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Quality assurance

14 June 2022

Rapid measurement method for assessing the haze stability of beer

Colloidal stability | The colloidal stability of beer is an important criterion in filtered beers as any haze or flocculation is not accepted by consumers. For this reason, timely knowledge of the state of the colloidal stability is not only of interest to export beer breweries, but also to small and medium-sized brewing companies. As existing methods of analysis either only take some of the beer constituents involved in the formation of haze into account or require long analysis times, a new rapid measurement method for checking the colloidal stability was developed.

As a result of increasing globalisation and a rise in exports in the food and beverage industries, consumers expect the longest possible chemical-physical product stability without the formation of haze – also for beer. Maintaining beer stability throughout its shelf life thus plays a central role in beer production. Analytically, the stability of beer can be separated into colloidal stability, taste stability, microbiological stability and foam stability. Here, the colloidal stability (also chemical-physical stability) is one of the most important characteristics, as the formation of haze or flakes can be quickly and simply identified by consumers and is often associated with microbiological spoilage of the product.

Status quo – summary of existing methods

Technologists have a number of measures at their disposal to increase the colloidal stability of beer, from the selection of the raw materials to the use of stabilising agents in the storage or filtration cellar. Possible measures that can be taken during the brewing process include, for example, the use of low-protein barley malts, correspondingly low pH values during mashing and wort boiling, vigorous primary fermentation and rapid refermentation at low temperatures [9]. Another common procedure is to use different stabilisers in the cold range or directly during filtration, such as silicic acid compounds (silica sols/gel), bentonite or PVPP or a combination of these to reduce potential haze formers and thus improve the colloidal stability [1]. This is necessary as the beer is sometimes subjected to extreme stress from fluctuations in temperature and/or motion during transportation.

Shelf-life requirements (BBD) result from the chemical composition of the beer and the expectations consumers and the brewery have of the product. In German-speaking countries, shelf lives for beer range from six week to twelve months, during which no haze or flakes should appear.

Raw material quality

Haze consists of complex molecules, mostly proteins and polyphenols, which are subject to natural movement (Brownian molecular motion) [10]. Collision of the particles leads to a gradual coarsening of the degree of dispersity that makes the haze particles visible. In addition to protein and polyphenol compounds, oligosaccharides, highly-molecular polysaccharides (starch, β-glucan or arabinoxylan) or associations of polypeptides and minerals may be involved in the haze formation [10]. Haze agglomerates range from 500 nm and 50 µm in size. Their starting products only measure between 10 and 500 nm, however [5]. Haze problems are often a seasonal occurrence and heavily depend on the quality of the raw materials. At the end of 2018/beginning of 2019 especially, increased hazing and flocculation was observed in filtered beers. The reasons for this undoubtedly lay in the change of harvest in conjunction with the specific processing criteria necessitated by the warm weather, such as higher gelatinisation temperatures and nitrogen contents, plus the long, hot summer that in many breweries boosted sales but also shortened storage times.

Tests for predicting the haze stability in beer according to MEBAK “Wort, Beer and Mixed Beer Beverages (WBBM)” [3, 4, 6]

In order to be able to quickly and simply test the colloidal stability of beer – also in these special circumstances – various methods of analysis have been developed over the years (see table 1) [4]. These tests are based on either the precipitation or interaction of the beer haze formers with chemical reagents or the formation of haze under the influence of heat (alternate heating and cooling). The key methods for determining the colloidal stability are given in table 1. These can be split into direct methods of measurement, such as the forced ageing or alcohol-chill test, and various indirect measurement methods [3]. The stabilisation effect of the protein fractions can be checked with the help of the ammonium sulphate precipitation limit (MEBAK WBBM 2.14.2.4) and the Esbach reaction test. A similar procedure is adopted by the formaldehyde test (MEBAK WBBM 2.14.2.3) that tests the stabilisation of the polyphenols. The disadvantage of this method is that only one haze-inducing substance is recorded. These methods can be used to check the functionality of the various stabilisation methods but seldom allow information to be gathered on the total resulting colloidal stability. Direct methods such as the forced ageing test (with many available variations) include all influencing factors on the formation of haze [2, 7]. However, these methods require a long time for analysis (up to several weeks) and are thus not suitable as rapid tests.

