived by chemical or microbial manipulation. Theoretically therefore dates could be a source of many derived products though as has already come out before, dates are not a typical sugar crop. Production of fermentation products is therefore restricted where the availability of suitable (low cost) dates and market demand create a favourable climate for producing these types of products, a number of which are listed below:
Wine is a beverage derived from fruits (and occasionally also vegetables) in which all or part of the sugar is converted into alcohol. All other constituents of the solubles of the fruit are part of the final product and determine, together with newly formed components, its colour, taste, flavour and consistency. The alcohol content most commonly is about 11-13% (by volume). Wine is mostly known and connotated with grapes, but also other fruits may be used including dates. Apart from the use of dates as a reinforcement and flavouring agent in beer making in Ancient Egypt (128) references to date wine can also be found from early history, although the subtle difference between a pure date extract or a fermented one, does not always come out clearly (445, 128). Dates are not an outspoken fruit for wine making; they lack natural acidity and the typical flavour and the slight astringency such as found in grapes. It should also be remembered that almost 40% of the total date crop is produced in Muslim countries where prohibition is in force, thus reducing the marketing possibilities considerably. Basically wine making is a simple process, but to produce high quality wines through the correct choice of raw materials, process conditions and aging, one needs a thorough know-ledge of the processes that are taking place. Research on date wine focusing on selection of active yeast strains, density of the solution, process conditions and additional nutrients has been active and continuing over the last decades (209, 482, 419, 197, 26, 210, 26, 415, 73).
Zahdi dates, the common variety in Iraq, were found to be produce good quality light-coloured dry wine whilst date syrup (dibs) proved better suited for a darker, sweeter wine. The addition of 0.025% urea reportedly aided the fermentation process. Aging improved the quality of the wines (73).
It would seem logical that the vast experiences gained in wine making from grapes could be used fruitfully to further explore and improve the use of dates as a raw material for a fermented beverage.
Whilst the alcohol produced during wine making is an integral part of the final product, alcohol can also become a target product and separated from the fermented mother liquor (mash) by distillation and rectification. Perhaps a transitional product between wine and distilled pure alcohol was date "sherry" (a misnomer because the original sherry made from grapes is a specific type of (strong) wine and not a distilled product). The process consisted of fermentation of a date extract and a one-run distilling off without much concern for cutting the "heads" (the light fractions) and the "tails" (the heavy fractions) resulting in a drink of about 25-30% alcohol content with a flavour determined by the volatiles of the raw material which distilled together with the alcohol (62).
Pure alcohol production is better known as the production of a chemical produced in different strength and purities to be used subsequently in beverages, the chemical industries (solvent), medicinal purposes (disinfection), cleaning and as a fuel for small stoves and alcohol lamps. In the seventies, in the wake of the fossil fuel crisis it gained importance as a fuel for internal combustion engines as a renewable energy source. In large scale industry alcohol is also used as the primary material for the production of numerous derivatives such as acetaldehyde (further used in the production of acetic acid and butanol) and ethylene (which is the base for the manufacture of polyethylene, polystyrene and poly vinyl chloride (PVC)), not to mention the numerous alcohol distillates which derive their typical flavour from the raw material they are made of such as "Kirsch" (cherries), "Schlivovitch" (plums), Gin (juniper), Tequila (cactus), Grappa (grapes), Brandy (grapes), Rum (molasses) etc. Alcohol is therefore a very versatile chemical but as an industrial raw material requires a well developed down-stream chemical industry. In the date producing countries alcohol making with dates as the raw material exists but its use is mainly restricted to medicinal purposes, small scale household use and in some countries for the preparation of beverages. It should also be remembered that alcohol used as a feedstock for the chemical industry is mostly synthetically produced.
