Analysis of aflatoxin M1 contamination in milk and cheese produced in Mexico: a review

Due to the carcinogenic character of aflatoxins when present in foods, these compounds are considered a risk to human health. This systematic review aimed at compiling the available research data on detection and quantification of aflatoxin M1 (AFM1) in milk and common types of cheese produced in Mexico in the past two decades. A limited number of studies were found that matched the purpose of our review. Only ten research works focused on the evaluation of AFM1 content in milk while three studies analysed the occurrence of this mycotoxin in oaxaca and panela cheeses. HPLC-FD and ELISA were the methods of choice utilised to detect AFM1. Concentrations higher than 0.5 μg AFM1/kg, a maximum limit set in current food regulation in Mexico, were found in major dairy brands consumed in Mexico. Analysis of raw milk produced during the rainy season in the states of Jalisco (2007) and Chiapas (2013) showed mycotoxin levels within the regulation limits while milk samples obtained during the dry season in the Mexico City and the State of Mexico (2008) exceeded that threshold. For cheeses, 33% of the artisanal produced oaxaca type samples from Veracruz (2016) and 55% of those acquired in Mexico City (2019) were found above the limit set for milk. In contrast, the panela cheese samples obtained in Baja California and Guanajuato (2009) complied with the AFM1 regulation. Additionally, the presence of AFB1 and its hydroxylated metabolites other than AFM1 were determined in the major milk brands at concentrations that could be of high risk for human health. Similar results were reported for both artisan and industrially produced oaxaca cheese. Finally, mycotoxins enter human food chain through animals fed with contaminated fodder. Our systematic review demonstrated the urgent need to amend the existing food regulation in Mexico to include mycotoxins as potent contaminants in cheese.

Fluorescence and Textural Characterization of Ortho-Amine Tetraphenylporphyrin Covalently Bonded to Organo-Modified Silica Xerogels

Most of the studies performed with porphyrins involve these species functionalized with peripheral substituents lying on the same macrocyclic molecular plane. The main objective of this work deals with the successful preservation and optimization of the fluorescence of a uncommonly used porphyrin species, i.e. tetrakis-(ortho-amino-phenyl)-porphyrin; a molecule with substituents localized not only at one but at both sides of its molecular plane. In cases like this, it must be stressed that fluorescence can only be partially preserved; nevertheless, intense fluorescence can still be reached by following a twofold functionalization strategy involving: (i) the bonding of substituted macrocycles to the pore walls of (ii) organo-modified silica monoliths synthesized by the sol-gel method. The analysis of both absorption and emission UV spectra evidenced a radiation energy transfer taking place between the porphyrin and the host silica matrix. Our results showed that the adequate displaying of the optical properties of macrocyclic species trapped in SiO2 xerogels depend on the polarity existing inside the pores, a property which can be tuned up through the adequate selection of organic groups used to modify the surface of the pore cavities. Additionally, the pore widths attained in the final xerogels can vary depending on the identity of the organic groups attached to the network. All these facts finally demonstrated that, even if using inefficient surface functionalization species, such as ortho-substituted tetraphenylporphyrins, it is still possible to modulate the pore shape, pore size, and physicochemical environment created around the trapped macrocycles. The most important aspect related to this research deals with the fact that the developed methodology offers a real possibility of controlling both the textural and morphological characteristics of a new kind of hybrid porous materials and to optimize the physicochemical properties of diverse active molecules trapped inside the pores of these materials.

Thermal denaturation of porcine pepsin: a study by circular dichroism

Thermal denaturation of porcine pepsin in 10% ethanol was studied by circular dichroism (CD) spectroscopy. It was observed that the process is markedly irreversible. The denaturation unfolding process was strongly dependent on the heating rate, as is expected for an unfolding process kinetically controlled due to the presence of an irreversible reaction. Experimentally, we demonstrate the existence of an unfolded (U) state in equilibrium with the native (N) state. The U state is observed to exist at temperatures lower than 45 degrees C. The van't Hoff enthalpy, DeltaH(vH), was determined from direct estimation of the equilibrium constant at several temperatures (DeltaH(vH)=304.3 kJ/mol). To explain the observed behavior, we have considered a Lumry-Eyring model, which takes into account the presence of the U state in addition to N and denatured (D) states (i.e. N<-->U-->D).

Circular dichroism studies of acid proteinases from Aspergillus niger and Aspergillus awamori

Acid proteinases produced by strains of Aspergillus niger and Aspergillus awamori were isolated by means of ethanol precipitation, gel filtration and anion-exchange high resolution chromatography. In each case, the purified proteinase showed a single band in polyacrylamide gel electrophoresis. Their molecular weights were almost identical (approx. 45,000). However, the proteinase from Aspergillus awamori contained 16% of neutral hexoses while the other enzyme (Aspergillus niger) showed negligible amounts of these carbohydrates. Both enzymes displayed circular dichroism spectra that share a number of features with that of penicillopepsin. This suggests that proteinases from Aspergilli possess the structural folding pattern typical of aspartic proteinases. Proteolytic-activity pH optima were different, thus distinguishing one enzyme from another. This variation seems to be related to the particular resistance of the proteinases to acid denaturation, as indicated by changes in their circular dichroism spectra when the pH is decreased.