Wort Boiling in Relation to Beer Quality


By J. R. Hudson, Ph.D., F.R.I.C. and S. E. Birtwistle, B.Sc.
(Brewing Industry Research Foundation, Nutfield, Redhill, Surrey)

Received 15th June, 1965

Comparisons of beers from worts mashed with pale ale malts (nitrogen 1-3%) at 150° F. (65-5° C.) and boiled for ½, I, and 2 hr. show that increase in boiling time leads to slightly increased hop-utilization, decreased head-retention, improved non-biological stability and slight differences in flavour.  Findings were very similar when the mashing temperature was either 145° F. (63° C.) or 155° F. (68° C.) except that, in the latter case, no difference in flavour was detectable between beers from long and short boiled worts.  Stirring can replace vigorous boiling to secure normal hop-utilization and shelf-life but simmered worts give beers of characteristic flavor probably as a result of enhanced contents of hop oil.

Use of a lightly kilned malt of high nitrogen content (1-8%) produced a characteristically different flavour from that obtained with pale-ale malt and reduced the utilization of hop substances by provoking a greater loss of this material during fermentation.

The function of wort-boiling is to extract bitter and other substances from hops, to precipitate unwanted nitrogenous material, to terminate enzymic action and to sterilize the wort (see 1, 5). Additionally, excess hop oil and some constituents of malt are eliminated by volatilization.

Kolbach & Esser 2, 3, 4 have reported on the effects of varying conditions of wort-boiling in relation to the head-retention and non-biological stability of lager.  The present investigations were undertaken to establish what changes in beer quality ensue from alterations in boiling conditions applied to infusion worts of various qualities.

The trials were carried out using infusion mashing and top-fermentation.

Wort-production.—Distilled water to which calcium sulphate (200 mg./litre) had been added, was employed.  Malt (201b., 9 kg.) was mashed with water (5-2 gal., 22-5 litres) and stood for 1½ hr. before starting to run off the wort.  The grain bed was then sparged (165° F., 74° C.) and drained during 3 hr. at such a rate that the total volume of wort collected was 13 gal. (59-5 litres).  Thus the mashing ratio was 2-4 brl./qr. and the total ratio of liquor to grist was 6-8 brl./qr.  Two pale malts, one containing 1*3% nitrogen and the other 1*8% nitrogen were each mashed at 145° F. (63° C), 150° F. (65-5° C.) and 155° F. (68° C).

Boiling.—Sweet worts were boiled with hops (1 lb./brl.) for 0-5, 1, 1-5 or 2 hr. in a steam-jacketed vessel with free evaporation.  On completion of boiling, the wort was run into a hop-back, stood for 10 min., circulated for 10 min., and then transferred to a wort-receiver where the specific gravity was adjusted to 1-040 with treated water. Alternatively, some worts mashed at 150° F. (65-5° C.) were simmered at either 200° F. (93° C.) or 210° F. (99° C.) and comparisons were made between stirred and un-stirred bulks and between free and restricted evaporation in this connection.

Fermentation.—Cooled wort (60° F., 15-5° C.) was pitched with yeast (N.C.Y.C. No. 240, 1 Ib./brl.) and during fermentation attained a maximum temperature of 68° F..(20° C).  When the apparent attenuation was 75%, the green beer was filtered with the aid of kieselguhr (Celite 545) to remove yeast and repitched in cask with the same strain of yeast at 0-2 Ib./brl.  Draught beers were conditioned for 5 days at 58° F. (14-5° C.) and fined (2 pt./brl.).  Beers for bottling were conditioned in the same way, and stored at 34° F. (1° C.) for 13 days prior to filtration and bottling.  Air in the head- space of bottles was reduced to approximately 0-5 ml. by fobbing and the beer was subsequently pasteurized at 138° F. {59° C).

Results and Discussion
Initially a comparison was made between beers produced from normal pale-ale malts mashed at 150° F. giving worts which were boiled for 0-5, 1 and 2 hr. (see Table I).  The most notable features observed were that prolonged boiling (i) improved non-biological stability, (ii) caused a significant reduction in head-retention coinciding with a small loss of nitrogenous material, and (iii) increased the ratio (isomerized materials / a acids) in wort and the amount of bitter substances in the resulting beer.  This ratio gives a measure of the unchanged ac acids which are present after boiling and which represent part of the hop-material lost during fermentation.  More over a taste-panel, using the triangular technique, detected a small difference in taste, which was presumably due to a combination of bitterness and the existence of a slight flavour and aroma of hop-oil related to the short boil.

