Purification and characteristics of a new milk-clotting enzyme from Bacillus licheniformis BL312

Invited review: Role of Bacillus licheniformis in the dairy industry- friends or foes?

Bacillus licheniformis is one of the major spore-forming bacteria with great genotypic diversity in raw milk, dairy ingredients, final dairy products, and is found throughout the dairy processing continuum. Though being widely used as a probiotic strain, this species also serves as a potential risk in the dairy industry based on its roles in foodborne illness and dairy spoilage. Biofilm formation of B. licheniformis in combined with the heat resistance of its spores, make it impossible to prevent the presence of B. licheniformis in final dairy products by traditional cleaning and disinfection procedures. Despite the extensive efforts on the identification of B. licheniformis from various dairy samples, no reviews have been reported on both hazard and benefits of this spore-former. This review discusses the prevalence of B. licheniformis from raw milk to commercial dairy products, biofilm formation and spoilage potential of B. licheniformis, and its potential prevention methods. In addition, the potential benefits of B. licheniformis in the dairy industry were also summarized.

Role of Clay Substrate Molecular Interactions in Some Dairy Technology Applications

The use of clay materials in dairy technology requires a multidisciplinary approach that allows correlating clay efficiency in the targeted application to its interactions with milk components. For profitability reasons, natural clays and clay minerals can be used as low-cost and harmless food-compatible materials for improving key processes such as fermentation and coagulation. Under chemical stability conditions, clay materials can act as adsorbents, since anionic clay minerals such as hydrotalcite already showed effectiveness in the continuous removal of lactic acid via in situ anion exchange during fermentation and ex situ regeneration by ozone. Raw and modified bentonites and smectites have also been used as adsorbents in aflatoxin retention and as acidic species in milk acidification and coagulation. Aflatoxins and organophilic milk components, particularly non-charged caseins around their isoelectric points, are expected to display high affinity towards high silica regions on the clay surface. Here, clay interactions with milk components are key factors that govern adsorption and surface physicochemical processes. Knowledge about these interactions and changes in clay behavior according to the pH and chemical composition of the liquid media and, more importantly, clay chemical stability is an essential requirement for understanding process improvements in dairy technology, both upstream and downstream of milk production. The present paper provides a comprehensive review with deep analysis and synthesis of the main findings of studies in this area. This may be greatly useful for mastering milk processing efficiency and envisaging new prospects in dairy technology.

Purification and characteristics of a novel milk-clotting metalloprotease from Bacillus velezensis DB219

Milk-clotting enzyme (MCE) is the essential active agents in dairy processing. The traditional MCE is mainly obtained from animal sources, in which calf rennet is the most widely used in cheese industry. Traditional MCE substitute is becoming necessary due to its limited production and increased cheese consumption. A novel traditional MCE substitute was produced from Bacillus velezensis DB219 in this study. The DB219 MCE exhibited a notable specific activity of 6,110 Soxhlet units/mg and 3.16-fold purification yield with 28.87% recovery through ammonium sulfate fractionation and DEAE-Sepharose Fast Flow. The purified DB219 MCE was a metalloprotease with a molecular weight of 36 kDa. The DB219 MCE was weak acid resistance and stable at pH 6.0 to 10.0 and temperature <45°C. The highest milk-clotting activity was observed in substrate at pH 5.5 added with 20 to 30 mM CaCl2. The Michaelis constant and maximal velocity for casein were 0.31 g/L and 14.22 μmol/min. The DB219 MCE preferred to hydrolyze β-casein instead of α-casein. The DB219 MCE hydrolyzed α-casein, β-casein, and κ-casein to generate significantly different peptides in comparison with calf rennet and ES6023 MCE (fungal MCE) through SDS-PAGE and reversed-phase HPLC analysis. The DB219 MCE mainly cleaved Thr124-Ile125 and Ser104-Phe105 bonds in κ-casein and had unique casein cleavage sites and peptide composition through LC-MS/MS analysis. The DB219 MCE was potential to be a new milk coagulant and enriched kinds of traditional MCE substitute.

Characteristics and application in cheese making of newly isolated milk-clotting enzyme from Bacillus megaterium LY114

Milk-clotting enzyme (MCE) is a crucial active agent in cheese making. It is necessary to find traditional MCE substitutes due to the limited production of traditional MCE (e.g., calf rennet) and increased cheese consumption. Bacillus megaterium strain LY114 with good milk-clotting activity (MCA) (448 SU/mL) and a high MCA/proteolytic activity (PA) ratio (6.0) was isolated and identified from agricultural soil in Laiyang (Shandong, China) through 16S rRNA sequencing of 45 strains. The Bacillus megaterium LY114 MCE had a remarkable specific activity (7532 SU/mg) and displayed a 4.83-fold purification yield with 34.17% recovery through ammonium sulfate fractionation and DEAE-Sepharose Fast Flow. The purified LY114 MCE was a metalloprotease with a molecular weight of 30 kDa. LY114 MCE was stable at pH 5.0-7.0 and temperature <40 °C. The highest MCA appeared at a substrate pH of 5.5 with 30 mM CaCl2. The Michaelis constant (Km) and maximal velocity (Vm) for casein were 0.31 g/L and 14.16 μmol/min, respectively. LY114 MCE preferred to hydrolyze α-casein (α-CN) rather than β-casein (β-CN) and had unique α-CN, β-CN and κ-casein (κ-CN) cleavage sites. LY114 MCE hydrolyzed casein to generate significantly different peptides compared with calf rennet and fungal MCE as determined by SDS-PAGE analysis. Chemical index analysis and sensory evaluation confirmed the usefulness of LY114 MCE in cheese making. LY114 MCE had the potential to be used in dairy processing and enriched traditional MCE substitutes.