Alcohol fermentation is a biochemical process whereby the enzymes necessary for the conversion of sugars are procured from living yeast cells, usually of selected strains of Saccharomyces cerivisiae. Through a chain of reactions, fermentation results ultimately and chiefly into ethyl alcohol (ethanol) and carbon dioxide according to the following formula:
C6H1206 ________ 2C2H50H + 2C02 + 27 Kcal
From this formula the theoretical yield of 51.1 kg ethyl alcohol (64.1 litres) and 48.9 kg carbon dioxide can be calculated. However, because the yeast cells for their own growth need material and some by-products are formed, the practical yields rarely exceed 90% of these theoretical figures. To produce 95% (v/v) alcohol from dates the following processes are involved: extraction of the juice; fermentation; distillation and rectification. Since yeast cells thrive best in solutions of 18 to 20o Bx and the extracted juice does not necessarily have to be completely free of non-solubles, a batch-wise extraction is much practised. In this system the whole dates remain stationary in a vessel until after a series of step-wise extractions with juices of decreasing Bx content the dates are almost completely stripped of their sugars. A schematic representation of such a system is given in Figure 82 (587) which shows increasing sugar content from vessel 1 to 4. Live steam is added to keep the extraction temperatures at the correct level. Once the dates in vessel 1 are exhausted and removed the extraction water will be fed to vessel 2 and fresh dates fed to vessel 1. Juice from vessel 4 which had the highest sugar content will be diverted to vessel 1 which will now become the producer of the final juice. Thus by intermittently emptying and filling and changing of valves a semi-continuous extraction system is created with few moving parts and no separation equipment. A variation of the above process is whereby juice is circulated by a pump in the same vessel until equilibrium is reached, after which the juice is redirected to another vessel with higher Brix in the dates. In this way 3 or 4 extractions may suffice to yield a suitable fermentation liquor with minimal sugar loss in the dates. Another advantage is that dates in their stationary position release very little fibre and start to act as a filter for the circulating juices. Juice for fermentation is also produced in a 2-stage countercurrent continuous system (Iraq) but in that case the recovered juice is filtrated before being fermented. The collected juice of around 18-20o Bx is now ready for fermentation. It is advisable to keept it at an elevated temperature (60oC) to avoid infection by undesirable microbes. Just before adding the prepared starter of the selected yeast strain (428) the juice has to be cooled, because optimal fermentation temperature lies around 32oC. Fermentation will, for the purposes of crude alcohol making, last for a couple of days, when the liquid now called DM (distilling material) contains about 9% alcohol. The next phase aims at separating the alcohol from the mother liquor. Distillation can be done batch-wise in pot stills whereby alcohol/mixtures are distilled and collected in between set boiling temperatures range in order to remove the non-desired (and sometimes poisonous) light fractions and the heavier by-products (a.o. fusel oils). Repeating this process 4 times alcohol strengths of up to 90% can be obtained. In industrial alcohol production the process has been made continuous, and consists of two main stages: distillation and rectification (Fig. 83).
In the distillation part the DM is counter currently brought into contact with rising steam under vacuum in a stripping column over a number of plates. The net result of the continuous process is that at the top of the column a predominantly alcohol/water vapour mixture is drawn off, whilst at the bottom the remainder of the DM, practically free of alcohol, now called slop or stillage, is discarded. The alcohol/water vapour mixture is drawn into the base of the rectifying column in which by a step-wise condensing and boiling process the vapour mixture separates into fractions of different boiling points. The lightest fractions (heads, mainly aldehydes and esters) are drawn off at the top, pure alcohol of 95% (side product) somewhere at the upper part of the column, heavier fractions (fusel oil, mainly higher alcohols) at the lower end and a residue called reflux at the bottom which is returned to the distillation column. By this process a very pure alcohol is produced from pure culture fermentation in contrast to for instance rum making which purposely introduces micro-organisms other than yeast to create aromas which are distilled with the alcohol to impart specific flavour to the final product.
Four main by-products are produced during alcohol production:
i. Carbon dioxide, which can be (and sometimes is) recovered and liquified or sold as dry ice. In countries where the soft drink industry is well developed the use of CO2 for this purpose should be considered.
ii. Fusel oil is a generic term for a mixture of alcohols with a higher boiling point. They are normally removed from quality drinks (the ratio ranges for 0.1-0.5% of total alcohol) and can be used as solvent for lacquers and resins (466).
iii. Yeast cells have, during fermentation, fed on sugars and excreted (mainly) alcohol whilst increasing in size and number. The microbial biomass, high in protein, is therefore increasing and represents a valuable resource in the magnitude of 3-4 kgs dry yeast per 100 l of alcohol produced. Its validation will depend much on the size of the alcohol operation and the marketability of the recovered product.
iv. Slop or stillage, also called vinasse is the watery remains of the distillation process and may contain 7-10% solids. It is rich in minerals and yeast (if not centrifuged out). It can be used as fertilizer but low solids content soon puts a limit to its application because of transport costs. It has therefore been concentrated to be used in feedstuffs. This method is partially induced by the need to find an outlet for this high polluting waste product. In smaller plants where disposal of limited quantities of stillage does not create environmental problems, the stillage is normally discarded without further use.
