A novel electrochemical assay for chymosin determination using a label-free peptide as a substrate

Chymosin is a predominant enzyme in rennet and is used in cheese production because of its excellent milk-clotting activity. Herein, we proposed a facile and label-free electrochemical method for determining chymosin activity based on a peptide-based enzyme substrate. The synthesized substrate peptide for chymosin was assembled onto the surface of the Au-deposited grassy carbon electrode. The current was proportional to chymosin activity, and thus chymosin activity could be determined. The detection ranges of chymosin activity were 2.5 to 25 U mL^-1. The detection limit of chymosin activity was 0.8 U mL^-1. The sensing platform was used to quantify chymosin activity in commercial rennet with high selectivity, excellent stability, and satisfactory reproducibility. We developed a facile, fast, and effective electrochemical assay for detecting chymosin activity, which has potential applications in cheesemaking.

Impact of Rodenticides on the Coagulation Properties of Milk

In this study, we investigated the impact of the rodenticides (strychnine, bromadiolone, and brodifacoum) on milk pH, rennet coagulation time (RCT), and coagulum strength. Sub-lethal amounts of strychnine and bromadiolone produced an unnaturally large change in milk pH, compared to brodifacoum and brodifacoum on milk coagulation properties. All three studied rodenticides significantly affected RCT and coagulum strength. The presence of sub-lethal amounts of each individual rodenticide increased RCT by an overall mean of 17% (p < 0.001). Rodenticide contamination decreased coagulum strength by an overall mean of 26% (p < 0.05). Our results suggest that such changes could be noticeable at the farm, thus, potentially averting the mixture of contaminated milk with the tanker supply, and preventing downstream distribution to consumers.

Simultaneous interaction of pH, CaCl2 addition, temperature and enzyme concentration on milk coagulation properties / Interacción simultánea del pH, CaCl 2 añadido, temperatura y concentración enzimática sobre la coagulación de la leche

The simultaneous interaction, of the pH (5.6-6.8), CaCl2 addition (100-300 mg/L milk), tempera ture (30-40 °C) and enzyme concentration (0.02-0.05 rennet units/ml milk) on curd firmness (CF) and clotting time (CT) during milk coagulation were followed. The coagulation was performed in the specially designed vessel which was coupled to a rheometer. Whole milk was coagulated by the addition of two types of enzyme, one of microbial and another of bovine origin. The effect of the four variables on CT and CF was evaluated using the response surface design by Box and Wilson, consisting of 30 combinations of factors on four levels, grouped into three blocks. Surface response analysis lead to a polynomial function that describes CT and CF as a function of pH, temperature, added CaCl2 and enzyme concentration. The simultaneous interaction demonstrated that for curd firmness development the main factors were a decrease in pH and an increase in temperature. The firmness of the gel produced with microbial enzyme depended on temperature; conversely, the pH affected the curd firmness achieved with bovine rennet. Under the test conditions, maximum firm ness values were obtained over a small range of pH and temperature. Experimental tests on firm ness showed a better fit for the microbial model than for the bovine one.

Exploring the milk-clotting properties of a plant coagulant from the berries of S. elaeagnifolium var. Cavanilles

Solanum elaeagnifolium (trompillo or silverleaf nightshade) is an endemic plant from the northeast of Mexico and southwest of United States. This plant is considered as a weed with negative impact on agriculture and livestock production. Nevertheless, in some places of Chihuahua, Mexico, the berries of this plant have been used for decades in the manufacture of artisanal filata-type asadero cheese. The milk-clotting enzyme of S. elaeagnifolium has been scarcely studied; for this reason, the aim of this work was to explore some properties of this plant coagulant. Protein extracts (PEs) from ripe berries of S. elaeagnifolium were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and assessed for milk-clotting activity. In addition, milk gels and miniature fresh-type cheeses manufactured with the plant coagulant were analyzed for their texture properties. The PE from the berries of S. elaeagnifolium contained 8 proteins with molecular weights from 22 to 62 kDa. Some bands observed in the PE had similar molecular weights as reported for aspartic proteinases such as chymosin. The extracts from the berries of S. elaeagnifolium had lower milk-cloting activities than observed with rennin or chymosin, but this plant coagulant produced firm gels under acidic conditions. The mini-cheeses manufactured with this coagulant were softer than cheeses manufactured with rennin or chymosin. For this reason, the coagulant from the ripe berries of S. elaeagnifolium could be suitable not only for the manufacture of filata-type cheeses but also for the manufacture of soft cheeses such as cream cheese.

