What are the reasons for the formation of a fishy soy flavor and 3 ways to deodorize soy milk?

Although the consumption of soymilk has increased significantly, its odor is still unacceptable to many national consumers, and the problem of removing the soya odor has been one of the key technical difficulties in improving product quality during bean processing. This paper cites some of the research literature for your reference.
The formation of soybean odor A large number of studies have proved that soybean odor is mainly composed of volatile odor and non-volatile odor. Some of these odor substances, some show grassy, fishy, some show bitter, astringent, spicy, sour, fragrant, and a variety of different irritating odors. The combined effect of all these undesirable odors, the formation of soybeans unique soy flavor. The formation of soy odor, has its extremely complex reasons and reaction process. Summarized, there are mainly the following aspects:
1. Soybean itself contains undesirable odor components Soybean itself contains undesirable odor components, volatile odor substances are formaldehyde, acetaldehyde, hexanal, isovaleraldehyde, n-heptanal, acetone, ethylheptanone, n-hexanol, n-heptanol, acetic acid, propionic acid, valeric acid, capric acid, caprylic acid, methylamine, dimethylamine, hydrogen sulfide, etc.. Non-volatile odor presenting substances are mainly phenolic acid, chlorogenic acid and soybean phosphatidylcholine (SPC). These undesirable odor components and soybean protein combined together, so that soybeans have a green odor and soybean odor. 2. autooxidation reaction of soybean fat Soybean contains a large number of unsaturated fatty acids. Among them, oleic acid (9-octadecenoic acid) accounts for about 20%, linoleic acid (9,12-octadecadienoic acid) accounts for about 52%, and linolenic acid (9,12,15-octadecatrienoic acid) accounts for about 10%. Due to the presence of oleic acid, linoleic acid, linolenic acid in the unsaturated double bond, they are highly susceptible to oxidation reactions, generating hydroperoxides and a series of bad odor substances. 3. Enzymatic oxidation reaction of soybean fat Soybean contains a variety of enzymes, they can promote the decomposition of nutrients in soybean, which, in particular, the highest content of fat oxidase, the highest vitality. Soybean rich in linoleic acid and linolenic acid is a good substrate for fat oxidase.
Fat oxidase acts on the primary product of unsaturated fatty acids is hydroperoxides, and then after further complex changes, generating aldehydes, ketones, alcohols, phenols and other volatile flavor substances. Among them, hexanal, hexenal, nonadienal, 3-cis (trans)-hexanal, cis and trans-pentyl vinyl vinyl, etc., all of which exhibit a strong soy flavor. In addition to fat oxidase, there are four kinds of soybean fat oxidase isoenzyme, prompting the oxidative degradation of soybean fat to produce a variety of fishy-smelling substances. 4. Reaction between amino acids and sugars Soybean contains a variety of amino acids and low-grade sugars. Under certain conditions, amino acids and sugars react. This reaction belongs to the category of Maillard reaction. During the reaction, amino acids are decomposed into formaldehyde, acetaldehyde and other most hydroxyl compounds, as well as ammonia and carbon dioxide; sugar is formed into furfural and hydroxymethyl furfural.
Next, a Strecker degradation reaction occurs between the dihydroxy compound and the amino acid, in which the amino acid is decarboxylated and deaminated to produce an aldehyde with one less carbon. Sulfur-containing amino acids, such as cysteine and cystine, after Strecker degradation, in addition to aldehydes, hydrogen sulfide is also generated. The resulting aldehydes and hydrogen sulfide, both produce bad smell.
5. Amino acids and aldehydes, ketones reaction produced by the oxidation of soybean fat aldehydes and ketones, as well as amino acids and sugar reaction or amino acid degradation of aldehydes, continue to amino acids and soybean proteins, non-enzymatic browning reaction, this reaction is still belonging to the United States of America Larder (Maillard) reaction of the category. The products of this type of reaction are amines, ammonia, and new hydroxyl compounds with undesirable odors. At the same time, it also produces brown or black pigments with high molecular weight and complex structure. 6. Hydrolysis of soybean protein soybean special bad smell and soybean protein peptide chain originally combined with other flavor groups. Enzymatic hydrolysis of proteins, the release of these flavor-presenting substances; but also lead to the production of bitter taste of proteins. The bitter taste of hydrolyzed proteins depends on the original amino acid composition of the protein.
