L-arabinose (C5H10O5, Mr = 150.13), an aldopentose, is initially isolated from arabic gum via complex chemical and physical methods [5] [6]. L-arabinose is an odorless white needle-like crystalline at room temperature, and it is soluble in water, slightly soluble in alcohol, insoluble in ether, methanol and acetone. Some property of L-arabinose: Melting point: 154˚C - 158˚C, Specific rotation [α] 20D (H₂O, 24 h) +100 ~ + 104˚, relative density 1.585 [4] [7]. The sweetness of L-arabinose is about half of sucrose.

Linear form of L-arabinose is having a carbonyl group (C1) at the top of the carbon chain. The C-5 linked oxygen atom of 1.1 carries out nucleophilic attack on the carbonyl to give a cyclic structure 1.2, which forms β-L-arabinose 1.3 and α-L-arabinose 1.4 epimers due to the different spatial arrangement of the newly formed chiral C-1 atom (Figure 1).

Free L-arabinose rarely exists in the nature, it only been found in the core of coniferous trees [8]. As a component of heteropolysaccharides that is found in gum, hemicellulose, bacterial polysaccharides and some glycoside, L-arabinose exists in the form of β-L-arabinose. Figure 2(a) shows the presence of L-arabinose in corn cob hemicellulose: the backbone is β-1,4-conjugated D-xylan, which can be combined with the hemiacetal hydroxyl of L-arabinose in the side chain at both the 2 and 3 positions of the xylose residue. Typically, more than 61% of L-arabinose is present at the non-reducing end [9]. However, L-arabinose exists mostly in the form of L-arabinose-oligosaccharides in sugar beet pectin: L-arabinose oligosaccharides (with a degree of polymerization of 10 - 20 in multiple forms and shapes) combined with the backbone of the pectin

that consisted by D-galacturonic acid and rhamnose both at the 3 or 4 positions of the rhamnoseresidue [10] [11] (Figure 2(b)).

As a newly developed functional sugar, L-arabinose generally produced by hydrolysis of biomass that rich in L-arabinose residue such as vegetable gums [5] [12], corncob [13] [14], beet pulp [15], bagasse [16] and other materials [17]. L-arabinose can also prepared by chemical or biological synthesis methods, but both methods have shortcomings of high costs, difficult to separate, so it is rarely used in commercial production [18].

Domestic enterprises with a certain scale mainly include: Jinan healtang Biotech Co., Ltd., Xiamen Thomson Biotech Co., Ltd., Shandong Futase Co., Ltd., Jilin Huakang Pharmaceutical, Danisco (China) Co., Ltd., and Shandong Longlive Biotech Co., Ltd., Shandong Tanglikang Co., Ltd. Among them, Jinan Healtang Biotech Co., Ltd. is the largest producer of L-arabinose sugar in China which has the world’s first 1000 ton production device.

As a new functional sweetener, L-arabinose has huge market space in people with diabetes, obesity, and health care food products. With the improvement of people’s awareness, and deepening of research, L-arabinose will be more widely used in the fields of food industry, medicine and cosmetics, it will make a greater contribution in promoting human health and improving people’s life quality.

L-arabinose can be prepared by enzymatic hydrolysis of hemicellulose, chemical synthesis, and biotransformation. However, certain problems exist in those methods.

1) Acid hydrolysis of corn bran, sugar cane pulp and other high L-arabinose content biomass: this method includes alkaline extraction and acid hydrolysis of Arabinoxylan. it is the most mature method with relative high yields, however obvious drawbacks also exists: ① harmful substances generates in high alkaline concentration and extraction temperature environment; ② a lot of salts generated in neutralization process makes subsequent crystallization process more complex, consumes more energy and time leading to higher production costs; ③ large-scale use of strong base and acid not only cause corrosion of equipment, but also has the potential of polluting the environment since a large amount of industrial waste is produced.

2) Acid hydrolysis of Arabic gum: the mixture of L-arabinose, xylose, rhamnose, galactose and other monosaccharides is obtained by acid hydrolysis of Gum Arabic. After the mixture is neutralized, yeast fermentation is used to remove fermented carbohydrates (such as galactose and glucose). It was then crystallized in methanol to give crude L-arabinose which was subsequently recrystallized from 90% ethanol to give pure L-arabinose crystal that approximately have a yield of 15.6% and 97.5% purity.

This way also exists some drawbacks: ① large-scale use of acid not only cause corrosion of equipment, but also has the potential of polluting the environment since a large amount of salts is produced. ② Yeast fermentation process is the most time and energy consuming step in the whole process as sterilization (consumes large amount of energy), yeast growth and consumes fermented carbohydrates (needs more than 24 h) is required, leading to lower productivity and higher production costs. ③ It is costly to use gum arabic to produce L-arabinose since gum arabic itself is in low production and high price.