Haze formed by external factors

The formation of haze can also be accelerated by various external factors besides the beer constituents involved in the same. Alongside the known facilitators such as temperature or oxygen, these include movement, light or the catalytic action of heavy metal ions [2].

In order to provoke faster haze formation, a combination of these facilitators was to be applied. Despite this, the equipment needed should not be too complicated so that the test can also become an established practice in brewery laboratories. For this reason, in addition to temperature the impact of oxidation on the beer sample was first examined. The aim was to devise a simple, rapid test with results comparable to those of the classic forced ageing test (0/60 °C) that provides information on the colloidal stability of beer.

Fig. 1 Investigation of the temperature interval and number of temperature changes of the new forced ageing test at a dosage of 0.06 % hydrogen peroxide as the oxidising agent, determined as a mean haze increase (n = 3) at a light scattering angle of 90° and 25° [7]

Development of the rapid test

Compared to classic forced ageing tests, several steps in the methodology were modified to achieve a faster final result. Despite this, the maximum temperature of 60 °C was maintained, as various investigations have not been able to determine any acceleration in the hazing reaction nor considerably larger deviations in the measurement accuracy at temperatures of over 70 °C [11, 12]. In contrast, the optimum length of the forced ageing interval was studied in order to achieve the largest possible increase in haze in the shortest possible time for analysis. Fig. 1 shows the tested intervals, the number of changes in temperature and the time required to change the temperature (heating and cooling phases). It was proved that an interval of 26 hours with five temperature changes permitted an optimum increase in haze. The selected temperature regime resulted in a total analysis time of 26 hours. This would therefore enable information on the stability of a beer to be available on the next working day.
Simply applying this forced ageing test did not result in an increase in haze in the stabilised beer samples, however. Another method therefore had to be found to speed up the formation of haze. As haze formation is increased by oxygen or oxidation, hydrogen peroxide was to be tested as the oxidising agent. According to Uchida and Ono [13, 14], oxygen radicals are formed from atmospheric oxygen that cannot be completely removed during filling. These radicals (O2, O21-, O22-, H2O2 and OH radicals) are largely responsible for beer ageing, as various experiments – also on the taste stability – have proved [13, 15, 16]. This natural formation during the ageing process was to be accelerated with the help of 30 % hydrogen peroxide. For this reason, different concentrations of H2O2were injected into the beer and the samples forced. The largest haze increase during the analysis time was measured with the addition of 1 millilitre of H2O2 to 500 millilitres of beer (see fig. 2). This result was confirmed using further samples.

Fig. 2 Haze increase (n = 3) in a bottom-fermented beer (500 ml) on addition of various concentrations of hydrogen peroxide with three changes in temperature per measurement point [7]

Optimisation of the method

Furthermore, the size of container used was optimised, as owing to the short forced ageing intervals a delay in the core bottle temperature could lead to a lower increase in haze. Here, it was shown that on average the 0.5-litre bottle deviated from the temperature of the water bath by 8.3 °C and that the core temperature of 0 °C following incubation at 60 °C was only reached 60 minutes later than the water bath. A 200-millilitre container was thus also considered for the test. These bottles achieved a core temperature of 0 °C within 20 minutes. The sample in these bottles was therefore provided free of foam for the test. The experiments were carried out in triplicate, resulting in a test volume of 600 millilitres of beer per sample to be examined. As small bottles are used, it is also possible to take samples directly from the pressure tank and check the stability of the beer prior to filling.

Distribution of the beer styles and mean starting haze in the 39 beers tested

Once the method had been successfully optimised, 39 beers from ten German breweries were compared with the results from the warm days of the classic 0/60 °C forced ageing method. The distribution of beer styles is given in table 2. The haze meter used was the LabScat 2 from Sigrist (Ennetbürgen, Switzerland) with a scattering angle of 90 ° and 25 ° applied for both measurements.

Comparison of the methods

In order to better assess the colloidal stability of the samples, the nitrogen, polyphenol and anthocyanogen content was determined. Here, clear distinctions in the amount of soluble nitrogen were detected of between 560 and 1160 mg/l (mean: 810 mg/l, n = 39). This degree of fluctuation was also evident for the polyphenols (59–277 mg/l) and anthocyanogens (12–129 mg/l). The selection of samples thus provided a sound basis for testing a wide range of beers with differing colloidal stability.

