原著論文
卵殻存在下でのマイクロ波加熱を利用したガラクトースの異性化による希少糖の生産
2026 年 27 巻 2 号 p. 15-22
詳細
2026 年 27 巻 2 号 p. 15-22
In this study, we optimized the production of rare sugars, such as tagatose, using galactose aqueous solution and eggshell powder as raw materials through isomerization by microwave heating. Similar to the conventional subcritical water treatment, isomerization of galactose to tagatose proceeded efficiently by the microwave heating, achieving a maximum tagatose yield of 16 mol% after a treatment at 600 W for 210 s. A high tagatose selectivity (up to 76%) was obtained in the early reaction stage. However, as the treatment time increased, byproducts (organic acids and ketones) accumulated, causing a decrease in both the yield and the selectivity of tagatose. Analysis of the byproducts suggested a reaction pathway in which galactose and tagatose decompose into ketones and organic acids through the pathways such as retro-aldol condensation. Furthermore, increasing the initial galactose concentration improved weight-based concentrations of the produced rare sugars although their molar yields decreased. Based on these results, microwave heating treatment was concluded to be an effective method for the production of tagatose in a short time, with optimization of microwave output, time, and feedstock concentration being key to achieving high yields.
本研究は，卵殻粉を加えたガラクトース水溶液をマイクロ波で加熱し，希少糖であるタガトースを効率的に生産する方法の開発を目的としている．従来のタガトース合成法には，アルカリ処理による化学反応や酵素法があるが，前者は副生成物が多く，後者は触媒の安定性などに課題があった[12,13]．一方，著者らはこれまでに亜臨界水処理による中性条件下での異性化を報告してきたが[14]，高収率には長時間加熱（60 min以上）が必要であり，処理効率の改善が求められていた．そこで本研究では，短時間かつ高効率な加熱が可能なマイクロ波加熱を利用し，卵殻粉を加えることで酸性副生成物によるpH低下を抑制しつつ，タガトースの収率向上を図る新規プロセスの開発に取り組んだ．
まず，マイクロ波加熱出力（200または600 W）によるタガトース収率の変化を調査した（Fig. 1）．卵殻を添加しない条件では収率は1 mol%未満にとどまったが，卵殻粉を加えることで最大16 mol%の収率が達成された．600 Wの高出力では約90 sで収率は14 mol%に達し，従来の外部加熱による亜臨界水処理[15]と比較して加熱時間を大幅に短縮できた．また，ガラクトースの変換率とタガトース収率の関係を評価した結果（Fig. 2），マイクロ波加熱と亜臨界水処理の反応機構は類似していると考えられた．
マイクロ波出力によるタガトースの選択率への影響も明らかにした．600 Wでは短時間で高収率が達成できるが，加熱を延長するとタガトースの分解が進行し，選択率が大きく低下した．一方，200 Wでは反応時間が長くなるものの，選択率は比較的安定していた．このことから，高出力では厳密な加熱制御が求められ，低出力では柔軟な運用が可能であると考えられる．
副生成物の分析から，加熱条件により有機酸（グリコール酸，ギ酸，酢酸）やケトン（ジヒドロキシアセトン，ヒドロキシアセトン）が生成することが明らかとなった（Fig. 3）．これらは，糖の加熱により生じる逆アルドール縮合や酸化還元反応により生成したと推定された（Fig. 4）．とくにジヒドロキシアセトンの生成タイミングは，タガトースの収率が最大に達した直後と一致し，タガトースが逐次的に分解されていることが示唆された．
また，異性化により他の希少糖であるタロースやソルボースも生成した（Fig. 5(A)）．ソルボースはとくに200 Wで240 s以上加熱した際に生成が顕著になり、タガトースからの逐次的生成が示唆された．同時に，pHの急激な低下も観測され（Fig. 5(B)），グリコール酸などの酸性副生成物による影響と考えられた．タガトース収率が16 mol%程度に達すると分解や異性化が優勢となり，それ以上の収率向上は困難と示唆された．
次に，原料であるガラクトースの初濃度（1～30 wt%）が異性化に与える影響を調査した（Fig. 6～8）．初濃度が高くなるほど最終的なタガトースの重量濃度は上昇した．一方，初期濃度が高くなるほど希少糖の物質量基準の収率は低下したが， 15 wt%以上のガラクトース濃度では収率はほぼ一定であった（Fig. 6）．このように，初期濃度が高くても希少糖収率の大幅な低下が生じないことから，生産性の観点からはガラクトースの初濃度を高く設定することが望ましい．一方，選択率は低下傾向を示し，初濃度が30 wt%では選択率は46%にまで低下した（Fig. 7）．これは副反応の進行によるもので，反応後のpH低下から有機酸の蓄積による影響が示唆された（Fig. 8）．
最終的に，マイクロ波加熱処理によるタガトースの製造では，従来法と比べて加熱時間を大幅に短縮しつつ，同等の収率（最大16 mol%）を達成できた．高出力短時間加熱では副反応が進行しやすいため，適正な出力と処理時間の最適化が重要である．また，副反応生成物の分析から，反応機構の理解が深まり，今後の反応制御への応用が期待される．
Rare sugars are defined as “monosaccharides and their derivatives that are present in limited quantities in nature” [1], with d-tagatose and d-allulose being representative examples. Tagatose possesses 92% of the sweetness relative to sucrose while having low calorie content [2] and is recognized as GRAS. Tagatose has been confirmed to improve blood glucose control in type 2 diabetes [3,4] and demonstrates beneficial metabolic effects in healthy individuals [3,5,6]. In addition, it is resistant to utilization by oral bacteria and possesses anti-caries properties [7], making it a promising candidate for application as a food additive and sugar substitute [8,9].
