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Fresh juice from C grade longan (mg/g fresh fruit; total sugars 51.6 ± 0.5, nitrogen 0.021 ± 0.004; gallic acid 0.025 ± 0.001; ellagic acid 0.016 ± 0.001) with addition of 8.52 g ammonium sulfate/l was the optimal medium for cultivation processes of Saccharomyces cerevisiae TISTR 5606 and Candida tropicalis TISTR 5306 capable of producing ethanol and phenylacetylcarbinol (PAC) among five longan grades. S. cerevisiae TISTR 5606 produced ethanol (33.4 ± 3.2 g/L) at a significantly higher level (p ≤ 0.05) than C. tropicalis TISTR 5306 (22.3 ± 1.1 g/L) with similar ethanol yields (Yp/s) between 0.21 and 0.22 g ethanol produced/g sugars consumed. Whole cells of C. tropicalis TISTR 5306 produced a significantly higher (p ≤ 0.05) PAC level (27.2 ± 0.7 mM) than S. cerevisiae TISTR 5606 (3.59 ± 1.33 mM) after 6 hr in an equivalent volume biphasic biotransformation system. Practical applications Longan is one of the important economic fruit of Thailand with production volume in 2017 reaching one million tons. Less than 10% of these are domestically consumed while more than 90% are exported to several countries whose population frequently consume longan as a nutritious food supplement. This study can help solving overproduction problem of longan fruit by processing C grade fresh longan, which is accounted for 5% of the overall production volume, in the form of juice extract with the relatively high sugars content. Microbial fermentation of this juice and subsequent whole cells biotransformation process result in ethanol and PAC, respectively. Ethanol can be used as an alternative biofuel or important industrial solvent while PAC is a precursor for production of commercial nasal decongestant (ephedrine) or anti‐asthmatic compound (pseudoephedrine).

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Viral infections may cause serious human diseases. For instance, the recent appearance of the novel virus, SARS-CoV-2, causing COVID-19, has spread globally and is a serious public health concern. The consumption of healthy, proper, functional, and nutrient-rich foods has an important role in enhancing an individual's immune system and preventing viral infections. Several polysaccharides from natural sources such as algae, bacteria, and fungi have been considered as generally recognized as safe (GRAS) by the US Food and Drug Administration. They are safe, low-toxicity, biodegradable, and have biological activities. In this review, the bioactive polysaccharides derived from various microorganisms, including bacteria, fungi, and algae were evaluated. Antiviral mechanisms of these polysaccharides were discussed. Finally, the potential use of microbial and algal polysaccharides as an antiviral and immune boosting strategy was addressed. The microbial polysaccharides exhibited several bioactivities, including antioxidant, anti-inflammatory, antimicrobial, antitumor, and immunomodulatory activities. Some microbes are able to produce sulfated polysaccharides, which are well-known to exert a board spectrum of biological activities, especially antiviral properties. Microbial polysaccharide can inhibit various viruses using different mechanisms. Furthermore, these microbial polysaccharides are also able to modulate immune responses to prevent and/or inhibit virus infections. There are many molecular factors influencing their bioactivities, e.g., functional groups, conformations, compositions, and molecular weight. At this stage of development, microbial polysaccharides will be used as adjuvants, nutrient supplements, and for drug delivery to prevent several virus infections, especially SARS-CoV-2 infection.

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