Erythritol Powder What Is It Made From？

Erythritol, also known as (2R,3S)-butane-1,2,3,4-tetrol, belongs to the family of polyhydric alcohols (also known as sugar alcohols). Erythritol is a four-carbon sugar alcohol with a molecular weight of 122.12, the smallest in the sugar alcohol family. It is not optically active and only exists in a symmetrical form, i.e. as a racemate [1].

Erythritol, a four-carbon polyol, is widely found in nature. It can be isolated from fruits (pears, grapes, melons), mushrooms, alcoholic beverages (beer, wine, sake) and fermented foods (soy sauce, bean paste), and is also found in the body fluids of humans and animals, such as lens tissue, serum, plasma, fetal fluid and urine [2]. Erythritol was first isolated in 1852, but it was not until 1990 that it appeared on the Japanese market as a new natural sweetener. Erythritol has now been approved as a food additive in at least 55 countries. Like other polyols, such as xylitol, sorbitol, mannitol, lactitol or maltitol, erythritol has sweetening properties. Its sweetness is about 60% to 70% that of sucrose, and its taste and texture are similar to those of sucrose. However, due to its small molecular weight, the metabolism of erythritol in the human body is different from that of other sugar alcohols, giving it unique physiological properties such as low calories, high tolerance, few side effects, and suitability for diabetics, as well as being non-cariogenic [3]. In addition, erythritol is also a free radical scavenger with antioxidant properties [4].

1. Erythritol production methods

Erythritol production methods mainly include direct extraction, chemical synthesis and microbial fermentation. The direct extraction method refers to the extraction of erythritol from natural sources such as fruits or vegetables. However, since the content of erythritol in nature is too low, the direct extraction method is rarely used. Compared with other polyols, erythritol is not suitable for chemical synthesis. The high temperature conditions and nickel catalysts required make it difficult to produce, with low product yield and poor economic benefits, so it is difficult to be adopted in large-scale industrial production [5]. As early as 1950, erythritol was found in the residue after fermenting black molasses with yeast, and this discovery opened up a new way of producing erythritol, namely microbial fermentation. At present, the microbial fermentation method for producing erythritol is becoming more and more mature, and it is the main method for industrial production.

2 Optimization of erythritol fermentation production method

2.1 Optimization of culture medium

The composition of the culture medium has an important regulatory effect on the growth of microorganisms. Therefore, when preparing the culture medium, it is necessary to ensure that the growth needs of microorganisms are met, while also facilitating the efficient production of the target product. At the same time, consideration should be given to avoiding the production of by-products. To this end, many scholars have conducted research on the optimization of erythritol fermentation culture media, with more research focusing on carbon and nitrogen sources, as shown in Table 1.

2.2 Alternative carbon sources

The commonly used substrate for industrial production of erythritol is glucose, which has good fermentation results but is relatively expensive. At present, a more common approach is to further reduce production costs by using alternative substrates. Currently reported alternative carbon sources for glucose include glycerol, xylose, molasses, inulin, restaurant industry oils and fats, fructose, sucrose, etc., as shown in Table 2. Among these, glycerol is a research hotspot for alternative carbon sources for glucose.

There have been many studies on glycerol as a new carbon source for the fermentation of erythritol, which mainly includes pure glycerol and crude glycerol. Crude glycerol is mainly a by-product of the biodiesel industry. Using it in the fermentation production of erythritol not only effectively reduces the production cost of erythritol, but also solves the waste disposal problem for the biodiesel industry. At present, more research is being done on the Yarrowialipolytica strain. In addition, glycerol conversion to erythritol can also be observed in Moniliellamegachiliensis [11].

The Y. lipolytica strain not only effectively converts pure glycerol to erythritol, but can also use crude glycerol from industrial waste. Moreover, the chemical composition of crude glycerol is complex, with many impurities. The main impurities vary depending on the source, and it may be contaminated with compounds such as methanol, salts or metals.