Conventionally, chemical techniques using NaOH or Ca(OH)2, or biochemical approaches using enzymes or microorganisms, have been employed to produce tagatose. Tagatose is chemically formed by isomerizing galactose under alkaline conditions through Lobry de Bruyn-Alberda van Ekenstein (LBAE) transformation [10,11]. However, the chemical techniques produce numerous byproducts due to the strong alkali and require laborious purification, while the biochemical approaches face challenges regarding catalyst selection, stability of enzyme, and production cost [12,13]. These issues necessitate the development of efficient and selective methods to minimize byproduct formation and simplify the purification process.
We have investigated the application of subcritical water treatment for the production of rare sugars, such as tagatose [14]. Although this method enables isomerization under neutral conditions, it suffers from longer reaction time and a decrease in the yield due to pH drop caused by the formed organic acids. Therefore, we developed a method to stabilize pH by adding eggshell powder, primarily composed of calcium carbonate, to neutralize acidic byproducts, thereby improving the yield of tagatose [15]. However, achieving the maximum tagatose yield of 16 mol% required 60-min heating, necessitating further improvements in production efficiency. Therefore, significantly shortening the heating time while maintaining high yield remained the major challenge for practical application.
Aiming to shorten the heating time, we are focusing on the application of microwave heating. Microwaves promote the movement of water molecules and charged particles, enabling uniform heating from within, which allows for faster and more efficient temperature control compared to the conventional conduction heating [16,17]. It has also been widely used as a promising technology from the perspective of green chemistry and is being applied to various chemical reactions [18]. Recently, we succeeded in synthesizing corresponding rare sugars from various reducing sugars by microwave heating [19-22]. However, the production of rare sugars such as tagatose in the presence of eggshells by microwave heating has not been studied in detail. Therefore, in this study, we systematically optimized the reaction conditions for microwave heating, quantified byproducts, and evaluated the effect of initial galactose concentration. By analyzing the reaction behavior in detail from the perspective of the reaction mechanism, we proposed an efficient and selective method for the production of tagatose.
Chicken eggshell powder was purchased from Leaf Corporation (Gunma, Japan). d-Galactose (abbreviated as ‘Gal’ if necessary), d-tagatose (Tag), d-talose (Tal), d-sorbose (Sor), and acetonitrile, all of which were of special grade, were purchased from FUJIFILM Wako Pure Chemical (Osaka, Japan). Note that because all the sugars treated in this study are d-sugars, the prefix ‘d-’ indicating the d-configuration is omitted hereafter. Other chemicals were purchased from FUJIFILM Wako or Nacalai Tesque (Kyoto, Japan).
