Screening of goaty flavor-inhibiting lactic acid bacteria and their effects on the flavor profiles of goat milk cakes

The goaty flavor of traditional fermented Yunnan goat milk cake can be reduced by fermentation,but the bacteria species related to such functions have not yet been explored.This study therefore screened lactic acid bacteria(LAB)for their ability to inhibit goaty flavor in goat milk cake and sour juice samples.Three strains capable of inhibiting goaty flavor were identified as Lactococcus lactis subsp.lactis SD-3,Pediococcus lactis DN-1,and Lactiplantibacillus plantarum GM-6.Goat milk cake was then fermented using orthogonally designed mixed strains.The optimum inoculums of strains were determined based on orthogonal experiment,sensory evaluation score and gas chromatography-mass spectrometry(GC-MS)analysis,which were 6.7×10^(6),1.0×10^(7) and 2.0×10^(7) CFU/mL for strain SD-3,DN-1 and GM-6,respectively.The GC-MS analysis of samples fermented with such artificial adjunct starter combination confirmed that free fatty acids(FFA)with goaty flavor decreased.These included propionic acid,valeric acid,hexanoic acid,heptanoic acid,octanoic acid,and decanoic acid.In contrast,the content of aroma compounds with goaty flavor-inhibition effects increased,such as benzaldehyde,ethyl butyrate and hexanal.This study provides a reference for screening goaty flavor-inhibiting LAB and improving the flavor quality of goat dairy products by using artificial adjunct starters.

Cloning,purification,and characterization of branched-chain α-keto acid decarboxylases from Lactococcus lactis strains with different 3-methylbutanal production abilities

Lactococcus lactis is an important food-grade microorganism that has been successfully applied as a starter to increase the level of 3-methylbutanal produced during the ripening of cheese.Three variants of branched-chain α-keto acid decarboxylase (KADC) were discovered in L.lactis strains with different 3-methylbutanal production abilities.Three genes encoding KADCs of varying lengths (KADC-long,KADC-middle,and KADC-short) were cloned and heterologously expressed into Escherichia coli.KADC activity was only detected in the E.coli cloned with the KADC-long-encoding gene.Homology modeling of the three KADC recombination proteins showed that an active-site residue (Glu462) and an S-pocket structure were necessary for the ability to catalyze substrates.KADC-long showed maximum activity at pH 7.0 and 30 ℃.The substrate hydrolysis and kinetic parameters demonstrated that KADC-long efficiently produces 2-methylbutanal and 3-methylbutanal.The heterologous expression of the full-length kdcA in low-3-methylbutanal-yield L.lactis strains increased their production yields.The results of this study demonstrate the function of the complete KADC in 3-methylbutanal production.

Improvement of stability and in vitro bioaccessibility of nervonic acid by nonionic surfactant in protein-based nanoemulsions

Nervonic acid(NA,ω-9)is beneficial for brain health,but its poor water solubility limits its bioaccessibility.The protein-based emulsions were unstable under environmental stress.In this study,the effects of a nonionic surfactant(Tween 20,T20)on the characteristics,stability,and in vitro digestion properties of NA-loaded nanoemulsions prepared with proteins(whey or soybean protein isolate)were evaluated.The emulsions stabilized by protein-T20 mixtures had smaller droplets with more even distributions,more negative charges,and higher encapsulation efficiencies for NA.The addition of T20 improved the interfacial properties of the systems,as confirmed by spectral and rheological analyses.Emulsion stability under various conditions was enhanced by the improved interfacial composition and emulsifying ability.In particular,the effective interfacial layers produced by the protein-T20 mixture decreased lipid digestion but increased NA bioaccessibility.Therefore,the coadsorption of proteins with suitable nonionic surfactants at oil-water interfaces is a promising approach for designing functional food delivery systems with improved stability and NA bioaccessibility.

Effect of microwave radiation combined with cellulase treatment of soybean residue on the culture of Aspergillus oryzae

Soybean residue (SR) is a by-product of bean product processing with potential usefulness as a fermentation medium. However, the fermentation efficiency of SR is limited due to its low reducing sugar content. In this study, a method of microwave radiation combined with cellulase hydrolysis (MCH) was developed to increase the reducing sugar content of SR, and the effect of treated SR on fermentation by Aspergillus oryzae was evaluated. First, the process parameters, including the reaction time, pH and temperature, were optimized. Second, the effect of treated SR on fermentation by A. oryzae was evaluated, including changes in cell proliferation and protease viability. The results showed that the MCH method capitalized on the advantages of microwave radiation and cellulase and promoted the enzymatic hydrolysis of fibrous polysaccharides in the crystallization zone of SR. The reducing sugar content of SR significantly increased from 0.42 to 0.98 mg/mL after MCH treatment. In addition, treated SR effectively promoted the proliferation of A. oryzae , and the protease activity reached 111 U/mL after 3 days of fermentation. The results of this study suggested that the MCH method can effectively increase the reducing sugar content of SR to expand its use as a fermentation medium.