It is worth mentioning that the strain Yarrowialipolytica can take advantage of glycerol and can grow on crude glycerol from different sources. Another advantage of using glycerol as a fermentation substrate for erythritol is that it can effectively reduce the production of by-products after fermentation. Glycerol is one of the main by-products during fermentation using glucose as a carbon source [12]. Moreover, glycerol is particularly difficult to separate from erythritol during purification. When glycerol is used as a fermentation substrate, it can be completely consumed before the end of the fermentation process, and the content of other by-products can be reduced to less than 10% [13]. A comprehensive analysis shows that the maximum erythritol concentration levels for glucose and glycerol fermentation processes are comparable, but the yield of the latter is lower. The amount of glycerol added varies depending on the culture system. Although glycerol is a by-product of the biodiesel industry and has a high impurity content, its commercial value is low. However, many reports have shown that it has great potential as a carbon source for bioprocessing.

Xylose is the main component of hemicellulose and is abundant in nature. In recent years, xylose has gradually attracted attention as a potential carbon source for microbial fermentation, especially for the possibility of using low-cost substrates such as xylose-rich industrial waste. Molasses, as an inexpensive industrial by-product, has been studied as a carbon source for erythritol production. It should be noted that molasses is not directly used in erythritol synthesis, but for the growth of bacterial cells. Molasses is used to accumulate bacterial cells at the beginning of microbial fermentation, and glycerol is added later to increase osmotic pressure and trigger erythritol production [14].

Inulin is a polysaccharide found in the roots and tubers of plants such as Jerusalem artichoke, chicory, dahlia and yacon. Like molasses, inulin is renewable and inexpensive, making it an ideal carbon source for microbial fermentation. Similar to molasses, inulin has also been used in a two-step fermentation process with the Y. lipolytica strain to produce erythritol [15]. In addition, taking advantage of Y. lipolytica's ability to grow on oil, it was added to waste cooking oil, and after fermentation, erythritol was extracted from the fermentation system [16].

It has been reported that the C. magnoliae KFCC 11023 strain prefers fructose as a carbon source over glucose. When fermented in a batch fermentation mode with fructose as the carbon source, the erythritol concentration is 21.25 times that of glucose as the carbon source, but the by-product glycerol production is as high as 77 g·L-1 [17]. Moreover, when this strain is used to ferment sucrose as a substrate, the erythritol concentration is 65 g·L-1, the conversion rate is 0.21 g·g-1, and the yield is 1.0 g·L-1·h-1, which means that the inexpensive industrial by-product molasses, which is mainly sucrose, can be used as a carbon source to further reduce costs.

2.3 Alternative nitrogen sources

Nitrogen sources are nutrients that provide nitrogen for microbial cells and metabolites. Commonly used nitrogen sources can be divided into two categories: inorganic nitrogen sources (ammonium sulfate, nitrate, ammonia and urea, etc.) and organic nitrogen sources (soybean cake meal, peanut cake meal, cottonseed cake meal, corn syrup, peptone, yeast extract and fish meal, etc.). The nature and concentration of the nitrogen source are very important parameters in the fermentation production of erythritol. In order to obtain a large erythritol production capacity, it is necessary to optimize the type and amount of the best nitrogen source for different strains.

Under the condition of constant culture with pure glycerol as the carbon source, the effect of inorganic and organic nitrogen sources on the production of erythritol by the Y. lipolytica Wratislavia K1 strain was studied. It was found that the erythritol content in a constant culture medium containing 4.6 g·L-1 ammonium sulfate was as high as 103.4 g·L-1, and the optimal erythritol yield and conversion rate were also obtained under the conditions of inorganic nitrogen source (4.6 g·L-1 ammonium sulfate), 1.12 g·L-1 and 99.6%, respectively.  L-1. The best erythritol yield and conversion rate were also obtained under the culture conditions of inorganic nitrogen source (4.6 g·L-1 ammonium sulfate), 1.12 g·L-1·h-1 and 0.52 g·g-1, respectively [18]. Rywińska et al. used Y. lipolytica Wratislavia K1 as the experimental object, and carried out erythritol fermentation experiments using a variety of inorganic and organic nitrogen sources. The results showed that ammonium chloride, ammonium sulfate and yeast extract were the best nitrogen sources, and the best erythritol yield and conversion rate were also obtained under ammonium sulfate as the nitrogen source [9]. In contrast to the above results, it has been reported that organic nitrogen sources are more suitable for the fermentation of erythrulose by the Y. lipolytica Wratislavia K1 strain [21].

When cultured with glucose as the carbon source, it was also confirmed that ammonium sulfate is more suitable for the fermentation of erythrulose by the Y. lipolytica mutant 49 strain than yeast extract. However, when the concentrations of glucose and ammonium sulfate were lower or higher than the optimal concentrations, the erythritol yield varied greatly [8].