3) Enzyme hydrolysis of high L-arabinose content biomass: this method effectively integrates the above two methods and avoids its’ disadvantages. It is a more practical, environmental protection and efficient way. This method uses cellulase and arabinoxylanse to hydrolyze high L-arabinose content biomass and the hydrolysate is then fermented by yeast to get rid of fermented carbohydrates. The method has the advantages of mild reaction conditions, strong specificity, and simple purification steps. The problem with this method is that the activity of cellulase or arabinoxylanase is not high and the hydrolysis time is relatively long, and the price of cellulase or arabinoxylanase is too high compared to hydrochloric acid and sulfuric acid, resulting in higher investment cost.

4) Chemical or biological synthesis of L-arabinose: L-arabinose can also be prepared by chemical synthesis or biotransformation. For example, through a series of addition reaction, ring opening, dehydrogenation and other steps, L-arabinose is obtained from dihydropyran. L-arabinose can also be obtained from L-ribose catalyzed by L-arabinose isomerase, while the L-ribose is obtained from L-ribitol catalyzed by acetobacter. A variety of optical isomers is produced no matter by chemical synthesis or biotransformation resulting in low production efficiency and high production costs, this is because separation and purification of optical isomers remains the bottleneck of global industrial crystallization separation.

Compared with the other functional sugar, the representative physiological activity of L-arabinose is that it specifically inhibits intestine sucrase activity, thereby inhibiting the hydrolysis and sbsorption of sucrose, and reducing the magnitude of hyperglycemia. G. Semenza [19] found that the rabbit intestinal sucrase can be non-competitive inhibited by L-arabinose in both 10 mM and 50 mM concentration, with the inhibition rate 65% and 80%, respectively.

Researches revealed that only L-arabinose can non-competitive inhibit intestinal mucosa sucrase activity, while the D-arabinose or L-arabinbiose had no such activity, and the binding constant of L-arabinose with sucrase is 12 times that of sucrose with sucrase. At the same time, L-Arabinose has no significant inhibition effect on pancreas amylase, intestinal mucosa maltase, isomaltase, trehalase, lactase [2] [20] [21].

L-arabinose has hypolipidemic and hypoglycemic effect which can prevent effectively from some metabolic diseases, such as obesity, diabetes. The intervention experiment of L-arabinose on high fat and high fat feeding rats and mice shows that L-arabinose can significantly reduce the fasting blood glucose, enhance glucose tolerance, and can effectively slow the growth of weight, compared with the control group [22] [23].

Han [24] confirmed that L-arabinose can reduce the fat index, inhibit and eliminate the accumulation of fat, thus to achieve the effect of weight loss on high fat and high fat feeding rabbits. Liu [3] and Shibanuma [25] tested the effects of L-arabinose on adults postprandial glucose levels and body weight index. Results shown that 1) the postprandial glycemic index of the experimental group supplemented with ≥3% of L-arabinose were significantly lower than that of the control group after 30 days in the postprandial blood glucose test; 2) compared with the normal diet obese group, the intervention group supplemented with ≥ 5% of L-arabinose (20 obese subjects) average decreased 5.5 kg after 6 months, indicating that L-arabinose significantly reduce postprandial glycemic index and has a slimming effect.

Research shows that the L-arabinose content in the various dietary fiber residue decreases significantly when it go through rat or human digestive tract, in where L-arabinose is dissolved and absorbed as a monosaccharide or oligosaccharide and then used by intestinal bacteria [26].

This is because L-arabinose residue is mainly present at the non-reducing end of the side chain (Figure 2(a)), which easily hydrolysis to free sugars in the acid environment of the stomach and then implement the functional characteristics of inhibition of blood glucose elevation, reduction of serum neutral fat and cholesterol concentration, proliferation of intestinal bifidobacteria, and so on. Studies show that L-arabinose promotes the growth of bifidobacteria thereby enhancing the body’s immune system and control weight gain in mice [27]. To the other hand, L-arabinose itself is difficult to be absorbed and utilized to generate energy in the body and the non-utilized part can be excreted from the urine so it will not lead to obesity.

Skeletal muscle is the place using sugar and insulin, change of skeletal muscle composition affects the insulin sensitivity. Research shows that the number of type I muscle fibers increases significantly while type II muscle fibers decreases after long-term utilization of L-arabinose, so taking L-arabinose can ameliorate type 2 diabetes mellitus symptoms since type I muscle fibers effectively improve insulin resistance [23] [28] [29].