2.2 Treatment of galactose solution by microwave heatingAll reactions were conducted in a pressure-resistant jar (47 mm I.D. × 130 mm, 60 mL capacity, FLON INDUSTRY, Tokyo, Japan) made of perfluoroalkoxy alkanes (PFA). The jar was tightly sealed after adding a 1-30 wt% galactose aqueous solution (40 mL) and eggshell powder (0 or 4.0 g). The amount of eggshell powder (4.0 g) was determined based on the results of our preliminary investigation in a previous report [15], which confirmed that this amount is sufficient. The jar was placed in a microwave oven (DMW-P96W, DAEWOO SALES, Tokyo) at the center of the turntable inside the oven, and the heating process was conducted while the turntable was rotating at 6 rpm for 0-270 s. The output of the microwave oven was set to either 200 or 600 W. Note that heating was performed for 90 s at 600 W and 270 s at 200 W; however, we could not prolong the heating time further because the pressure relief valve on the jar activated (relief pressure: ca. 0.7 MPa). After the treatment (hereafter referred to as the heating time), the jar was immediately immersed in an ice bath with occasional shaking for rapid cooling until it reached room temperature. The reaction mixture was filtered through a filter paper (No. 2, TOYO ROSHI KAISHA, Tokyo) to remove the powdery eggshell. The composition of the resulting filtrate was quantified by HPLC as described later.
2.3 Measurement of liquid temperature during the microwave heatingChanges in liquid temperature inside the jar by heating were measured as follows: Pure water (40 mL) was poured into a jar. The jar was then tightly sealed and subjected to microwave heating at 200 W or 600 W. Heating time was set within the range where water does not boil at ambient pressure (0-90 s), and the liquid temperature was measured immediately after heating.
2.4 Analysis of sugarsGalactose and formed rare sugars (tagatose, talose, and sorbose) in the treated solution were quantified by HPLC. An HPLC pump (LC-20AD, Shimadzu, Kyoto) was connected to a COSMOIL Sugar-D column (3.0 mm I.D × 250 mm, Nacalai Tesque) and a refractive index detector (RID-20A, Shimadzu). The mobile phase consisted of 80 vol% acetonitrile, with a flow rate of 0.4 mL/min. Column temperature was controlled at 40°C using a column oven (CTO-10AVP, Shimadzu).
2.5 Analysis of byproductsOrganic acids and ketones formed as byproducts during the treatment were quantified by HPLC. An RSpak KC-811 column (8.0 mm I.D × 300 mm, Resonac, Tokyo), an RSpak KC-G8B guard column (8.0 mm I.D × 50 mm, Resonac), and a refractive index detector (RID-20A) were connected to the HPLC pump (LC-20AD). The mobile phase was a 0.1 vol% phosphoric acid aqueous solution, with a flow rate of 0.8 mL/min. The column temperature was controlled at 40°C. The byproducts were determined by comparing the retention times with those of commercially available standards.
2.6 pH measurementThe pH of each treated solution was measured at room temperature using a pH meter (D-71, HORIBA, Kyoto).
Microwave treatment of galactose solutions was performed in a pressure-resistant jar at a 40-mL scale with a microwave output of 200 W or 600 W. Figure 1 shows time courses of the liquid temperature (below 100°C) inside the jar during heating and of the yield of tagatose in the presence or absence of eggshell. It is inferred that the isomerization proceeded even after the microwave heating ended, as long as the temperature remained above 100 °C. Therefore, the yields reported here include the progress of the reaction during the cooling process. The yield of tagatose was defined as follows:
 | (1) |
In the absence of eggshell, tagatose was hardly formed (yield <1 mol%). In contrast, the yield increased by heating in the presence of eggshell, finally reaching 14-16 mol%. At 600 W, the yield reached 14 mol% after around 90 s. At 200 W, however, it took approximately 200 s to reach the maximum yield of 16 mol%, and further prolongation of the treatment time did not increase the yield.
Time courses of the molar yield of tagatose and liquid temperature in the pressure-resistant jar during the microwave heating treatment.
Conditions for the treatment: Initial concentration of galactose: 5 wt%; solution volume: 40 mL; amount of eggshell: 0 or 4.0 g; microwave output: 200 or 600 W; heating time: 30-270 s. Experiments were performed in triplicate (n ＝ 3) for samples in the presence of eggshell (mean±SD (standard deviation)); once (n ＝ 1) for samples without eggshell.
In both cases, no formation of tagatose was observed during the initial heating phase. The formation was observed after 30 s at 600 W and after 90 s at 200 W. In both cases, the reaction proceeded gradually below 100°C, and the yield of galactose was 0.84% (200 W) at 90 s. Furthermore, at both microwave outputs, the reaction became prominent only after reaching 100°C or higher.
