MEETING of the YORKSHIRE AND NORTHEASTERN SECTION,
HELD at the QUEEN’S HOTEL, LEEDS, on THURSDAY, Nov. 25th, 1926.
Mr. T. C. Radcliffe, B.A., in the Chair.
The following paper was read and discussed: —
WORT BOILING
By Leonard R. Skinner.
Wort boiling, its objects and idiosyncrasies, has been a much-discussed subject, and no doubt most brewers have their own opinions as to the actions and interactions which take place in the copper, and as to which are necessary for the production of a sound and finely-finished beer.
The main objects in boiling are, sterilisation, precipitation of proteins, or, in other words, getting a good “break” extraction of soft resins in which are the preservative principle of the hops, obtaining a sufficient concentration of the wort, which goes hand in hand with the real cooking and extraction of a truly malty flavour from the wort constituents, and, finally the production of a stable fermentable wort with some retention of hop aroma.
The principal factor in sterilisation is temperature. The higher the temperature of the boiling wort, the more likely is it that organisms and spores are destroyed.
Precipitation of proteins is a somewhat vexed question, but assuming the wort to be properly balanced with respect to its various constituents, a vigorous ebullition is the desideratum for a sufficient and satisfactory break.
The complete extraction of the soft resins, whereby the maximum benefit is obtained from the hops added to the copper, is an operation requiring judgment and experience. Agitation of the wort is necessary so that the hops are completely disintegrated, and every particle of resin is utilised; that is to say, a good and vigorous boil is necessary, but if the boiling of the wort is prolonged there is a danger of extracting too many hard resins and producing an excessively bitter taste in the finished beer. The cooking effect on the wort and the wort concentration seem to be closely connected for a beer which has had a really good boil concentrating through some 8-9 lb. gravity, or about 25 per cent, bulk evaporation. Such a beer possesses an un mistakeable malty flavour on comparison with one which has not been so well boiled. There appear to be three factors upon which a good boiling depends:—
- The highest possible temperature for boiling.
- A vigorous and continuous ebullition.
- The maximum amount of heat transmission in any given time.
It is necessary to raise the actual temperature at which the wort will boil to the highest point practicable, so that the maximum sterilising effect is obtained. A vigorous and continuous ebullition is self-explanatory, but to do this maximum efficiency is necessary in transferring the heat from its secondary source, the furnace, to the wort. This leads up to the question, “Why has copper been universally chosen as the container in which to boil wort?” Reference to a few of the physical properties of copper may not be out of place. Pure copper, after mechanical treatment and annealing, has a Sp. Gr. of 8*89 at 20° C, ordinary commercial copper ranges between 8*2—8*6. Copper melts at 1,083° C, when it rapidly oxidises, and the resulting cuprous oxide is dissolved in the metal. In the molten state hydrogen, carbon dioxide and sulphur dioxide are absorbed, but are given off as the copper cools thus pitting the surface of the copper and making copper casting a matter of some difficulty.
Copper has a thermal conductivity of 0-918 at 18° C. or 0″908 at 100° C, as compared with 0144 and 0142 for iron or 0204 and 0″254 for brass at these respective temperatures. Thus, as a heat conductor it is ideal, and it is perhaps for this reason that it was first used as a vessel for cooking purposes. It should be remembered that small quantities of impurities in the copper considerably reduce its thermos-conductivity. Perhaps the most interesting part about copper from the brewers’ point of view is its crystalline structure. The accompanying micro photographs which were kindly supplied by Messrs. The Broughton Copper Co., Ltd., of Manchester, show:
No. 1, the structure of copper in the cast state, the large crystalline and granular structure being very noticeable.
No. 2, the appearance of copper after a small amount of forging. The crystals have become more defined and regular.
No. 3 illustrates the structure of copper, after submitting it to a considerable amount of mechanical treatment.
No. 4 shows the structure of copper in a highly wrought state. The crystals have become very small and regular.
No. 5 depicts the usual appearance of wrought copper originally similar in structure to No. 2, but which has been afterwards seriously overheated or burnt. The crystals have grown larger and in places the formation of cracks may be discerned.
No. 6 shows copper heated in presence of reducing agents.