The hydrophobic amino acids in soybean proteins are important in causing the bitter taste of hydrolyzed proteins. Valine, leucine, isoleucine, and tryptophan belong to this group of amino acids. The degradation of cysteine produces ammonia, hydrogen sulfide and acetaldehyde.
Elimination of soybean milk fishy smell methods[16] The current methods and technologies to eliminate soybean milk fishy smell are mainly reflected in three aspects: first, through the improvement of raw materials, to explore and cultivate new soybean varieties; second, to reduce the smell of soybean fishy smell during processing, by blunting the activity of fat oxidase in soybeans or inactivating it; third, to improve the storage conditions. 1. excellent soybean varieties of screening consumers in different regions of the soybean milk soy flavor acceptance of large differences, so in the preparation of soybean milk before the “local conditions” to select the soybean varieties is particularly important.
Soybean varieties on the soybean taste of soybean milk through the chemical composition to achieve, for soybean milk traits unknown soybean varieties, as long as its physical and chemical indicators are known, you can use the two discriminant function of the flavor traits predicted, so as to target the establishment of the consumer’s favorite flavor evaluation method [1].
In one study, based on the relationship between protein and fat content and the sensory score of soya flavor, it was initially determined that soya milk prepared from soybean varieties with a protein amount fraction of less than 40% and a high fat mass fraction had the weakest soya flavor and the best overall sensory quality [2]. In addition, from the fundamental pathway of the formation of soybean odor can be seen, lipoxygenase is the formation of soybean odor indispensable substances, cultivation of non-lipoxygenase soybean varieties is to eliminate the root of the soybean odor treatment method. At present, many domestic crop research institutes have bred a variety of fat oxidase deficiency varieties, such as Sui-Nui-Fishy 1, Sui-Nui-Fishy 2, Dongnong 56, # 1, # 2, etc., of which Sui-Nui-Fishy soybean selected by the Heilongjiang Academy of Agricultural Sciences Suihua Agricultural Institute of traits are good, the preparation of soybean milk when the soybean smell is low, good taste [3].
2. Production process improvement (a) soybean pretreatment soybean epidermis and cotyledons between the collection of a large number of lipoxygenase [4], in the pulping before the pretreatment process should be increased in order to reduce the opportunity of these lipoxygenase and air contact. The most direct method is to peel soybeans, and the higher the peeling rate, the best effect of soy odor reduction [5].
In addition, the use of freezing technology to treat soybeans can also play a good effect of enzyme inactivation. Soybeans soaked in water for 9 h [bean to water ratio of 1:3 (m/V)], and then use gauze to dry the surface water in -18 ℃ under the condition of freezing 20 h after the preparation of soybean milk, sensory evaluation results show that the soybean milk soybean odor is significantly weakened, the reason is that the freezing process of ice crystal generation and growth not only will make the soybean tissue structure has been mechanically damaged, but also able to destroy the protein and fat oxidation enzyme molecular spatial structure, so that the activity of the enzyme is inhibited or even inactivated [6].
At the same time, soybeans can also be treated by ultra-high temperature transient method, studies have shown that: after two-stage ultra-high temperature transient (120 ℃ / 80 s + 140 ℃ / 4 s) treatment of soybeans prepared by the raw soybean milk in about 99% of the loss of lipoxygenase activity, most of the soybean odor compounds reduced to below the limit of detection, soybean odor is significantly weakened [7].
(ii) Pulping process In the pulping process, the soy odor can be attenuated by changing the conditions of the lipoxygenase-induced enzymatic reaction, such as through the addition of acid or alkali, adjusting the pH value of the soaking soybean solution so that it deviates from the optimal pH value of lipoxygenase.
Yang Daoqiang [8] replaced the traditional aqueous solution for soaking soybeans with a solution that added a certain mass fraction of NaHCO3, resulting in a higher overall sensory score of soymilk, a lower soya odor score and an increase in the content of proteins, polysaccharides and other nutrients.
