In physics and chemistry, the development of problem-solving skills is necessary to become an expert. A simple cognitive model to analyse such development is proposed and tested. An exploratory research was conducted with expert professors and students in initial and advanced years. A think aloud procedure was used to obtain relevant data while the participants tried to solve undefined, open problems. Solving these problems required a particular skill representative of expertise: modelling reality using science. More than 1350 solving actions were collected and related to the mental representations elaborated, developed and inter-related by solvers. The proposed model was able to account for expert-novice differences in terms of the respective distributions of solving actions among the mental representations. Large differences appeared in the mental representation of Conceptual scientific Model. In addition, advanced and initial students showed similar and significant averages of unproductive actions, while experts took very few. Experts showed high convergence in their distributions of actions among the mental representations. If the outcomes were replicated with higher external validity, the model could help researchers to analyse the cognitive mechanisms in problem-solving, and teachers to better focus their efforts on specific students’ lacks. Keywords: cognitive model, expert-novice differences, mental representations, problem-solving skills, solving actions

With the objective of evaluating the quality parameters of raw milk in Ecuador between 2010 and 2020, a systematic review and meta-analysis of 73 studies on raw milk produced in different regions of Ecuador was performed. Under the random effects model, effect size and heterogeneity were determined vs. climatic region both among analyses and studies, with Cochran’s Q, I2 and Tau (π) statistics. For all the variables, it was observed that there was great heterogeneity (I2 > 90%) among the studies; additionally, it was found that climatic region had an influence only among the variables arsenic, mercury, pH and total solids, and it was greater in the coastal region than the Inter-Andean region. The mean values of the physicochemical characteristics of the milk (titratable acidity, ash, cryoscopy, fat, lactose, pH, protein, non-fat solids and total solids) in the great majority of these studies were within the range allowed by Ecuadorian regulations. As for the hygienic quality of raw milk (total bacterial count, somatic cell count and presence of reductase), although the mean values were within those determined by local legislation, it should be noted that the range established by Ecuadorian regulations is relatively much higher compared to other regulations, which possibly means that there is a high presence of bacteria and somatic cells in raw milk. Finally, the presence of several adulterants (added water) and contaminants (AFM1, antibiotics and heavy metals) was confirmed in the milk, in addition to other substances such as eprinomectin, zearalenone and ptaquilosides, whose presence can be very dangerous, because they can be hepatotoxic, immunotoxic and even carcinogenic. In conclusion, there is great variability among the studies reviewed, with the physicochemical characteristics being the most compliant with Ecuadorian legislation; the hygienic characteristics, adulterants and contaminants of raw milk require greater attention by producers and local authorities, so that they do not harm the health of consumers and the profitability of producers in Ecuador.

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