Of particular interest is No. 6, which shows the structure of copper after it has been heated, not necessarily to red heat, in the presence of reducing agents such as carbon monoxide or coal gas.
Now these are conditions which readily obtain in the ordinary fired copper. What happens is that the reducing agent attacks the oxygen which is always present in good copper and removes it. This reduces the cohesion between adjacent crystals and leaves the copper in a brittle condition, so that it breaks under slight strains. This process is technically known as “gassing,” and the question arises to what extent does this “gassing” take place in the coal-fired copper? ” It is conceivable that there is a certain admixture of carbon monoxide in the flue gases, and if it does occur, to what extent has it a direct bearing on the alleged superior cooking value of a fired copper over a steam-heated one? It may have been noticed by some users that it is only after a new fire copper has been installed some few months that the best cooking effect has been obtained.
It may be of interest to consider what is the actual process known as transmission of heat, how is it connected with this physical property of “gassing” of copper, and what relation has the source from which we obtain our energy necessary for boiling, to the whole complex question.
Has the fact that when a copper full of wort is boiled by stoking shovelful after shovelful of coal underneath the copper, the sun’s energy is being utilised in one of its most directly available forms, anything to do with that ultimate superiority in cooking and malty flavour obtained in the resultant beer. Energy directly transmitted to the wort from the combustion of coal is the sun’s energy second-hand, but energy transmitted by steam is the sun’s energy third hand, for coal has to be burned to generate the steam.
Now what is the process of heat trans mission and heat radiation. Those who have listened to the recent lectures of Sir Oliver Lodge will have learned that there is a theory, applicable to all forms of manifested energy, known as the quantum theory of emission. Sir Oliver Lodge admits that very little is known about this theory, which is of a speculative nature, but briefly it is that matter is made up of an infinite number of molecules, the molecule Is formed from one, two, three, or more atoms according to its valency, and each atom in itself is constituted of one or more electrically positively charged nuclei surrounded by one or a number of negatively charged ions. Each of these nuclei has a definite orbit within the atom, and by all solar rules should adhere to it. But they do not. These nuclei occasionally jump their orbits, and in the resultant melange an ionic emission occurs, and it is the conglomerate effect of these miniature cataclysmic occurrences which manifests itself as heat, light and electricity. Thus, heat radiation is actually an ionic emission, and there is a definite negatively charged ion passing from the source of the heat—the furnace—to the conducting medium—the copper. This may explain why the hydrogen ion concentration is slightly increased in a wort after it has been boiled. The writer cannot say what resistance the copper offers to the passage of this ion of emission through to the wort, but it is reasonable to suppose that in the case where the copper has been partially gassed by the reducing action of the flue gas on the contained oxygen, a quicker passage will be offered than if the copper had not been gassed. In the light of this theory the exceptional heat conductive property of copper is explained by its crystalline structure. The large amount of free space which results from the aggregate of all the minute free spaces that exist where all the crystalline faces of the copper rest against one another, offers far less resistance to the passage of an ion of emission than would a substance which has no such free spaces. Thus, when small impurities are added to copper which upset its crystalline structure, its heat conductivity is consider ably impaired.
It is possible that the rate and velocity of the impingements of these ions has a direct bearing on the ultimate cooked flavour. This is merely a suggestion, but practice seems to substantiate it. What are the facts? The higher the furnace temperature and the more vigorous the boil, the better and more cooked is the wort. Higher furnace temperature means a greater rapidity in the vibration of the molecules with, it may be supposed a corresponding increase in the frequency and velocity of ionic emission. Vigorous ebullition necessitates rapid vaporisation of worts and this entails quick and efficient heat transmission, as so much latent heat is necessary to convert boiling water to steam. Therefore it is necessary to have the best possible heat conductor with the highest practicable furnace temperature, as the greater the difference between the outside temperature and the temperature at which the wort boils, the more rapid the trans mission and greater will be the quantity of heat transferred to the wort in any unit of time.
