Light struck
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The light struck flavor also contributes to changes in beer aroma during the storage of the bottled beer. It is attributable to the formation of sulfur compound (3-methyl-2-buten-I-thiol) from hydrogen sulphide, from light-activated nascent sulfhydryl compound which comes mainly from sulfur containing amino acids and iso prendiene from the 4-mcthyl-3-pentenoyl-side chain of the iso-alpha-acids. Very light colored beers are particularly susceptible to this effect. Investigation of great many beer samples indicated that a high polyphenol content and possibly certain amount of residual amino acids in beer diminished the sensitivity of sunstruck flavors. This photo reaction which is very rapid in green or colorless bottles also exists in brown bottles during longer storage time and the exposure to natural or artificial light.[1]
Even though they contribute to beer stability, iso-α-acids are partly responsible for nascent off-flavors of aging (stale and cardboard flavors) [131]. Beer exposure to light causes a decomposition of different chemical compounds, which results in various off-flavors, with the skunky flavor being one of them. To avoid the formation of skunky flavor, due to the light transmission into the packaging, beer must be stored in opaque cans, or green/brown bottles. Iso-α-acids are especially sensitive to light, and its decomposition results in a skunky (or lightstruck) flavor, a 3-methylbut-2-ene-1-thiol [132]. The flavor threshold of this compound is very low (4 ng/L) but can irreversibly spoil beer quality [114,132]. To avoid this, brown bottles should be used as beer packaging and they should be stored away from the light. Sometimes, it only takes seconds for the reaction to occur and start instilling the undesirable skunk off-flavor. Light beers and beers hopped with more hops are more prone to obtain the skunky off-flavor. This only occurs in the finished beer and is always perceived as an off-flavor [132,133,134].[2]
Photo-oxidation: A variety of carbonyls (saturated, monounsaturated, diunsaturated) was seen to be produced by photo-oxidation of oleic and linolenic acid in beer.69,71,97 Photosensitizers such as riboflavin (vitamin B2) are activated by light irradiation. These activated species excite triplet oxygen to singlet oxygen, which in turn reacts with fatty acids to form hydroperoxides and aldehydes (Figure 6). The reaction is independent of the temperature. Singlet oxygen is much more reactive than triplet oxygen, and so without the influence of light, this pathway is of little significance. Therefore, beer packaging should aim for a minimal passage of light.[3]
Ferulic acid and other phenolic compounds can absorb UV light and promote stability of the iso-alpha-acids.
References[edit]
- ↑ Narziss L. Technological factors of flavour stability. J Inst Brew. 1986;92:346–353.
- ↑ Habschied K, Košir IJ, Krstanović V, Kumrić G, Mastanjević K. Beer polyphenols—bitterness, astringency, and off-flavors. Beverages. 2021;7(2):38.
- ↑ Baert JJ, De Clippeleer J, Hughes PS, De Cooman L, Aerts G. On the origin of free and bound staling aldehydes in beer. J Agric Food Chem. 2012;60(46):11449–11472.