In addition, it was found that the highest erythritol yield was obtained when the strain P. tsukubaensis and Moniliella sp. were mixed with corn soaking powder and yeast extract as the fermentation nitrogen source [22, 23]. For the Torula sp. strain, the highest erythritol yield was obtained when yeast extract was used as the sole nitrogen source [6]. This shows that different strains have different choices of nitrogen source, and the type of nitrogen source directly affects the yield and productivity of erythritol produced by the strain during fermentation.

3 Fermentation process improvement

The culture system is one of the important factors determining the final concentration of erythritol after fermentation. In order to improve efficiency, the erythritol fermentation process has been continuously improved, mainly including batch fermentation, fed-batch fermentation, two-step fermentation and continuous fermentation, as shown in Table 3.

Erythritol production is usually carried out in batch mode, and a higher initial glucose concentration can increase erythritol production. In batch fermentation, all the necessary substrates are introduced at the start of the fermentation, and the product and by-products are extracted after all the substrates have been depleted. Simple batch fermentation is easy to operate, but the yield and concentration of erythritol are low.

The most common culture process for fermenting erythritol is batch feeding. Batch feeding can maintain a high osmotic pressure throughout the culture process. The most efficient erythritol production process reported to date is a fed-batch fermentation using the P. tsukubaensis strain as the fermenting strain, with a productivity of up to 2.86 g·L-1·h-1, and an increase in erythritol production of 73% compared to the same strain under batch conditions [22].

High osmotic pressure can increase the yield of erythritol, but it also inhibits the growth of bacterial cells. To solve this problem, a two-step fermentation process has been developed, which promotes the growth of bacterial cells under low osmotic pressure conditions in the early stage of fermentation, and then increases the osmotic pressure in the later stage of fermentation to promote the metabolism of erythritol by the bacteria [24].

In addition, it has been reported that a continuous fermentation method has been applied to the microbial fermentation method for the production of erythritol. In the continuous fermentation process, part of the culture medium is regularly replaced with fresh medium. This method can improve productivity by extending the effective production phase. The disadvantage is that there is a greater chance of contamination by foreign bacteria, and the strain is prone to degeneration due to mutation. The solution is to reduce the pH to prevent bacterial contamination [13].

4 Optimization of culture conditions

The production efficiency of erythritol depends to a large extent on the culture conditions: osmotic pressure, temperature, pH, dissolved oxygen, etc. are all important technical indicators related to the fermentation of erythritol.

Osmotic pressure stress is one of the main causes of erythritol production. There are two main ways to regulate osmotic pressure: one is to use a high concentration of substrate, such as glucose or glycerol, and the other is to add extra salt [28]. Osmotic pressure regulation is a relatively complex process. An increase in osmotic pressure leads to an increase in erythritol production and a decrease in by-product formation. However, too high an osmotic pressure prolongs the lag phase of the development of the fermentation strain, resulting in a decrease in erythritol productivity [24].

Studies have shown that temperature, pH and dissolved oxygen have a significant effect on the production of erythritol by different strains, and the optimal values vary, as shown in Table 4.

5 Prospects

Nowadays, more and more people are paying attention to a healthy lifestyle, and the demand for polyols is also growing. How to achieve the efficient and low-cost industrial production of erythritol has become the focus of attention of many scholars. Erythritol has many of the same properties as other sugar alcohols. However, it is the only sugar alcohol that is industrially produced through a natural fermentation process. Scholars have conducted a lot of research to increase production capacity. First, based on obtaining a high-performance erythritol-producing strain, the composition of the culture medium and the culture conditions have been well optimized and adjusted in order to maximize yield and conversion.

A lot of effort has also been put into effectively reducing production costs, especially through the consideration of cheap renewable carbon sources, and even from the perspective of by-product recycling, research and development of erythritol co-production has been carried out with some success. However, there has been relatively little research on the regulation of erythritol metabolic gene expression. Although some key enzymes in metabolic processes have been identified, the erythritol microbial metabolic pathway is still unclear at the genetic level. In the future, the field of gene regulation should be targeted to comprehensively understand the genes and regulatory factors involved in the synthesis of erythritol. This will undoubtedly serve as a powerful reference for effectively regulating the microbial fermentation of erythritol, thereby further achieving efficient, industrial production of erythritol.

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