To consider side reactions, degree of isomerization to tagatose, i.e., the selectivity, was chosen as the evaluation factor. Here, the yield of tagatose was plotted against the conversion of galactose (Fig. 2), where the conversion was defined as follows:
 | (2) |
Previous results obtained using a batch reactor are also shown [15]. Both plots showed convex curves (Fig. 2A). As the conversion of galactose increased, the yield of tagatose also rose. However, once the conversion exceeded 15 mol%, deviation of the curve from the diagonal straight line became significant, where this line represents the path taken when no side reactions occur at all. These results indicate the progression of the side reactions. Moreover, both treatments by microwave heating and conventional subcritical water gave nearly identical plots. This indicates that these treatments are essentially similar in their reaction mechanisms. Considering the heating time required to achieve 10 mol% or higher of tagatose yield, it took approximately 30-min heating in a batch treatment [15]. In contrast, microwave treatment at 600 W achieved 10 mol% yield within 180 s (the maximum yield was 16 mol%, achieved at 210 s). Therefore, microwave treatment achieved the yields equivalent to the conventional batch treatment within a short time.
Behavior of the isomerization of galactose to tagatose during the microwave heating treatment.
(A) Relationship between the molar yield of tagatose and the conversion of galactose; (B) Relationship between the selectivity of tagatose and microwave heating time.
Conditions for the treatment: Galactose solution (5 wt%): 40 mL; amount of eggshell: 4.0 g; heating time: 30-270 s; microwave output: 200 or 600 W. Experiments were performed in triplicate (n ＝ 3, mean±SD). Results for the batch type subcritical water treatment were from the previous study [15]. Conditions for the batch reaction: Galactose solution (5 wt%): 5 mL; eggshell: 500 mg ; heating time: 5-120 min; heating temperature: 120°C. The diagonal line in (A) corresponds to the case in which no side reactions occur and the selectivity of tagatose is 100%. Regarding (B), since the conversion of galactose was small for the short-time microwave heating treatments (600 W for 30 s and 200 W for 60 s), leading to significant error, the selectivity was not calculated.
Next, we investigate the effect of microwave output on the selectivity of tagatose. Here, the selectivity was calculated by dividing the yield of tagatose by the conversion of galactose:
 | (3) |
At 600 W, the treatment showed a 76% selectivity at 60 s (Fig. 2(B)). However, the selectivity decreased considerably with further prolongation of the heating time (57% at 90 s), with the side reactions being particularly promoted in the latter stages of the reaction. In contrast, treatment at 200 W showed slightly higher selectivity (65-78%) compared to the 600-W treatment, although this difference was not statistically significant. Moreover, the selectivity was maintained even at prolonged treatment time.
Therefore, while high-power treatment enables high-yield tagatose production within a short time, it requires precise reaction control. Conversely, low-power treatment suppresses the side reactions and offers flexibility in processing time.
3.2 Formation behavior of other rare sugars and byproductsNext, we investigated the formation of other rare sugars (talose and sorbose) and byproducts, since both isomerization and decomposition occur simultaneously during the microwave heating treatment.
Figure 3 shows time courses in the yields of organic acids and ketones that were identifiable and quantifiable among the byproducts during the reaction. Here, the yields of byproducts are expressed as weight percent relative to the initial galactose weight. This is because the molecular mass of each byproduct species differs, making mol% representation inappropriate. We observed the formation of at least glycolic acid, formic acid, and acetic acid as organic acids, and dihydroxyacetone and hydroxyacetone as ketones. Notably, by the 200-W treatment, formation of formic acid, acetic acid, and dihydroxyacetone became significant after 180 s.
Time courses of the weight-based concentration of organic acids and ketones in the solutions subjected to microwave heating treatment.
Conditions for the treatment: Galactose solution (5 wt%): 40 mL; amount of eggshell: 4.0 g; microwave output: 200 or 600 W; heating time: 30-270 s. Experiments were performed in triplicate (n ＝ 3, mean±SD); SD is the standard deviation relative to the total amount of acids and ketones.
Figure 4 depicts the possible decomposition pathways for galactose and tagatose. It is known that reducing sugars decompose through pathways such as retro-aldol condensation (RAC) upon heating [23,24]. On the basis of the double-bond rule, galactose decomposes through RAC into primarily C4 compound (threose) and C2 compound (glycolaldehyde), although these were unidentified. It is considered that the latter was oxidized by trace dissolved oxygen to form glycolic acid, which was detected in this study.
