Ethanol and phenylacetylcarbinol production processes of Candida tropicalis TISTR 5306 and Saccharomyces cerevisiae TISTR 5606 in fresh juices from longan fruit of various sizes

Abstract

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).

Batch and continuous cultivation processes of Candida tropicalis TISTR 5306 for ethanol and pyruvate decarboxylase production in fresh longan juice with optimal carbon to nitrogen molar ratio

“…The comparison between different C/N molar ratios from the current and previous studies (Nunta et al, ) allowed the investigation of specific sugars and AMS composition spanning five different ratios in the range of 3.88 ± 0.04 to 39.07 ± 0.54 as indicated in Table . Although FLJ‐C from Nunta et al () had the similar level of sucrose composition to the current study (25–26 mg/g fresh longan fruit for molar C/N ratio at or above 9.88 ± 0.14), the specific total sugar composition was relatively higher (51.6 ± 0.7 instead of 38.0–40.4 mg /g fresh longan fruit).…”

“…The comparison between different C/N molar ratios from the current and previous studies (Nunta et al, ) allowed the investigation of specific sugars and AMS composition spanning five different ratios in the range of 3.88 ± 0.04 to 39.07 ± 0.54 as indicated in Table . Although FLJ‐C from Nunta et al () had the similar level of sucrose composition to the current study (25–26 mg/g fresh longan fruit for molar C/N ratio at or above 9.88 ± 0.14), the specific total sugar composition was relatively higher (51.6 ± 0.7 instead of 38.0–40.4 mg /g fresh longan fruit). The presence of relatively high specific glucose (13.9 ± 0.5 instead of 7.38–7.92 mg/g fresh longan fruit) and fructose (11.9 ± 0.3 instead of 6.73–7.23 mg/g fresh longan fruit) compositions contributed to this observation which illustrated variation of individual and total sugar compositions among different batches of FLJ‐C.…”

“…An example calculation for the average C/N molar ratio of 21.88: mol C per g fresh longan fruit = (26.0 mg sucrose /MW sucrose = 342 g/mol) × 12 C‐mol in 1 mol of sucrose + (7.59 mg glucose /MW glucose = 180 g/mol) × 6 C‐mol in 1 mol of glucose + (6.88 mg fructose /MW fructose = 180 g/mol) × 6 C‐mol in fructose = 1.39 m C‐mol in 1 g of FLJ‐C; mol N from added AMS per g fresh longan fruit = (4.20 mg AMS /MW AMS = 132 g/mol) × 2 N‐mol in 1 mol of AMS = 0.06353 m N‐mol in 1 g of FLJ‐C, the original N content in FLJ‐C prior to AMS addition was not included as the results from Nunta et al () indicated negligible content; C/N was thus = 1.39 m C‐mol/0.06353 m N‐mol = 21.88 with calculation of corresponding propagation errors as described by Skoog et al () resulting in the value of 0.20.…”

“…Nitrogen source could be either organic (such as yeast extract, tryptone, and peptone) or inorganic (e.g., ammonium sulfate [AMS], ammonium nitrate, and ammonium chloride) forms (Kalil, Alshiyab, & Yusoff, ). Our previous study indicated that 1 kg of C‐grade fresh longan fruit contained 25.8 ± 0.3 g sucrose, 13.9 ± 0.5 g glucose, 11.9 ± 0.3 g fructose, 51.6 ± 0.3 g total sugar, and only 0.021 ± 0.004 g of nitrogen (Nunta et al, ). This corresponded to the very high carbon to nitrogen (C/N) molar ratio of 1,177 ± 225, an indication of nitrogen shortage.…”

“…The subsequent addition of an inorganic nitrogen source in the form of AMS to FLJ‐C could bring the C/N molar ratio down to 39.07 ± 0.48 or the decrease by 30.12 ± 5.77 times in relation to nonsupplemented FLJ‐C. The cultivation of C. tropicalis TISTR 5306 in this medium could produce 12.9 ± 0.5 g/L dried biomass and 22.3 ± 1.1 g/L ethanol (Nunta et al, ). The cessation in cells growth might be observed in the situation where C/N molar ratio was too low due to the generation of extraneous toxic ammonia (Kalil et al, ; Smith & Holtzapple, ).…”