Comparative Transcriptomic Analysis of the Flavor Production Mechanism in Yogurt by Traditional Starter Strains

Synergistic fermentation of milk by Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus is one of the key factors that determines the quality of yogurt. In this study, the mechanism whereby yogurt flavor compounds are produced by mixture of S. thermophilus SIT-20.S and L. delbrueckii ssp. bulgaricus SIT-17.B were investigated by examining these strains' flavor production, growth, and gene transcription. The results showed that yogurt produced by a 10:1 mixture of the aforementioned strains had the highest abundance of acetoin, whereas yogurt produced by a 1:1 mixture had the highest abundance of diacetyl and acetaldehyde. In addition, the growth of S. thermophilus SIT-20.S was enhanced in the 10:1 mixture. Transcriptomic analysis revealed differentially expressed genes in the flavor-compound-related pathways of S. thermophilus SIT-20.S and L. delbrueckii ssp. bulgaricus SIT-17.B in yogurts produced by 10:1 and 1:1 mixture compared with those produced by either strain alone. Mixed fermentations regulated the expression of genes related to glycolysis, resulting in an increase of pyruvate, which is an important precursor for diacetyl and acetoin synthesis. The gene encoding the acetoin reductase (SIT-20S_orf01454) was decreased in S. thermophilus SIT-20.S, which ensured the accumulation of acetoin. Besides, gene encoding the acetaldehyde dehydrogenase (SIT-20S_orf00949) was upregulated in S. thermophilus SIT-20.S, and the expression of alcohol dehydrogenase (SIT-20S_orf01479; SIT-17B_orf00943) was downregulated in both strains, maintaining the abundance of acetaldehyde. In addition, the gene encoding the NADH oxidase (SIT-17B_orf00860) in L. delbrueckii ssp. bulgaricus SIT-17.B were upregulated, which promoted the accumulation of diacetyl and acetoin. In conclusion, we characterized the mechanism by which S. thermophilus and L. delbrueckii ssp. bulgaricus synergistically generated yogurt flavor compounds during their production of yogurt and highlighted the importance of appropriate proportions of fermentation starters for improving the flavor of yogurts.

Olfactory impact of guaiacol, ortho-vanillin, 5-methyl, and 5-formyl-vanillin as byproducts in synthetic vanillin

To better control the quality of synthetic vanillin obtained by using the guaiacol synthesis method, the olfactory impacts of byproducts on the aroma of the synthetic vanillin samples were evaluated and their optimum concentration ranges were determined. Four byproducts (guaiacol, ortho-vanillin, 5-methyl-vanillin, and 5-formyl-vanillin) were identified by gas chromatography-mass spectrometry (GC-MS) and quantified by gas chromatography-flame ionization detection (GC-FID) in the synthetic vanillin samples with different degrees of purity. The aroma intensities (AIs) of the four byproducts obtained by gas chromatography-olfactometry (GC-O) were: guaiacol (AI: 3.5-4.0, smoke), ortho-vanillin (AI: 1.6-2.5, almond), 5-methyl-vanillin (AI: 2.5-3.3, aldehyde), and 5-formyl-vanillin (AI: 3.2-3.8, green). The aroma perceptual interactions of the four byproducts and the vanillin in the synthetic vanillin samples were determined by S-curve analysis. Guaiacol and 5-methyl-vanillin showed synergistic effects by Feller's additive model. Combined with the results of an addition experiment, when the contents of guaiacol, ortho-vanillin, 5-methyl-vanillin, and 5-formyl-vanillin were within 50, 10, 400, and 1,000 mg/kg respectively, the byproducts had no effects on the aroma quality of the synthetic vanillin samples. PRACTICAL APPLICATION: Synthetic vanillin is one of the most commonly used food additives. Currently, the purity of synthetic vanillin can reach 99.9%, but trace byproducts are still present. Continuing to improve the purity of synthetic vanillin will significantly increase its production costs. Therefore, it is necessary to determine whether the presence of these byproducts affects the aroma quality of the synthetic vanillin samples or not. If they have a negative effect on its aroma, it will be important to reduce their content. If they have no influence or positive role, there is no need to control the content of these byproducts to very low levels. This study determined the content of the byproducts produced during the synthesis of vanillin by guaiacol glyoxylic acid method, judged the perceptual interaction between the byproducts and the vanillin in the synthetic vanillin samples, and determined the optimum range within which the byproducts had no effects on the aroma quality. This study provides a theoretical basis for improving the aroma quality of synthetic vanillin while controlling the production costs.

Aroma perceptual interactions of benzaldehyde, furfural, and vanillin and their effects on the descriptor intensities of Huangjiu

Aldehydes are important in the aroma of Huangjiu and contribute the almond and sweet aromas to Huangjiu. The perceptual interactions of 3 important aldehyde compounds were investigated using S-curves. Three volatiles, benzaldehyde, furfural, and vanillin, reduced the "olfactory threshold" of an aromatic reconstitution (AR) from 24.2 mL/L to 11.0, 14.8 and 9.00 mL/L (AR/matrix, mL/L), respectively. Furthermore, synergistic effects were observed in a benzaldehyde and furfural mixture (71:29, m/m) wherein the overall olfactory threshold value was reduced from 17.0 mL/L to 2.60 mL/L. In addition, these compounds could increase or decrease the intensity of sensory descriptors even at subthreshold concentrations, and the profiles confirmed that they had synergistic effects with other compounds in Huangjiu. In particular, when benzaldehyde and furfural were present in the actual concentration ratio determined in Huangjiu (71:29, m/m), the enhancement of the Qu and sweet aromas became significant, which indirectly indicated synergy between them.

Effects of natural ingredients on the shelf life of chicken seasoning

The effects of the natural ingredients Angelica sinensis (AS) and Codonopsis pilosula (CP) on the shelf life of chicken seasoning were investigated. Color differences and sensory evaluation were used to indicate sensory differences. Changes in volatiles were monitored. The rate of increase in the color value a* of the AS and CP samples was lower than that in the control. Rancid flavor appeared later in the AS and CP samples than in the control. The levels of aldehydes, ketones, and alkenes increased during storage. A kinetic model was built based on the proportion of aldehydes (main marker), to predict shelf life. The predicted shelf life at room temperature was 60 days for the control, 114 days for AS, and 89 days for CP. The shelf life of chicken seasoning could be prolonged with AS and CP. This kinetic model can be used to predict the shelf life of chicken seasoning.

Assessment of main factor causing sensory quality defects in chicken seasoning during storage

BACKGROUND

Chicken seasoning is a widely consumed palatable seasoning made with chicken meat. Quality, and especially sensory quality, may determine the consumer choice of food. The same bag of chicken seasoning will be stored by the consumers over a long period of time when it is in use, so it is particularly important to be able to assess its sensory quality. However, the sensory quality defects of chicken seasoning during storage remain unknown. This study evaluated flavor changes in chicken seasoning during storage using sensory evaluation and gas chromatography-mass spectrometry (GC-MS).

RESULTS

The sensory evaluation indicated a perceptible change in rancidity during storage. The GC-MS results showed increases in the content of aldehydes, heterocyclic compounds, ketones, and sulfur compounds associated with lipid oxidation. A random forest model was constructed to predict the storage time based on the data for volatile compounds related to lipid oxidation. The low average predicted error indicated a good correlation between lipid oxidation and storage time.

CONCLUSION

These results suggest that lipid oxidation is the main factor behind sensory quality defects in chicken seasoning during storage. This can be used as the basis for further evaluation of sensory quality and the shelf life of chicken seasoning. © 2018 Society of Chemical Industry.