Apart from medicinal and household use date alcohol is well known, in those countries where its use is permitted, for the production of arak, a strong alcoholic beverage in the same group of drinks as the Greek "Ouzo", the French "Pernod" or the Italian "Mistral". It is a "made up" drink, i.e. it is composed of pure distilled alcohol with little flavour reference to the original dates it was made from, and added essences (anise), and mastic (a natural resin). It is a clear beverage of around 50% (v/v) alcohol strength with a pronounced anise flavour. Upon dilution with water the anise oil separates out of solution and forms an emulsion, giving the long drink its milky appearance. Mastic is added to give viscidity ("body") and extra flavour to the drink. The purity and wholesomeness of arak depends on the purity of the components and the way these are made (94, 567, 83). A proposal for manufacture of date brandy, in which the volatile date flavours would participate in the aroma of the final product has also been made (285).
c. Organic acids
The number of organic acids that directly or indirectly can be derived from sugars is large but for practical reasons of economics of scale and market limitations only a few have been considered for date extracts.
The most known and widespread is acetic acid in the form of household vinegar. The principles of manufacture are much similar as that for wine making, that is both products can be made (and used) at the household (or cottage) level incorporating the flavours of the date extract and secondary fermentation products. Acetic acid (like alcohol) can however also become the target product and separated from the mother liquor by distillation. Acetic acid formation is a sequence to alcohol making (well known from wines that turn sour ("vin-aigre" actually means sour wine). The basic chemical formula is:
C2H5OH + O2
______ CH3COOH + H2O + 118
alcohol acetic acid
It shows the base material is alcohol, the process needs oxygen (in contrast to yeast fermentation) and is exothermic, i.e. heat is formed. The microbes, in this case bacteria, belong to the genus Acetobacter. Like for grape vinegar very acceptable household date vinegar can be made inoculating a strain of Acetobacter into a date wine supplemented with some nutrients (urea or malt) and let it stand with an access to oxygen (for instance a barrel on its side with the bunghole left open). After a first formation period vinegar can be drawn off at the bottom for use and at regular intervals the feedstock can be supplemented by adding new date wine. In this way a perpetual production takes place which can go on for years. To speed up the process of acetic acid formation, generators have been developed in which the inoculated mother liquor is sparged over a filling material (ceramics, beechwood shavings, pieces of date midribs), which provides a large contact sur-face between forced circulated air and the liquid. The collected liquid under the perforated false bottom in the generator is recirculated by pump passing through a water cooler to keep the temperature at the correct level (27-30oC) (Fig. 84). Another method of accelerated vinegar production is by submerged culture in a vessel heavily oxygenated by air bubbles. One batch will take 24-48 hours to ferment after which half the tank will be emptied and new feedstock added for another run. Vinegars can be produced in this way from a multitude of raw materials each having its own flavour and contain about 4-5% acetic acid.
Concentrated pure acetic acid used in industry and other chemical use is made from pure synthetic or fermentation alcohol and concentrated by distillation.
In the date producing countries date vinegar is well known, but there are only one or two industrial scale production units. Literature has few references on the subject (197, 436).
With respect to other potential organic acids which are quite numerous as compared to the product lists for molasses (466), for date juice or date syrup most attention in research and project development has been given to citric acid (408, 407, 69, 446, 406, 517). The reason probably is that citric acid would have an assured market potential for the soft drink and food industry. But it is a more complex fermentation than for alcohol and acetic acid which are almost spontaneous and with minor precautions can be optimized with simple means. Instead citric acid is formed by Aspergillus Niger under stress and the conditions to optimize the process are stringent. The feedstock may need treatment to bring it in a condition to yield the highest amount of citric acid versus another common fermentation product, oxalic acid. It is therefore clear that apart from the possible alternative feedstocks available, and the market potential for organic acids, the suitability of date juice also has to be investigated for these fermentation. It is not just a matter of technology transfer.