In order to ensure that the beers had no haze prior to the beginning of testing, all starting haze was analysed (see tab. 2). According to Analytica-EBC 9.29, beers with an EBC formazine unit of up to one count as brilliant to clear. The starting haze of the samples fluctuated from 0.14 to 0.95 EBC at a scattering angle of 90 ° and from 0.04 to 0.69 EBC at a scattering angle of 25 °. The samples were therefore all brilliant to clear and did not have any haze. The ascertained amounts of starting haze were used to define the colloidal stability with the help of the classic 0/60  °C forced ageing test. In this test, the beers are alternately incubated at 60 °C and 0 °C for 24 hours and the increase in haze determined before the end of the cooling phase. If this exceeds a value of 2 EBC, the sample is considered to be opaque and the test can be concluded.

The estimated shelf life can be determined in practical circumstances with the help of a formula. However, some beers considerably exceed the value of 2 EBC during this procedure, causing the colloidal stability to be overestimated. For this reason, a conversion was also developed to adjust the measurements, resulting in a reduction of the warm days measured with the 0/60 °C forced ageing test. This conversion can be used both for comparing the methods and for predicting the colloidal stability. The number of warm days for the tested beer samples ranged from one to 22. On average the 39 samples had a stability of 6.5 warm days. Analysis of the colloidal stability using the forced ageing test thus took an average of 13 days. The beer with the highest stability even needed 44 days for analysis until a final result was produced. As the resulting calculated stability would amount to over two years, these high values are not practicable, as the taste stability would have certainly suffered considerably beforehand. Nevertheless, beers with a BBD of twelve months should have between six and eight warm days in order to guarantee the desired stability [8,12]. This is equivalent to an analysis time of ca. two weeks.

The new rapid method has the advantage, however, that results are available after just 26 hours. In order to effect a comparison between the two methods, the 39 beer samples were examined using the new rapid test. The increase in haze in the samples ranged from 0 to 9 EBC (90 ° scattering angle) following addition of H2O2 and 26 hours of incubation with five temperature changes. These haze increases were compared with the measured warm days of the classic forced ageing test. The results of comparison of the methods are shown in fig. 3. An exponential correlation between the warm days of the classic forced ageing test and the haze increase in the new rapid test was calculated with a coefficient of determination of R2 = 0.871.

Fig. 3 Correlation between the warm days of the 0/60 °C forced ageing test (n = 3) and the haze increase (n = 3) of the new method of the beer samples tested (n = 39) and classification of the colloidal stability [7]

It was shown that with an increasing difference in haze in the new rapid test the number of warm days in the classic forced ageing test decreased. Moreover, no haze increase could be measured in the new test for beers with a stability of over eight warm days in the classic forced ageing test. The results also demonstrated that the new rapid test was not dependent on the brewery, type of beer or starting haze. The test thus provides information on the expected stability regardless of the brewing company and beer style. Only non-stabilised beers demonstrated such a high increase in haze that the results could not be compared to stabilised samples. The colloidal stability was assigned for the new rapid test with the help of the determined results (see table 3). This assignment enables a simple differentiation to be made between beers with a low, average or high stability.

Haze stability in warm days according to the results of the new method

Statistical comparison of the increases in haze in the new rapid test and the results of the wet chemical testing of the beer samples only yielded a significant correlation with the total amount of polyphenol (r = 0.779, P < 0.05) and anthocyanogen content (r = 0.557, P < 0.05). The soluble nitrogen in the samples neither correlated with the warm days of the forced ageing test nor with the haze increases in the rapid test.

Conclusion and outlook

The new forced ageing method enables conclusions to be drawn as to the colloidal stability of a beer sample quickly and without a great deal of effort being involved. The parameters for analysis can also be individually tailored to a brewery’s requirements. A gentler temperature of just 40 °C is thus feasible, for example, as is adaptation of the time for analysis.

In addition to comparison with the forced ageing test, comparison with retained samples should also be made to achieve a genuine correlation with the stability of the beer. As the number of exports and, accordingly, the beer distribution channels covered throughout the world are likely to also further increase in the next few years, fast prognosis of the haze stability of a beer is of growing significance. With the method outlined in this study, predictions on a product’s colloidal stability can be given within 26 hours and a decision thus made as to whether a beer satisfies the requirements for export or not.

Acknowledgements

The authors would like to thank all breweries who have provided samples for the development of this method!

References

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