Wort coppers can be divided into two general classes, steam heated and fire heated. Each of these classes can be divided into sub-classes according to whether they are open, domed, or closed pressure coppers, and finally comes the question of the necessity for agitators or fountains. The possibilities of electrically boiled worts has not yet been explored, but those who desire to experiment in this direction should remember that electrical furnaces require expert manipulation as the least extra voltage may possibly fuse all the lighting system in a building right down to the main supply. There appear to be great possibilities in electrical boiling, as by the insertion of a thermo-couple in the wort direct contact would be established between the source of heat and the wort, without the’ intermediate influence of the conducting copper.
The open steam copper can be either jacketed or heated by one of the types of steam heaters that are on the market to-day. A dome or some sort of fountain is essential if the copper is deep, as this exercises some control on the proneness of coppers to bump during the preliminary stage of heating and is also a useful aid to vigorous ebullition at the later stages of boiling. A possible dis advantage of the fountain is that the hops may get too rough a treatment, but as fountains and domes are quite generally adopted, this would hardly appear to be a serious consideration. In an open steam copper of this description heating can either be by super-heated steam, or ordinary steam of some 80-100 lb. pressure at the boiler.
The writer has found that when using a steam jacketed copper considerably better results are obtained with a boiler pressure of 80-100 lb. than 50-60 lb. although the actual pressure of steam in the jacket is the same. This is probably due to the fact that the steam is drier and consequently imparts a greater amount of heat per unit volume at the higher boiler pressure. With super-heated steam, still better results should be evident.
Complete sterilisation in a copper of this description is questionable, as a boiling temperature of about 212°F —214°F. is all that is possible. An advantage of the open copper is that the steam is able to get away more readily than in a pressure copper and this means a more satisfactory ebullition and protein precipitation. There is a more ready wort concentration as there is no drip back from the condensed steam, but on the other hand’ the temperature and rate of evaporation is not so high as in a pressure copper. It may be suggested that the cooking value of these coppers is inferior for two reasons, firstly, the energy is third hand, secondly, there is not sufficient difference of temperature between the steam and the wort to get a true cooking effect. The ordinary steam heater has only a temperature of some 400° F. True super-heated steam gets nearer the mark as temperatures up to 900° F. are possible, but is the quality of the heat the ‘ same as in a fired copper? Again, with the open copper it is quite possible that full use of the hops is not obtained, for it is doubtful whether all the soft resins are fully extracted.
Much that has been said about the open steam copper also applies to the closed pressure steam-heated copper. There is still the important question of the quality of the heat. Sterilisation should be better as higher boiling temperatures are obtained. Ebullition may not be quite so free, as the pressures would have a tendency to restrict the free production of steam. This might result in a poorer precipitation of proteins. There will certainly be a more efficient extraction of soft resins, greater concentration will also be obtained, as steam is produced at a higher temperature which consequently increases the rate of evaporation. The cooking effect will be better as higher temperatures obtain, but still the steam heated pressure copper or even super-steam heated pressure copper will not compare favourably in this respect with the fired coppers, chiefly for the reason first mentioned, namely, quality of heat, and also the fact that such high heating temperatures are not possible as with a furnaced copper.
Experimental work has been done in the matter of oil furnaces for supplying heat direct to the copper and beneficial results have been obtained. The fire copper proper can claim advantages to steam heating, but is far from perfect, as so much coal dust is involved. Oil would seem to be the best means for direct heating as a more intense heat is obtained and oil can be manipulated in a cleanly manner.
In other respects, oil and coal are similar in that they are both natural products and give us the sun’s energy second-hand. In the open fire-copper, sterilisation and soft resin extraction compare similarly with the open steam heated copper. A more vigorous ebullition probably occurs as there is a greater transference of heat for the source of heat is at a higher temperature. This should lead to a better precipitation of proteins, better cooking effect and a more stable beer.
Finally, the closed pressure copper results in a good sterilisation, as there is a high boiling temperature; good precipitation should be obtained if a dome or fountain is used, as a vigorous ebullition is obtained. Efficient wort concentration together with good cooking should also be obtained as there is a high heating temperature combined with a high boiling temperature. All these factors should help in the production of a stable wort. The one possible disadvantage of the fire-heated pressure copper is the con dense effect of the steam on the dome. This dripping back into the boiling wort may so influence the temperature at which certain re-actions are taking place, and alter their nature.
Crescent City Brew Talk 