The catalytic and kinetic characterization of Bacillus subtilis MK775302 milk clotting enzyme: comparison with calf rennet as a coagulant in white soft cheese manufacture

BACKGROUND

Calf rennet is considered the traditional source of milk clotting enzyme (MCE). However, increasing cheese consumption with decreasing the calf rennet supply had encouraged the quest for new rennet alternatives. The purpose of this study is to acquire more information about the catalytic and kinetic properties of partially purified Bacillus subtilis MK775302 MCE and to assess the role of enzyme in cheese manufacture.

RESULTS

B. subtilis MK775302 MCE was partially purified by 50% acetone precipitation with 5.6-fold purification. The optimum temperature and pH of the partially purified MCE were 70 °C and 5.0, respectively. The activation energy was calculated as 47.7 kJ/mol. The calculated Km and Vmax values were 36 mg/ml and 833 U/ml, respectively. The enzyme retained full activity at NaCl concentration of 2%. Compared to the commercial calf rennet, the ultra-filtrated white soft cheese produced from the partially purified B. subtilis MK775302 MCE exhibited higher total acidity, higher volatile fatty acids, and improved sensorial properties.

CONCLUSIONS

The partially purified MCE obtained in this study is a promising milk coagulant that can replace calf rennet at a commercial scale to produce better-quality cheese with improved texture and flavor.

Advances in research on calf rennet substitutes and their effects on cheese quality

Milk coagulation is an important step in cheese production, and milk-clotting enzymes (MCEs) play a major role in this process. Calf rennet is the most widely used MCE in the cheese industry. The use of calf rennet substitutes is becoming necessary due to the limited availability of calf rennet and the increase in cheese consumption. The objective of this review is to summarize the latest findings on calf rennet substitutes (animal MCEs, plant-derived MCEs, recombinant MCEs and microbial MCEs) and their application in cheese production. Special emphasis has been placed on aspects of the effects of these substitutes on hydrolysis, functional peptides, cheese variety and cheese yield. The advantages and disadvantages of different calf rennet substitutes are discussed, in which microbial MCEs have the advantages of less expensive production, greater biochemical diversity, easier genetic modification, etc. In particular, some of these MCEs have suitable characteristics for cheese production and are considered to be the most potential calf rennet substitutes. Moreover, challenges and future perspectives are presented to provide inspiration for the development of excellent calf rennet substitutes.

Simultaneous Hydrolysis and Extraction Increased Erucin Yield from Broccoli Seeds

Isothiocyanates (ITCs) are well-known chemopreventive agents that have received significant interest across the nutrition and pharmaceutical industries owing to their anticancer properties, thus it is essential to increase the conversion of glucosinolate to ITCs by myrosinase to maximize their health benefits. In this paper, using broccoli seed meals as a raw material, we comparatively analyzed the outcomes of two extraction methods: (i) hydrolysis followed by extraction (HFE) and (ii) simultaneous hydrolysis and extraction (SHE) in terms of the ITC yield. The results revealed that the SHE method showed a relatively greater erucin production from broccoli seeds and greater antitumor and antioxidant activities. A similar phenomenon was found for the hydrolysates of crude myrosinase and crude glucosinolate separated from broccoli seeds. However, when the crude glucosinolates were hydrolyzed by purified broccoli myrosinase, or when pure glucoraphanin was hydrolyzed by crude myrosinase, no significant effects were noted on the types and yields of ITCs between the SHE and HFE methods.

High-Level Expression and Substrate-Binding Region Modification of a Novel BL312 Milk-Clotting Enzyme To Enhance the Ratio of Milk-Clotting Activity to Proteolytic Activity

A novel BL312 milk-clotting enzyme (MCE) exhibited high-level expression and remarkable milk-clotting activity (MCA) (865 ± 20 SU/mL) that was 3.3-fold higher than the control by optimizing induction conditions in recombinant Escherichia. coli harboring pET24a-proMCE. Through substrate-binding region analyses and modification, MCE-G165A was identified from nine mutants and showed a proteolytic activity of 49.4 ± 2.4 U/mL and an MCA/PA ratio of 18.2, which were respectively 1.9-fold lower and 2.0-fold higher than those of the control. The purified MCE-G165A (28 kDa) exhibited weak α_s-casein, β-casein, and strong κ-casein (κ-CN) hydrolysis levels as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and reversed-phase high-performance liquid chromatography. The milk-clotting mechanism for MCE-G165A was the primary hydrolysis of Met106-Ala107 and Asn123-Thr124 bonds in κ-CN, as determined by mass spectrometry. MCE-G165A showed different hydrolysis sites in casein, leading to various functional peptides. Feasible methods for obtaining MCEs suitable as calf rennet substitutes are presented.