Characteristics of proteolytic microorganisms and their effects on proteolysis in total mixed ration silages of soybean curd residue

Objective

The objective of this study was to isolate proteolytic microorganisms and evaluate their effects on proteolysis in total mixed ration (TMR) silages of soybean curd residue.

Methods

TMRs were formulated with soybean curd residue, alfalfa or Leymus chinensis hay, corn meal, soybean meal, a vitamin-mineral supplement, and salt in a ratio of 25.0:40.0:30.0:4.0:0.5:0.5, respectively, on a basis of dry matter (DM). The microbial proteinases during ensiling were characterized, the dominate strains associated with proteolysis were identified, and their enzymatic characterization were evaluated in alfalfa (A-TMR) and Leymus chinensis (L-TMR) TMR silages containing soybean curd residue.

Results

Both A-TMR and L-TMR silages were well preserved, with low pH and high lactic acid concentrations. The aerobic bacteria and yeast counts in both TMR silages decreased to about 105 cfu g-1 FM (Fresh matter) and below the detection limit, respectively. The lactic acid bacteria count increased to 109 cfu g-1 FM. The total microbial proteinases activities reached their maximums during the early ensiling stage and then reduced in both TMR silages with fermentation prolonged. Metalloproteinase was the main proteinase when the total proteinases activities reached their maximums, and when ensiling terminated, metallo and serine proteinases played equally important parts in proteolysis in both TMR silages. Strains in the genera Curtobacterium and Paenibacillus were identified as the most dominant proteolytic bacteria in A-TMR and L-TMR, respectively, and both their proteinases were mainly with metalloproteinase characteristics. In the latter ensiling phase, Enterococcus faecium strains became the major sources of proteolytic enzymes in both TMR silages. Their proteinases were mainly of metallo and serine proteinases classes in this experiment.

Conclusion

Proteolytic aerobic bacteria were substituted by proteolytic lactic acid bacteria during ensiling, and the microbial serine and metallo proteinases in these strains played leading roles in proteolysis in TMR silages.

Optimisation of steam blanching on enzymatic activity, color and protein degradation of alfalfa (Medicago sativa) to improve some quality characteristics of its edible protein

The use of alfalfa protein in human food is limited by its low quality. Response Surface Methodology was employed to optimise the combined effects of different steam blanching conditions on the enzymatic activity, browning and protein degrading which cause undesirable characteristics. The optimum conditions were: steaming time 4.36 min, particle size 23 mm, time from harvesting to steaming 2 h leading to a residual activity of polyphenol oxidase of 1.31% and a completely inactivation of peroxidase. The Browning Index value was 108.3 and the non-protein nitrogen 170.2 (g kg^-1 TN). The browning and protein degradation rates of alfalfa treated under the optimum conditions were much lower than the control alfalfa after 60 days ensiling. This suggests that blanching of fresh whole alfalfa leaves under the optimum conditions was helpful for avoiding the appearance of the dark color and degradation of the extracted protein, improving its quality for human consumption.

Effects of four short-chain fatty acids or salts on the dynamics of nitrogen transformations and intrinsic protease activity of alfalfa silage

BACKGROUND

Short-chain fatty salts have been widely used as food and forage preservatives because of their antimicrobial properties. This study evaluated the effects of four chemical compounds with antimicrobial properties on nitrogen transformations and intrinsic protease activity of alfalfa silage.

RESULTS

Potassium diformate (PD) and formic acid (FA) rapidly reduced silage pH. Silages treated with sodium diacetate (SD) and calcium propionate (CAP) had higher final peptide N concentrations than other silage. The free amino acid N contents in PD and FA treated silages were lower than other silages at all intervals of ensilage. The ammonia N concentrations in FA and PD silages were the lowest, followed by SD and CAP silages. As ensiling progressed, the aminopeptidase activity was completely lost by day 5 for FA and PD silages and inactive by day 7 for SD silage, while it remained active after day 7 for control and CAP silage. The carboxypeptidase activities in FA and PD silages were already reduced below 50% by day 1 of ensiling.

CONCLUSION

Potassium diformate was as effective as formic acid in depressing the proteolysis, while sodium diacetate and calcium propionate were inferior to formic acid in protecting alfalfa proteins from being hydrolysed. © 2016 Society of Chemical Industry.

Effects of different source additives and wilt conditions on the pH value, aerobic stability, and carbohydrate and protein fractions of alfalfa silage

To improve the silage quality and reduce the silage additive cost, the present experiment was designed to evaluate the potential of applying the fermented juice of epiphytic lactic acid bacteria (FJLB) as an additive in alfalfa silage. The effects of FJLB on the fermentation quality, carbohydrate and protein fractions, and aerobic stability of alfalfa silage wilted under five different conditions were investigated and compared with commercial lactic acid bacteria (CLAB) and the control. The FJLB application decreased the pH value, the volatile fatty acids and non-protein nitrogen content, and the loss of sugar by 9.9%, 22.9%, 19.6% and 9.6%, respectively; it increased the lactic acid concentration by 29.5% and the aerobic stability by 17 h in comparison to the control. The FJLB application also decreased the pH value (4.44 vs. 4.66) and volatile fatty acid content (38.32 vs. 44.82) and increased the lactic acid concentration (68.99 vs. 63.29) in comparison to the CLAB-treated silage. However, the FJLB treatment had lower aerobic stability (254 h vs. 274 h) than the CLAB treatment. The FJLB application improved silage quality in comparison to the control; in addition, its effect as a fermentation stimulant may be comparable to or even better than CLAB.

Short communication: Characteristics of proteolytic activities of endo- and exopeptidases in alfalfa herbage and their implications for proteolysis in silage

The pH optimum and thermostability of both exopeptidases and endopeptidases were investigated in this study to elucidate the possible role of plant proteases in proteolysis during ensiling of alfalfa herbage. Proteolytic activities of 4 classes of endopeptidases (i.e., serine, metallo, aspartic, and cysteine peptidase) and 5 classes of exopeptidases (i.e., aminopeptidase, carboxypeptidase, dipeptidase, dipeptidyl-peptidase, and tripeptidyl-peptidase) were examined within pH values of 3 to 9, and within temperatures from 20 to 90°C. Serine and metalloproteases, the principal endopeptidases that hydrolyzed most of the protein to nonprotein nitrogen in alfalfa silage, had optimum activities at pH 4. Among the major exopeptidases contributing protein degradation in ensiled alfalfa, dipeptidase and tripeptidyl-peptidase had stable activities between pH 4 and 6, and carboxypeptidase activity was optimal at pH 5. The optimum temperature for most peptidase activities was 40°C. Proteolytic activities of both endo- and exopeptidases increased with the elevation of incubating temperature from 20 to 40°C. The pH value in well-preserved alfalfa silage is often above 4.0, and the temperatures in the ensiled mass range from 25 to 40°C. Therefore, high proteolytic activities between pH 4 and 6 and the temperature range of ensiled alfalfa suggest that plant peptidases play a role in hydrolyzing protein during prolonged storage.