Galactooligosaccharides (GOS) are documented prebiotic compounds, but knowledge of the metabolic and regulatory mechanisms of GOS utilization by lactic acid bacteria is still limited. Here we used transcriptome and physiological analyses to investigate the differences in the logarithmic growth phase of Lactobacillus plantarum and L. plantarum ΔccpA metabolizing GOS or glucose as the sole source of carbohydrate. In total, 489 genes (16%) were differentially transcribed in the wild-type L. plantarum grown on glucose and GOS and the value is decreased to 7% due to the loss of ccpA . Only 6% genes were differentially expressed when the wild-type and the ccpA mutant were compared on GOS. Transcriptome data revealed that the carbon sources significantly affected the expression of several genes, and some of the genes were mediated by CcpA. In particular, lac and gal gene clusters resembled the corresponding clusters in L. acidophilus NCFM that are involved in GOS metabolism, indicating that these clusters may be participating in GOS utilization. Moreover, reverse transcription-PCR analysis showed that GOS-related gene clusters were organized in five independent polycistronic units. In addition, many commonalities were found between fructooligosaccharides and GOS metabolism in L. plantarum , including differentially expressed genes involved in oligosaccharide metabolism, conversion of metabolites, and changes in fatty acid biosynthesis. Overall, our findings provide new information on gene transcription and the metabolic mechanism associated with GOS utilization, and confirm that CcpA plays an important role in carbon metabolism regulation in L. plantarum .

To evaluate the contributions of 3-methylbutanal, 2-methylbutanal, 2-methylpropanal, and benzaldehyde in cheddar cheese models, the threshold values, optimal concentration ranges, and perceptual actions of these compounds were determined at various concentrations. The thresholds for 3-methylbutanal, 2-methylbutanal, 2-methylpropanal, and benzaldehyde in the cheese matrix were 150.31, 175.39, 150.66, and 500.21 μg/kg, respectively, which were significantly higher than the corresponding values in water. The optimal concentration ranges of these aldehydes were determined as 150–300, 175–325, 150–350, and 500–1500 μg/kg, respectively. Based on the results of the threshold method and Feller’s model, five binary mixtures were found to have synergistic effects, and only the pair of 2-methylpropanal and benzaldehyde was determined to have a masking effect. In addition, the synergistic olfactory effects between the four ternary mixtures and the quaternary mixture of these aldehydes were also assesSsed using Feller’s model. In a σ-τ plot analysis, synergism was usually observed when these odor pairs were at their threshold levels. In summary, the results suggested that perceptual interactions among these aldehydes exist in a cheese model variably with different concentrations and threshold ratios. This study will be helpful to a further understanding of the nutty aroma and improving the aroma quality of cheddar cheese.

Aging plays an important role in the formation of aroma characteristics of Huangjiu, a traditional Chinese alcoholic beverage. Comprehensive two‐dimensional gas chromatography mass spectrometry (GC×GC‐qMS)‐based untargeted metabolomics combined with a multivariate analysis was used to investigate the dynamic variations in the aroma profile of Huangjiu during aging process and to establish the relationship between the changing volatile metabolite profiles and the age‐dependent sensory attributes. A total of 144 volatile metabolites were identified by GC×GC‐qMS and 63 were selected as critical metabolites based on variable importance in projection values and p‐values. Based on the results of principal component analysis, orthogonal partial least‐squares discriminant analysis, and hierarchical clustering analysis, the samples of six different ages were divided into three groups: 1Y and 3Y samples, 5Y and 8Y samples, and 10Y and 15Y samples. The partial least‐squares analysis results further revealed the relationship between the aromas attributes and variations of these volatile compounds. The high esters, aldehydes, and lactones contents contributed to the high intensities of the sweet and ester aroma attributes of the aged Huangjiu, while the high alcohols and ethyl esters contents contributed to the alcoholic and fruity aroma attributes of the newly brewed Huangjiu. These results improve our understanding of the chemical nature of the aroma characteristics of aged Huangjiu. Practical Application Huangjiu is often labeled with its age as a measure of quality, which influences consumers’ choice. Dynamic variations in volatile compounds of Huangjiu during aging and its contribution to the aroma characteristics of Huangjiu were figured out, which will assist the industry to produce better quality aged Huangjiu for consumers.

Excessive and uncoordinated bitterness of Shaoxing Huangjiu, a traditional Chinese rice wine, reduces its acceptance by consumers. To determine the compounds responsible for this bitterness, gas chromatography−mass spectrometry and highperformance liquid chromatography were performed on four types of Shaoxing Huangjiu (Yuanhong, Huadiao, Shanniang, and Xiangxue wine) for targeted quantitation of candidate compounds known to contribute to bitterness. Calculations of dose-overthreshold factors revealed that isoamyl alcohol, 1-hexanol, phenylethanol, ethyl butyrate, ethyl lactate, furfural, histidine, and arginine were important bitter compounds. Taste recombination experiments demonstrated that a recombination model constructed using the screened known bitter compounds showed good similarity with the original sample in bitter taste. Furthermore, omission experiments revealed that isobutanol, isoamyl alcohol, 1-hexanol, phenylethanol, ethyl acetate, ethyl butyrate, ethyl lactate, furfural, arginine, and valine were the compounds affecting the bitter taste perception. This study provides a certain guiding effect on the bitterness control and taste improvement of Shaoxing Huangjiu.

Diacetyl and acetoin are key aroma components of fermented milk but are produced in low concentrations by starter cultures. In this study, we expressed NADH oxidase, acetolactate synthase, and inactivated acetolactate decarboxylase in Lacticaseibacillus casei TCS to generate recombinant L. casei strains, and investigated the effects of the genes encoding these enzymes on diacetyl and acetoin production during milk fermentation. In the single-gene recombinant strains tested, diacetyl concentrations were highest in milk fermented by L. casei TCSI-nox (nox gene overexpressed, 3.68 mg/kg), whereas acetoin concentrations were highest in milk fermented by L. casei TCS-∆alsD (alsD gene deleted, 32.94 mg/kg). Moreover, diacetyl and acetoin concentrations were higher in the inducible strains than in the corresponding constitutive strains (e.g., TCSInox vs. TCSC-nox, and TCSI-∆alsD-nox vs. TCSC-∆alsD-nox). This phenomenon was also reflected in the protein expression levels and enzyme activities. In the double-gene recombinant strains tested, the highest concentrations of diacetyl and acetoin were produced by L. casei TCSI-∆alsD-nox (nox overexpressed and alsD deleted, 4.66 mg/kg, 69.62 mg/kg, respectively). The triple-gene recombinant L. casei TCS-∆alsD-nox-alsS produced the highest concentrations of diacetyl and acetoin, which were 2.38 and 11.19 times, respectively, the concentrations produced by the original strain. These results show that the nox, alsS, and alsD genes make key contributions to the biosynthesis of diacetyl and acetoin by L. casei. The modification of multiple genes had a synergistic effect, leading to greatly in-creased synthesis of diacetyl and acetoin by L. casei during its fermentation of milk.