Beany notes and green off-flavours are a major turn-off for many who want to adopt a more vegetarian diet. Pulses (legumes), especially peas and beans are very popular with vegetarians and vegans because they offer a suitable protein and high-fibre source. Who hasn’t toyed with soy milk for example ? Unfortunately, if the legumes and pulses are not handled properly, there is a tendency for the development of off-flavours and the problem has restricted commercial development. Not only are stored legumes prone to off-flavour formation, but also the protein extracts which limits the use of legume ingredients in product development.
Peas (Pisum sativum) can develop a ‘viney’ off-flavour caused by the volatiles released by lipoxygenase activity (Wagenknecht and Lee, 1958). These off-notes develop from compounds including peroxides from degraded lipids even during frozen storage (Wagenknecht and Lee, 1956). Other legumes prone to the beany notes are soybean (Glycine max), fava beans and Australian sweet lupin (Lupinus angustifolius).
Processing of peas also brings with it a number of issues. Not least is the phenomenon that canning causes producing noticeable changes in pea flavour. The vegetable no longer has the fresh flavour and colour of the freshly picked pea. One of those compounds is pyrrolidonecarboxylic acid (PCA) otherwise known as pyroglutamic acid, which is also found in other canned products including beets, even freshly processed tomato juice which increases in intensity as the product is stored when canned.
For quite a long time it was thought you simply couldn’t get away from these off-notes because it was inherent in the pulse it self and the way it was processed.
Most of the flavour in a pulse is derived from volatile compounds released from fatty acids. These are degraded in both enzymatic and thermal reactions. The main enzyme responsible is lipoxygenase otherwise known as lipoxidase in older references. Lipoxygenase activity was first examined in peas (Wagenknecht and Lee, 1956) at the New York Agricultural Experiment Station attached to Cornell University in New York State.
Soybean Flavours – Off-Notes
Soybean is the most popular pulse to be investigated because of the ‘beany note’. It is the principal reason for the low uptake of soy generally anywhere in the world except Japan. Bitter and astringent notes also tend to be associated with beany flavours although they are created by entirely different mechanisms. The beany flavours in soybean are:- (E)-2-hexanal, (E)-2-octenal, 1-octen-3-one, 1-hexanol and 1-heptanol.
The beany note is often mostly attributed to n-hexanal and n-hexanol production however don’t be fooled – it isn’t as straightforward as that. Beany notes are prevented from forming by inhibiting lipoxygenase. Measuring both hexanal and hexanol formation is a good measure of how lipid oxidation is progressing in a sample and how well it will store. It is possible to obtain triple-null soy flours as they are known which lack lipoxygenase presence.
Another compound associated with beany notes are 2-isopropyl-3-methoxypyrazine which is also known as bean pyrazine because of its association with a pea or earthy note and has been found in soy milk (Kaneko et al., 2011).
Soymilk Beany Notes
Soymilk which is one of those products derived from soya is bedevilled at times by the beany note. In one study, dynamic headspace dilution analysis (DHDA) and gas chromatography-olfactometry-mass spectrometry (GC-O-MS) was needed to identify the odour-active compounds as well as some non-beany notes.
The beany notes were eliminated on this occasion by processing the soya using hot water blanching for 10 minutes between 80 and 100 ºC to denature the enzymes that produce the off-notes then grind the chickpeas. However, the characteristic notes were also affected.
Control Of Off-Flavours
Heat treatments to inactivate the enzymes responsible are the most widely used control measure available for both canned and frozen pulses. Generally, very hot water or steam blanching is used to denature the enzymes that cause spoilage before freezing. If blanching is not employed then the off-flavours develop even on frozen storage. This phenomenon is well known. Peas that are not blanched develop a ‘hay-like’ note even when stored at -20 Cent. (Lee and Wagenknecht, 1951).
Germination of legumes is often used as part of the process to create a better flavoured ingredient for product development. When both lupin and soybean are germinated, the profile of beany notes based on (E)-2-hexanal, (E)-2-octenal, 1-octen-3-one, 1-hexanol and 1-heptanol lessens. Instead, 2-methylbutanal becomes more abundant with germination. In both legumes, there was an increase in meaty and brothy odors along with sweet, woody, mushroom and baked aroma notes.
Fermentation of pulse ingredients is one way to reduce the bitterness profile of legumes. Pea protein extracts for example can be fermented by lactic acid which reduces their off-flavour formation and even masks undesirable green notes. The volatiles were identified and quantified using dynamic headspace sampling with subsequently coupled TDS-GC-MS and TDS-GC-olfactometry flavor dilution analysis. Up to 23 odor-active compounds were identified (Schindler et al., 2012).
References
Kaneko, S., Kumazawa, K., & Nishimura, O. (2011). Studies on the key aroma compounds in soy milk made from three different soybean cultivars. Journal of Agricultural and Food Chemistry, 59(22), pp. 12204-12209
Lee, F. A., & Wagenknecht, A. C. (1951). On the development of off‐flavor during the storage of frozen raw peas. Journal of Food Science, 16(1‐6), pp. 239-244. https://doi.org/10.1111/j.1365-2621.1951.tb17377.x
Lv, Y. C., Song, H. L., Li, X., Wu, L., & Guo, S. T. (2011). Influence of Blanching and Grinding Process with Hot Water on Beany and Non‐Beany Flavor in Soymilk. Journal of Food Science, 76(1). S20 https://doi: 10.1111/j.1750-3841.2010.01947.x
Schindler, S., Zelena, K., Krings, U., Bez, J., Eisner, P., & Berger, R. G. (2012). Improvement of the aroma of pea (Pisum sativum) protein extracts by lactic acid fermentation. Food Biotechnology, 26(1), pp. 58-74.
Wagenknecht, A. C., & LEE, F. A. (1956). The action of lipoxidase in frozen raw peas. Journal of Food Science, 21(6), pp. 605-610. https://doi.org/10.1111/j.1365-2621.1956.tb16964.x
Wagenknecht, A. C., & Lee, F. A. (1958). Enzyme action and off‐flavor in frozen peas. Journal of Food Science, 23(1), pp. 25-31. https://doi.org/10.1111/j.1365-2621.1958.tb17534.x
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