The effects of mashing at 145° F. and 155° F. in relation to short (0-5 hr.) and long (2 hr.) boils were next examined.  As anticipated, those worts produced at low temperature were more fermentable and contained more nitrogen in comparison with those produced at 150° F. whereas opposite effects were noted for worts mashed at 155° F. (see Table II).  The findings with regard to beer quality were broadly similar to those made when mashing at 150° F. i.e., the longer boiled worts gave beers which were slightly more bitter, more stable and with lower head-retention.  It is seen that worts mashed at the lower temperature contained more hop-substances after boiling than did corresponding worts mashed at 155° F.  This extra resinous material was lost during fermentation.  Comments on differences in flavour caused by long and short boiling applied to worts mashed at 145° F. were similar to those made in conjunction with a mashing temperature of 150° F., i.e., part of the difference was subjectively ascribed to retention of hop-oil constituents in the shorter boil.  However, after mashing at 155° F. no significant differences in flavour of the beers were detected by the taste-panel.  Apparently the higher mashing temperature increases the complexity of beer flavour so that small differences are not readily discerned.  Furthermore, it is noteworthy that mashing temperature had an important influence on shelf-life and that prolonged boiling resulted in a marked stabilization in conjunction with high mashing temperatures, but only a small improvement when applied to worts mashed at 145° F.

A further aspect of this study was to observe the effect of replacing the conventional pale-ale malt with another containing relatively more nitrogen (1-8%).  Again short (0-5 hr.) and long (2 hr.) boils were applied to worts mashed at 145,150 and 155° F. and the effects of temperature on fermentability and soluble nitrogen were in the same sense as noted previously though the levels were markedly higher than had been obtained with a low-nitrogen malt (Table III).  Noteworthy features of the resultant beers were that low mashing temperatures gave rise to higher colours and that their flavours, though pleasant, were more complex than had been obtained when pale-ale malt was mashed at the corresponding temperatures.  Thus there emerged no clear-cut differences in flavour arising from changes in boiling time.  The relation of shelf-life to mashing temperature was again in the sense that at higher temperatures prolonged boiling effected the greatest improvement in stability.  An outstanding feature here was that the shelf-lives obtained with high-nitrogen malt were substantially better than had been obtained in corresponding brews with pale ale malt.

The findings concerning hop-substances are of particular interest in this case.  It is well known that the level of bitter substances in beer brewed from malts which contain much nitrogen is lower than when malt containing little nitrogen is brewed under the same conditions.  This effect has commonly been ascribed to the extra formation of break when high-nitrogen worts are boiled, leading to loss of bitter substances by adsorption.  However, it was found here that the worts leaving the hop-back contained much the same amounts of hop substances as did hopped worts from normal pale ale malts.  Thus the lower overall utilization found when high-nitrogen malt was employed was due to an enhanced loss of bitter material at the fermentation stage.

Finally some comparisons were made between worts and beers after boiling, simmering and stirring {see Table IV).  It is seen that simmering gives stable beer of normal character except that the bitterness is rather low perhaps owing to a lower rate of conversion of a acids in the copper.  However, this is compensated to some extent by extra aroma and flavour of hop-oil.  Continuous stirring in the copper with the temperature maintained at 200° F. caused very little evaporation and improved the level of bitterness as compared to simmering.  However, this beer possessed an exceptionally powerful aroma and flavour of hop-oil which might be unacceptable to many palates. Moreover the shelf-life, though reasonable considering that no special treatments were given to improve it, was not as good as that obtained by normal boiling.  Stirring in an open copper at 210° F. gave beer of good bitterness and pronounced hop oil character and, moreover, enhanced shelflife.

When stirring at the latter temperature was used in a closed copper for a brief period so that evaporation was again reduced, shelf life was again curtailed, bitterness low and the oil character rather too pronounced.

It is interesting to note that coagulation of the hot-break is achieved by movement of the wort rather than by temperature.  Thus worts simmered for 1½ hr. are cloudy whereas stirring either at 200° F. or 210° F. for 1½ hr., and also normal boiling produce bright worts from the hop-back.  Most significant, however, is the fact that no correlation has been found between the brightness of worts from the hop-back and the shelf-life of ensuing beers.

From these investigations it appears that relatively short boiling may be employed with advantage for beers which are not required to have a very long shelf-life (<10 weeks).  The aroma of such beers enhances quality, and differences in taste are unlikely to be significant if either high mashing temperatures or malts containing much nitrogen are used to prepare wort.

Acknowledgements.—The authors gratefully acknowledge the interest and encouragement of Dr. A. H. Cook, F.R.S.. throughout this work.

1. de Clcrck, J., Textbook of Brewing. Vol. 1. London: Chapman & Hall, p. 302.
2. Kolbach. P., & Esser, K. D., Brauerei, Wissensch. Beit.. 1058, 11. 31.
3. Kolbach, P., & Esscr, K. D., Brauerei, Wissensch. Beit.. 1068, 11. 91.
4. Kolbach, P., & Esser, K. D., Brauerei, Wissensch. Beit., 1068, 11, 221.
5. Steiner. K., & Stocker, H. R., Schweiz. Brau-Runds., 1062,73, 116.

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