Controlling off-flavors in plant-based food products

Plant-based proteins are economical and diverse alternatives to animal-derived proteins, there. Main sources of vegetable proteins that can be used for food products are the following:

  • Legumes (peas, soybeans, lupins, chickpeas, peanuts)
  • Cereals (wheat, corn, rice, barley, rye, oats)
  • Oilseeds (sunflower seeds, rape, sesame, cotton seeds)
  • Vegetables (potatoes, cassava, sweet potatoes)
  • Leaves (alfalfa, amaranth, various aquatic plants)
  • Fruits (grape seeds, tomato seeds, papaya seeds)

Unfortunately there are several drawbacks in using plant-based protein. Firstly antinutrients, secondly off-flavors. Latter will be discussed in this article.

Off-flavors in plant-based food

Off-flavors may be produced by heating in carbohydrate and amino acid reactions (Maillard reactions), thermal decomposition of phenolic acids, oxidative and thermal decomposition of carotenoids, and thermal decomposition of thiamine (MacLeod, Ames, & Betz, 1988).

Legumes and its off-flavors.

Volatile flavors in legumes include aldehydes, alcohols, ketones, acids, pyrazines, and sulfur compounds. The off-flavors of legumes are partly natural and partly generated during harvesting, processing and storage (Sessa and Rackis, 1977).

Off-flavors in soy

In soy, an important factor in the smell and taste is the oxidation of unsaturated fatty acids. Raw soybeans contain 20-23% lipids, of which oleic acid, linoleic acid and linolenic acid make up about 84% of the total fatty acids in soybeans. The main volatile compounds formed during the oxidation of lipids are alcohols, aldehydes, ketones and esters, and non-volatile compounds such as hydroperoxyepoxides, hydroperoxycyclic peroxides and dihydroperoxides. These compounds give soy its characteristic taste and aroma even in small concentrations (Damodaran and Arora, 2013).

Soy and pea off-flavors

Table 1 lists the soy and pea off-flavors and the classes of compounds that cause them. Volatile compounds give soybeans mainly beany off-flavors, fatty notes and freshness, non-volatile compounds give bitterness and astringency.

VolatilitySoy off-flavorPeas off-flavorsMolecule class
VolatilesBeany*Green*Aldehydes
Fatty*Beany*Ketones
Green*Pea*Alcohols
GrassyEarthy*Pyrazine
RancidHay-like*Others
LeafyLeafy
EarthyMetallic
CardboardBrothy
AcridAcrid
PungentPungent
MedicinalFatty
Non-volatilesBitter*Bitter*Isoflavones
Astringent*Astringent*Saponins
MetallicMetallicPhenolic acids
Peptides/amino acids
Table 1. * Most frequently mentioned off-flavors (Roland et al., 2017).

Off-flavors in other plants

Some of the most mentioned potato protein off-flavors are grainy, soily, acid, astringent and bitter. Glycoalkaloids are compounds that contribute to the characteristic bitterness of potato protein (Alting et al., 2011). Oats contain saponins and avenanthramides (phenolic alkaloids), which give a bitter and astringent aftertaste (Günther-Jordanland et al., 2016). The volatile substances in oats are mainly aldehydes, ketones and alcohols. Pyrazines and pyridine derivatives are often formed as a result of heat treatment (Heiniö, 2003).

Techniques to remove off-flavors

One possibility is to remove off-flavoring molecules using, for example, soaking, heat treatment, germination, enzymatic treatment, ultrafiltration, solvent extraction, or fermentation (Roland et al., 2017).

Another option is to mask off-flavors. The use of masking agents allows the desired taste to be enhanced or inhibited in the food. Usually, the use of one component to hide off-flavor is not enough, so different masking agents are often combined to achieve a better effect (Gascon, 2007). Traditional masking methods involve the addition of sugars, salts, acids, and flavors. Fermentation can also provide a good masking effect (Roland et al., 2017). Most commercial masking agents are mixtures of various modifiers, inhibitors and enhancers.

Sodium glutamate and 5′-nucleotides

Sodium glutamate and yeast extracts, for example, are widely used as flavor enhancers. In the case of monosodium glutamate, glutamic acid gives the umami sensation, and in the case of yeast extract, glutamate and 5′ nucleotides. Table 2 lists some foods that are high in free glutamic acid. These include Parmesan cheese, salami, tuna, mushrooms, tomatoes, algae and soy sauce. Fermented foods, such as soy sauce, contain large amounts of free glutamate, which is caused by hydrolysis of proteins during fermentation (Kurihara, 2015).

