Pin-Rou Lee

Profile of volatile compounds during papaya juice fermentation by a mixed culture of Saccharomyces cerevisiae and Williopsis saturnus

This study investigated the formation and utilization of volatile compounds during papaya juice fermentation by a mixed culture of Saccharomyces cerevisiae and Williopsis saturnus. Time-course papaya juice fermentations were carried out using pure cultures of S. cerevisiae var. bayanus R2 and W. saturnus var. mrakii NCYC2251 and a mixed culture of the two yeasts at a ratio of 1:1000 (R2:NCYC2251). Changes in S. cerevisiae cell population, Brix, sugar consumption and pH were similar in the mixed culture and in the S. cerevisiae monoculture. There was an early growth arrest of W. saturnus in the mixed culture fermentation. A range of volatile compounds were produced during fermentation including fatty acids, alcohols, aldehydes and esters and some volatile compounds including those initially present in the juice were utilized. The mixed culture fermentation of S. cerevisiae and W. saturnus benefited from the presence of both yeasts, with more esters being produced than the S. cerevisiae monoculture and more alcohols being formed than the W. saturnus monoculture. The study suggests that papaya juice fermentation with a mixed culture of S. cerevisiae and W. saturnus may be able to result in the formation of more complex aroma compounds and higher ethanol level than those using single yeasts.

Impact of amino acid addition on aroma compounds in papaya wine fermented with Williopsis mrakii.

The impact of amino acid addition on aroma compound formation in papaya wine fermented with yeast Williopsis saturnus var. mrakii NCYC2251 was studied. Time-course papaya juice fermentations were carried out using W. saturnus var. mrakii NCYC2251, with and without the addition of selected amino acids (L-leucine, L-isoleucine, L-valine and L-phenylalanine). Yeast growth and changes in sugars, °Brix, organic acids and pH were similar, regardless of amino acid addition. L-Leucine addition increased the production of isoamyl alcohol and some esters such as isoamyl acetate, isoamyl butyrate and isoamyl propionate, while L-isoleucine addition increased the production of active amyl alcohol and active amyl acetate. L-valine addition slightly increased the production of isobutyl alcohol and isobutyl acetate. L-phenylalanine addition increased the formation of 2-phenylethanol, 2-phenylethyl acetate and 2-phenylethyl butyrate, while decreasing the production of most other esters. This study suggests that papaya juice fermentation with W. saturnus var. mrakii NCYC2251 in conjunction with the addition of selected amino acid(s) can be an effective way to modulate the aroma of papaya wine.

Effects of Pure and Mixed-Cultures of Saccharomyces cerevisiae and Williopsis saturnus on the Volatile Profiles of Grape Wine

Non-Saccharomyces yeasts are being recognised for their ability to produce wines with unique character. This study assessed the production of volatiles by co-inoculating Saccharomyces cerevisiae and Williopsis saturnus. Laboratory-scale fermentations were carried out using pure and mixed-cultures of S. cerevisiae and W. saturnus at a ratio of 1:1000. The mixed-culture fermentation was dominated by S. cerevisiae, while W. saturnus had an early growth arrest. Changes of oenological parameters and volatiles were similar in both the mixed- and the S. cerevisiae monocultures. A range of volatiles was formed with alcohols and esters constituting the majority of volatiles produced. Volatiles initially present in the grape juice, particularly (E)-2-hexen-1-ol, 1-hexanol, trans-2-hexenal and n-hexanal, were metabolized. Wines produced using mixed-cultures closely resembled those fermented by the S. cerevisiae monoculture but acquired minor flavor characteristics from the initial presence of W. saturnus. These findings suggest that the ratio of S. cerevisiae and W. saturnus is critical to the survival of the non-Saccharomyces yeast and the impact on the perceivable characteristics of the resultant quality and flavor of wines.

Yeast ratio is a critical factor for sequential fermentation of papaya wine by Williopsis saturnus and Saccharomyces cerevisiae

The growth kinetics and fermentation performance of Williopsis saturnus and Saccharomyces cerevisiae at ratios of 10:1, 1:1 and 1:10 (W.:S.) were studied in papaya juice with initial 7‐day fermentation by W. saturnus, followed by S. cerevisiae. The growth kinetics of W. saturnus were similar at all ratios, but its maximum cell count decreased as the proportion of S. cerevisiae was increased. Conversely, there was an early death of S. cerevisiae at the ratio of 10:1. Williopsis saturnus was the dominant yeast at 10:1 ratio that produced papaya wine with elevated concentrations of acetate esters. On the other hand, 1:1 and 1:10 ratios allowed the coexistence of both yeasts which enabled the flavour‐enhancing potential of W. saturnus as well as the ethyl ester and alcohol‐producing abilities of S. cerevisiae. In particular, 1:1 and 1:10 ratios resulted in production of more ethyl esters, alcohols and 2‐phenylethyl acetate. However, the persistence of both yeasts at 1:1 and 1:10 ratios led to formation of high levels of acetic acid. The findings suggest that yeast ratio is a critical factor for sequential fermentation of papaya wine by W. saturnus and S. cerevisiae as a strategy to modulate papaya wine flavour.

Sugars, organic acids, and phenolic acids of exotic seasonable tropical fruits

Purpose – The purpose of this study was to characterise the physiochemical properties of selected exotic seasonal tropical fruits available in Singapore.Design/methodology/approach – A total of 11 seasonal tropical fruits including cempedak, chiku, custard apple, jackfruit, longkong, mangosteen, red jambu, pearl jambu, rambutan, salak and starfruit were analyzed for their sugars, organic acids and free phenolic acids composition using liquid chromatographic methods. Total phenolic content (free and bound), total soluble solids and pH were determined using Folin‐Ciocalteu method, refractometer and pH meter, respectively.Findings – Fructose, glucose and sucrose were the main sugars in all the fruits. Cempedak had the highest sucrose concentration, while custard apple had the highest content of fructose and glucose. Malic and citric acids were the major organic acids in most of the fruits, except for pearl jambu and red jambu where succinic acid was the dominant acid. The total phenolic content varied from 1...

Kinetics of volatile organic compounds during papaya juice fermentation by three commercial wine yeasts

Purpose – The purpose of this paper is to assess the potential of papaya juice fermentation and to evaluate the kinetic changes of volatile organic compounds (VOCs) produced during fermentation by three commercial wine yeasts.Design/methodology/approach – Laboratory‐scale fermentations were carried out in papaya juice using three commercial wine yeasts Saccharomyces cerevisiae var. bayanus strains EC‐1118 and R2, and S. cerevisiae MERIT.ferm. Brix, pH, optical density and yeast cell count were determined during fermentation. VOCs were analysed by headspace‐solid phase microextraction with gas chromatography‐mass spectrometry‐flame ionization detector (HS‐SPME‐GC‐MS/FID).Findings – During fermentation, the three wine yeasts grew actively and showed similar growth patterns. Changes in Brix and pH were similar among the three yeasts. A range of VOCs were produced during fermentation including fatty acids, alcohols, acetaldehyde, esters and acetoin. Esters were the most abundant VOCs produced. Some VOCs indig...