In the pulping process to increase the temperature can make the protein moderate thermal denaturation and lipoxygenase inactivation, and then inhibit the soybean fishy substances produced in the process, the aroma produced by heating can also mask part of the soybean fishy taste. Generally speaking, the heating temperature above 80 ℃ can make 99% of the fat oxygenase inactivation.
Common heating methods are straight boiling, steam, microwave, hot, etc., although these heating methods can help weaken the soy flavor, but individual ways will be on the other qualities of soymilk to the detriment. Such as direct-fire boiling will make the soybean milk due to continuous stirring and long exposure to light and air, a large number of single-linear oxygen-mediated generation of soybean fishy substances, such as the formation of 2-pentyl furan; steam heating can easily lead to localized overheating of soybean milk, resulting in the role of the bubble tumbling, so that soybean milk indicates that the production of many bubbles, affecting the overall quality. And microwave heating due to uniform heating, fast, and can effectively reduce the fishy smell of soybean and other advantages of the factory and the family’s most commonly used way of heating soybean milk, the United States has been the use of microwave to produce soybean milk patents [9-11].
In addition to heating, ultra-high-pressure homogenization and pulsed electric field treatment of raw soymilk or spray drying of cooked soymilk into a soluble soymilk powder can be used to produce soymilk without soy odor process. They have many advantages over traditional technologies, such as ultra-high-pressure homogenization can reach nanometer particles of soymilk, which has a more delicate taste and good stability; pulse electric field treatment time is short; spray drying can effectively remove the anti-nutritional factors in soymilk, etc. [12,13].
(C) soybeans and soybean milk storage conditions to improve the soybean after harvesting and drying should try to complete the production of soybean milk within 3 months, because the storage time of more than 3 months of soybeans prepared from soybean milk soybean flavor will not continue to weaken with the extension of the storage time, but will also increase the hidden danger of soybeans by moths [14].
The storage environment should be as low-temperature as possible, low humidity (commercial soybean storage humidity ≤ 13%), low oxygen, dry, ventilated, under such conditions soybean respiratory strength will be greatly reduced, the chemical composition of the changes in the magnitude of small. For the preparation of finished soybean milk, it is also generally recommended that low-temperature refrigeration, so that not only the soybean odor has a weakening effect, and the overall flavor without much impact.
Liu Fang et al [15] conducted a comprehensive physicochemical control test with plain soymilk as an example, and found that the combination of low temperature (4 ℃) and 0.10 g/Kg ε-polylysine, 0.10 g/Kg streptozotocin lactate storage can significantly inhibit microbial growth and reproduction in soymilk, extend the shelf-life of soymilk up to 130 d, and soymilk soybean fishy flavor substances in soymilk is reduced, and the loss of the non-soybean fishy flavor substances that are popular among consumers is smaller. The loss of soya-flavor substances was small. The formation of soymilk fishy flavor is closely related to the enzymatic reaction induced by lipoxygenase, and the production of related substances is affected by multiple factors.
The elimination methods of soybean odor that have been reported so far are mainly based on the starting point of blocking the enzymatic reaction through physicochemical methods, but it is worth noting that some of the methods can attenuate the soybean odor, but they also have side effects on the quality of soybean milk other than soybean odor. Such as no fat oxygenase system of soybeans due to the existence of linolenic acid into hydroperoxides to generate the unknown enzyme soy odor substances, can not fundamentally eliminate the soy flavor, and the lack of lipoxygenase soybean quality made of soybean milk taste rough, weak sweetness, not as good as ordinary soybeans made of soybean milk. Therefore, in order to reduce the soybean odor for the purpose of soybean milk preparation process improvement and innovation at the same time should also take into account the overall flavor and quality of soybean milk.
In addition, the use of ultra-high-pressure homogenization, spray drying, pulsed electric field and other physical methods instead of heat treatment in addition to soybean milk fishy flavor are good prospects for the application of technology; the use of a number of technologies instead of a single method is also the focus of future research in the improvement of fishy soybean flavor.