d. Single cell protein
The possibility of turning carbohydrates into proteins by microbial conversion in situations where the first occurs in abundance and the second is in deficit, has caught the imagination of many scientists over the last decennia. There are numerous proposals for utilizing waste and surplus products into high protein food and feed in the form of microbial biomass derived from a variety of micro-organisms, yeasts, micro-fungi and algae in particular. The exaltation perhaps reached its peak in the early seventies when hydrocarbons of fossil origin were included in the choice of raw materials with high expectations of alleviating world food shortages through the use of the "mighty microbe". But the enthusiasm was soon tempered for two main reasons: the sudden rise in oil prices (1973) and the increasing suspicions on the presence of noxious substances (for human health) in the final products, leading to the closing down of several large scale investments. Since then a more realistic view has been taken, because after all, microbial products have existed over the ages. Earlier reference was made to yeast as a by-product of beer, wine and pure alcohol production. In large scale operations these yeasts are recuperated, purified and sold, but the yield compared to alcohol production is small. By changing the process parameters (mainly a change from anaerobic to aerobic conditions), the selection of suitable yeast, almost exclusively of Saccharomyces Cervisiae strains, and providing appropriate nutrients, the process can be directed towards biomass production. Thus, expressed in a simple molar formula glucose is transformed into biomass following:
+ NH3 ______ C6H7O3NH2
glucose nutrients microbial biomass (ashfree)
which theoretically would mean an 80% conversion of glucose into biomass, but since the yeast needs glucose for metabolic energy (C6H12O6 + 602 ____ 6CO2 + 6H2O) the effective yield (dry basis) is not more than about 50% and about 3/8 of the glucose is converted into carbon dioxide and water. In the extreme case only biomass and no alcohol is produced, though there are systems whereby both biomass and alcohol are recovered, both substances at a lower rate than their maximum possible (466).
A well-known microbial product is baker's yeast available as pressed fresh yeast (27-29% dry substance) kept refrigerated or as active dry yeast (90-92% dry matter), the latter often imported and used as a leavening agent in bread making in the date producing countries of the Old World. It is a living product also in its dried form, and reactivated when put in solution.
There have been a number of investigations into the production of baker's yeast on date extracts (359, 412, 352, 410). This research was mainly directed to testing the suitability of date extracts versus molasses, which is the traditional raw material for yeast production. Good results are obtained and there does not seem any technological constraint in using date extract which would be of special interest to date producing countries which have no beet or cane sugar production.
Whilst baker's yeast is produced almost exclusively with selected strains of Saccharomyces Cerevisiae, food and fodder yeast is also produced with other genuses or mixtures of yeast strains, such as Candida Tropicalis and Torulopsis Utilis. The dried yeast (90% dry matter) is known for its high protein content and vitamin B as is shown in the Table 18, (466).
Feed or fodder yeast is fed mostly to monogastric animals as a protein and vitamin/mineral source. For human consumption it is mostly considered as a health food or used as condiment in the form of yeast extracts which are mainly hydrolyzed yeast proteins with a strong specific flavour.
When planning for a food or feed yeast industry note should be taken that the process needs a large amount of nutrients, in the form of minerals, electrical energy and cooling water. As a protein source it is normally not competitive with soy protein.
Apart from the experimental work on using date extracts (414, 186, 260, 43, 416), feed yeast production using dates has reached industrial level in Iraq.
Some research work has also been done on fungal protein using Aspergillus (390, 411), whilst the same organism was also tested on date extract for the production of amylase (562).
Composition of dried yeast
|Molasses||Spent liquor of beech|
|Water||8.8||6.0 - 8.5|
|Crude protein||43.5 - 56.6||46 - 54|
|Phosphate||2.6 - 2.9||2.7 - 4.3|
|Crude fate||-||4.4 - 8. 2|
|Ash||5.9 - 7.2||6.6 - 8.7|
|Vitamins in ppm|
|Thiamine||60 - 72||9 - 35|
|Riboflavine||39 - 45||39 - 67|
|Pantothenic acid||32||23 - 90|
|Nicotinic acid||476||427 - 685|
|Ergosterol||5 000 - 8 000||3 500 - 5 200|
By selection of the most appropriate micro-organisms and manipulating the nutrient supply (mainly minimize nitrogen and phosphorus) and the process conditions, production of lipids can be stimulated at the cost of protein. This method has been the subject of experimental work on many substrates, but has found no practical applications. Also date juice and syrup have been investigated mainly with fat producing fungi (Penicullum Lilacium, Pen. soppi Zaluski and Aspergillus Nidulans) with some success, but no practical follow-up has been reported (376, 375, 563).
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