PRACTICAL APPLICATION

Silverleaf nightshade (trompillo) is a plant that grows in northern Mexico and the southwestern United States. This plant is considered a weed with negative impact on agriculture and livestock production. However, the ripe berries of this plant have been used by ancient Pima Indians as a substitute of rennin in making cheese. In this work, it was observed that this plant coagulant had lower activity and produced softer cheeses than did rennin or chymosin. For this reason, the coagulant from berries of S. elaeagnifolium could be used for the manufacture of soft cheeses such as cream cheese.

Impedimetric biosensor for the assessment of the clotting activity of rennet

Cheese production is relied upon the action of rennet (a mixture of chymosin and pepsin) onto casein micelles of milk. For the first time, the monitoring of this interaction with electrochemical impedance spectroscopy (EIS) was used to develop a faradic impedimetric biosensor for the assessment of the clotting activity of rennet, using hexacyanoferrate(II)/(III) couple as a redox probe. Gold electrodes were modified with self-assembled monolayers of different thiols (thioctic acid, dithiobis-N-succinimidyl propionate, and cysteamine), and (artificial) casein micelles were immobilized on the modified gold surfaces. The proposed method is based on the measurement of charge-transfer resistance (R(ct)) changes attributed to the degradation of the negatively charged immobilized casein micelles by rennet to neutral biostructures. This action results in the increase of the flux of the redox probe, which exists in the bulk solution, to the surface of the electrode and, consequently, in the decrease of R(ct). Experimental parameters such as the micelle loading, the reaction time, the concentration of rennet, and the working pH, were optimized. Besides EIS measurements, cyclic voltammetry, FT-IR, and atomic force microscopy (AFM) experiments were also performed before and after the interaction of the immobilized micelles with rennet. Finally, the proposed biosensors were successfully tried for various commercial samples.

Measurement of milk clotting activity by rotational viscometry

Standard method for the determination of the activity of milk coagulants is the rotating bottle method, where clotting time is defined as the time when visually observable flocculation starts. Aim of this study was to verify whether it is possible to determine milk clotting time by rotational viscometry. Using three different coagulants and reconstituted milk of different pH and temperature, flocculation time and viscosity in steady shear was determined, and milk gelation was monitored by small amplitude oscillating shear rheometry. The results show that, independent of pH and temperature, milk clotting time is related to an apparent viscosity of 7·24±0·45 mPa.s, indicating that rotational viscometry can be used for the screening of flocculation time with an accuracy of approximately 6%.

Intermicellar relationships in rennet-treated separated milk: II. Process of gel assembly

The aggregation of casein micelles in milk by the action of rennet, examined by electron microscopy and viscometry, was correlated with visually-observed coagulation. The spatial relationships between micelles were quantified from measurements made on micrographs. Aggregation did not start until about 60% of the rennet-clotting time, when the enzymic action was almost complete. The micelles tended to form chains which then linked into a loose network. This became more extensive with time as the average size of the aggregates increased. By the clotting time, most micelles were in contact with another and a network was beginning to form. Initially, micelles probably linked by bridges. These then appeared to contract, bringing the particles into contact and eventually causing partial fusion.

Influence of processing on the molecular modifications of milk proteins in the course of enzymic coagulation

The formation of hydrophobic sites on the surface of casein micelles as a consequence of enzymic coagulation of industrially heat-treated milk was studied by following the distribution of a fluorescent hydrophobic probe between a free and an aggregated protein fraction. Results were compared with those obtained when coagulation of the same milk samples was followed rheologically in a Gelograph apparatus and by tristimulus colorimetry. Thermal treatment of milk appeared to affect the accessibility of casein to enzyme action, while homogenization influenced the rate of cooperative aggregation of casein subjected to proteolysis.

Rennet-Induced Milk Coagulation by Continuous Steady Shear Stress

The effect of continuous steady shear stress (CSSS) on rennet-induced coagulation of milk was studied by measuring the change in viscosity of the system with time. Continuous shear stress (< or =0.5 Pa) applied during coagulation did not counteract the network formation at standard cheese-making conditions. In fact, CSSS of 0.2 Pa promoted coagulation by possibly increasing diffusion, collision, and hydrolization rates due to near-field, attractive hydrodynamic reactions. This was evidenced by the high viscosity of the resultant coagulum. However, the viscosity profile of the coagulum formed in the presence of CSSS followed the same trend as that formed in the absence of CSSS. Viscosity versus time profiles in both cases displayed an initial lag phase followed by a steady increase until a plateau value. The viscosity plateau reached under CSSS was marked with several sudden peaks, indicating the dynamic structure of the coagulum. These peaks in viscosity profiles began to appear after about 1380 s since rennet addition at standard cheese-making conditions. The time at which viscosity first exceeds 40 kPa s was verified to coincide with manual determination of the time at which the coagulum is cut (i.e., cutting time) during cheese making.