Food productFreel L-glutamate concentration (mg/100g)
Parmesan cheese1200
Salami4944
Tuna fish3397
Mushrooms180
Tomatoes140
Brown algae2240
Soy sauce782
Table 2. Free L-glutamic acid rich foods (Laffitte, Neiers ja Briand, 2017)

Not all 5´-nucleotides produce umami sensation

Not all 5´ nucleotides produce umami sensation. In living cells, ribonucleic acid and ribonuclease (an enzyme that breaks down RNA) do not come into contact with each other and do not break down. When the cells are dead, contact between the ribonuclease and the ribonucleic acids occurs during cell disruption. (Kurihara, 2015). RNA in a yeast cell is degraded to nucleotides (5β-GMP, 5′-AMP, 5′-CMP, and 5′-UMP) (Noordam, Meijer, 2007). The feeling of Umami is provided by 5´-IMP and 5´-GMP. 5´-AMP can be converted to 5´-IMP by the enzyme deaminase, which also has taste-enhancing properties (Noordam, Meijer, 2007). 5´-IMP is also formed by the degradation of adenosine triphosphate (ATP) to adenosine monophosphate (AMP), which in turn is degraded to 5′-IMP (Kurihara, 2015).

Some food sources of 5´ nucleotides

Table 3 lists some sources of 5´ nucleotides. Animal products, such as pork, chicken and tuna, contain high levels of 5´-IMP. Shiitake mushrooms contain a lot of 5´-GMP. Tomatoes contain 5´-AMP, which does not cause umami sensation, but which can be converted to 5´-IMP by deaminase (Noordam, Meijer, 2007). 5′-IMP is only found in animal food and 5′-GMP is found mainly in mushrooms (Kurihara, 2015).

Food productIMP (mg/100g)GMP (mg/100g)AMP (mg/100g)
Pork20029
Chicken201513
Tuna fish2866
Tomato21
Dried shiitake mushrooms150
Table 3. 5′-nucleotide concentrations in some foods (Laffitte, Neiers ja Briand, 2017)

The presence of glutamate and 5´-IMP in food gives a more effective taste-enhancing effect (synergism). A similar effect has been observed for 5´-GMP. This is also the reason why, for example, yeast extracts are very effective flavor enhancers (Noordam, Meijer, 2007).

Used literature

Alting, A. C. Pouvreau, L. Giuseppin, M. L. F. van Nieuwenhuijzen, N. H. (2011). Potato proteins. Phillips, G. O. Williams, P. A, Handbook of Food Proteins (316–334). Cambridge, UK: Woodhead Publishing.

Damodaran, S. Arora, A. (2013). Off-Flavor Precursors in Soy Protein Isolate and Novel Strategies for their Removal. Annual Review of Food Science and Technology, 4, (1), 327–346.

Gascon, M. (2007). Masking agents for use in foods. Hort, T. Hort, J, Modifying Flavour in Food Cambridge, UK: Woodhead Publishing.

Günther-Jordanland, K. Dawid, C. Dietz, M. Hofmann, T. (2016). Key Phytochemicals Contributing to the Bitter Off-Taste of Oat (Avena sativa L.). Journal of Agricultural and Food Chemistry, 64, (51), 9639–9652.

Heiniö, R.-L. (2003). Influence of processing on the flavour formation of oat and rye. Doktoritöö. Espoo, Finland: VTT Publications. 494. 72 p. + app. 48 p.

Kurihara, K. (2015). Umami the Fifth Basic Taste: History of Studies on Receptor Mechanisms and Role as a Food Flavor. Biomed Research International, 2015, (6), Article ID 189402, 10 pages.

Laffitte, A. Neiers, F. Briand, L. (2017). Characterization of taste compounds: chemical structures and sensory properties. Guichard, E. Salles, C. Morzel, M. Le Bon, A.-M, Flavour: From food to perception. New Jersey, USA: John Wiley & Sons.

MacLeod, G. Ames, J. Betz, N. L. (1988). Soy flavor and its improvement. Critical Reviews in Food Science and Nutrition, 27, (4), 219–400.

Noordam, B., & Meijer, F. R. (2007). New developments in yeast extracts for use as flavour enhancers. Hort, T. Hort, J., Modifying Flavour in Food. Cambridge, UK: Woodhead Publishing

Roland, W. S. U. Pouvreau, L. Curran, J. Van de Velde, F. De Kok, P. M. T. (2017). Flavor Aspects of Pulse Ingredients. Cereal Chemistry, 94 (1), 58–65.

Sessa, D. Rackis, J. J. (1977). Lipid-Derived Flavors of Legume Protein Products. Journal of the American Oil Chemists´ Society, 54, 468–473.

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