Possible pathways for the isomerization and decomposition during the microwave heating.
RAC denotes retro-aldol condensation. Compounds enclosed by black borders were detected and quantified in this study.
In the case of tagatose, the cleavage position of the C-C bond differs from that of galactose based on the double bond rule. Therefore, tagatose is primarily decomposed into dihydroxyacetone and glyceraldehyde (both are C3 compounds). At both 200-W and 600-W microwave outputs, the yield of dihydroxyacetone increased sharply once the yield of tagatose reached a specific threshold, as described above (600 W: 80-90 s, 200 W: 240-270 s; Figs. 1 and 3). This result suggests that some of the formed tagatose could undergo further decomposition.
We also discuss the significant formation of formic acid and hydroxyacetone in the latter half of the reaction. It is presumed that dihydroxyacetone derived from tagatose was converted to hydroxyacetone by reduction. Concurrently, glyceraldehyde was decomposed into formic acid and glycolaldehyde through further RAC. Furthermore, in the 200-W treatment, the yield of glycolic acid also increased significantly between 210 and 240 s. This is also a reflection of the decomposition of glyceraldehyde derived from tagatose. In addition, acetic acid was detected in the latter half of the reaction; this would be formed through the sequential dehydration and rehydration of glycolaldehyde. As shown, the 200-W treatment allowed detailed confirmation of the side reactions, contributing to the interpretation of reaction mechanisms.
To verify the effect of the formed organic acids on LBAE transformation, we also investigated the formation behavior of other rare sugars such as talose and sorbose, along with the accompanying pH changes. Previously, we reported that LBAE transformation isomerizes galactose to talose and tagatose to sorbose [25]. However, the accumulation of organic acids is known to inhibit this transformation, leading to a decrease in the yield and selectivity of these rare sugars. Accordingly, we plotted the yields of talose and sorbose, as well as pH, against the yield of tagatose (Fig. 5). The yield of talose increased in parallel with an increase in the yield of tagatose (Fig. 5(A)), even though it remained at a considerably low level (<2 mol%) under both 200-W- and 600-W treatment. In contrast, sorbose was detected only at 200 W and began to form after heating for 240 s or longer. Comparing the composition of the solution after 240-s treatment with that at 210 s revealed a 1.4 mol% decrease in the yield of tagatose and a 1.0 mol% increase in the yield of sorbose (indicated by small arrows in Fig. 5(A)). This consistently demonstrates the formation of sorbose through the sequential isomerization of tagatose.
Changes in the yields of rare sugars and pH during tagatose production.
Dependence of (A) the molar yields of rare sugars and (B) pH on the molar yield of tagatose after the treatment. Small solid arrows in the figure indicate results at 210 s or 240 s in the 200-W treatment. Dashed arrows indicate the direction of increasing time for the treatment. Conditions for the treatment: Galactose solution (5 wt%): 40 mL; amount of eggshell: 4.0 g; microwave output: 200 or 600 W; heating time: 30-270 s. Experiments were performed in triplicate (n ＝ 3, mean±SD).
Furthermore, in Fig. 5(B), the pH dropped sharply at 240 s during the 200-W treatment (indicated by the small arrow). This fact coincided with the point of increased yield of dihydroxyacetone (Fig. 3), reinforcing the inference that while LBAE transformation is suppressed, formation of dihydroxyacetone is promoted through RAC from tagatose. Therefore, when tagatose is formed in high yield, further isomerization of galactose is suppressed, and instead, decomposition of tagatose driven by pH drop become gradually apparent. Consequently, it is hard to achieve the yield of tagatose exceeding 16%, resulting in an almost constant yield of tagatose under these reaction conditions.
3.3 Effect of the initial concentration of galactose on the isomerization behaviorThe weight-based concentration of tagatose in the solution obtained by treating a 5 wt% galactose solution was approximately 0.8 wt%, which was considered low. For practical applications, it is necessary to increase the concentration of tagatose. Therefore, the initial galactose concentration was set to 1-30 wt% (g-Gal/g-solution), where the solubility of galactose in water at 20°C is 30.2 wt%, and the solution was treated at 200 W for 210 s. Figure 6 shows effects of the initial concentration of galactose on the remaining galactose, the molar yield of rare sugars, and the weight-based concentration of each sugar. In all the treatments, 73-86 mol% of galactose remained (Fig. 6(A)). Furthermore, the yield of each rare sugar showed a decreasing trend as the initial concentration of galactose increased (Fig. 6(B) and (C); tagatose yield: 17.8 mol% at 1 wt%-Gal; 9.1 mol% at 30 wt%). The molar yield of tagatose was particularly pronounced in the low initial concentration range (≤10 wt%), but it remained nearly constant at initial concentrations of 15 wt% and above. Similar trends were observed for talose and sorbose. As shown, increasing the concentration of galactose above 15 wt% resulted in only small changes in the molar yields of the rare sugars, while their weight-based concentrations increased significantly. Therefore, to obtain each rare sugar at a high concentration, it is desirable to perform the reaction at the highest possible concentration of galactose.
Effect of initial galactose concentration on the molar yield, remaining molar fraction, and weight-based concentration of each sugar.
Yields and concentrations (A) of remaining galactose, (B) of tagatose, and (C) of talose and sorbose. Conditions for the treatment: Initial concentration of galactose: 1.0-30 wt%; solution volume: 40 mL; amount of eggshell: 4.0 g; microwave output: 200 W; heating time: 210 s. Experiments were performed once (n ＝ 1).
Figure 7 shows the effect of weight-based initial concentration of galactose on the selectivity of tagatose and pH. When the initial concentration was 20 wt% or less, the selectivity was approximately 60-80%. However, at 30 wt%, the selectivity decreased significantly to 46%. In addition, the pH dropped as the initial concentration increased, meaning that using galactose at high concentrations promotes the side reactions and hinders the isomerization.
Effect of initial galactose concentration on the selectivity of tagatose and pH of the treated solution.
Conditions for the treatment: Initial concentration of galactose: 1.0-30 wt%; solution volume: 40 mL; amount of eggshell: 4.0 g; microwave output: 200 W; heating time: 210 s. Experiments were performed once (n ＝ 1).
Here, we also investigated in detail the effect of initial galactose concentration on the formation of byproducts (Fig. 8). Figure 8(A) shows the dependence of molar concentrations of organic acids and ketones; here, note that the molar basis is adopted because the formation of organic acids is closely related to the pH drop due to dissociation. As the initial concentration increased, the molar concentration of byproducts also rose, reflecting the pH drop depending on the initial concentration.
Effect of initial concentration of galactose on the formation of byproducts.
(A) Molar concentrations of organic acids and ketones; (B) weight ratio of formed organic acids and ketones per unit weight of galactose. Conditions for the treatment: Initial concentration of galactose: 1.0-30 wt%; solution volume: 40 mL; amount of eggshell: 4.0 g; microwave output: 200 W; heating time: 210 s. Experiments were performed once (n ＝ 1).
Conversely, the weight-based yield of byproducts per unit weight of galactose decreased as the initial concentration increased (Fig. 8(B)). This indicates that, at high concentrations, the side reactions occurred less readily relative to sugar concentration, leading to only a slight, non-remarkable decrease in tagatose selectivity. This phenomenon is complex, involving changes in the reaction environment due primarily to the high sugar content, as well as the formation and dissociation changes of organic acids. Consequently, the reasons could not be fully elucidated in this study due to the involvement of numerous factors. However, since the isomerization yield did not decrease significantly even at high concentrations of galactose, the use of high-concentration solutions is desirable for the practical production of rare sugars at higher productivity.
This study demonstrated the efficient production of rare sugars, such as tagatose, from galactose and eggshell powder by the microwave heating treatment. Specifically, the treatment significantly shortened the reaction time compared to the conventional subcritical water treatment, achieving a maximum yield of 16 mol%. The reaction time and microwave output were determined to be the critical factors controlling the yield and selectivity, where a trade-off exists between high productivity and ease of process control. Moreover, the detailed analysis of byproducts (organic acids and ketones) and their influence on LBAE transformation rationally explained the reaction phenomena, providing a basis for optimizing the process conditions. Although increasing the initial galactose concentration lowered the molar yields, it was effective for improving the weight-based concentration of the rare sugars, which is essential for practical applications. Future developments, such as the construction of continuous reaction systems, scaling up, and advancing microwave control, are expected to lead to more realistic production.
This study was financially supported by Kieikai Research Foundation and partly by JSPS KAKENHI [grant numbers 21K05469 and 24K08807; T. K.].
