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Thermal conductivity of two types of nanobiodiesels (NBs) was investigated theoretically and experimentally. The first type of NBs (C4-Au) was composed of C4 biodiesel (purchased from Biofuels of Mexico) filled with Au nanoparticles (Au-NPs) and the second type (SB-Ag) was composed of soybean biodiesel (SB) filled with Ag nanoparticles (Ag-NPs). It has been demonstrated in the literature that the addition of Au-NPs or Ag-NPs to biodiesel can lead to a significant increase in thermal properties. The photothermal techniques were used to determine the thermal diffusivity (D), thermal effusivity (e), and thermal conductivity (k) of biodiesel filled with Au-NPs or Ag-NPs for different concentrations. For about two decades, researchers have made the effort to predict the enhancement of the thermal conductivity of nanofluids based on experiments and several theoretical models have been proposed. One of these analytical models, which have allowed researchers to calculate the thermal conductivity of the nanofluids, is the Hamilton–Crosser Model. This model is based on the classical theory of compounds and mixtures containing particles in the order of millimeters or micrometers and fails dramatically in predicting the thermal conductivity of nanofluids. In that sense, the Hamilton–Crosser model (H–C) cannot represent adequately the enhancement in k as a function of NP’s concentration. Then, the artificial neural network (ANN) modeling method was used to predict the thermal conductivity of the two NBs studied in this work.

Photoacoustic spectroscopy (PAS) has been widely used to analyze biological samples. For example, PAS has been applied to the study of blood which could be related to some diseases. In the present research, the optical absorption spectra of Fisher rat blood, with hepatic damage, were obtained by PAS. Also, the daily intake of turmeric (Curcuma longa), as a protection of liver damage, was evaluated through the characteristic optical absorption peaks of blood, α(585 nm), β(550 nm), and γ(420 nm), where it is possible to associate them with differences in hemoglobin concentrations of the studied samples, by means of the peak ratios γ/β and γ/α, for each analyzed group.

In recent years, has been reported an increasing interest to study vegetables and fruits in order to know their benefits to prevent different types of health problems. Curcuma longa has a great potential to prevent diseases as different types of cancer. In the present study, C. longa samples, of different trademarks, were optically and thermally characterized by using photoacoustic spectroscopy and photopyroelectric techniques, obtaining their optical absorption spectra, optical absorption length spectra, the thermal diffusivity, effusivity, conductivity and heat capacity per unit volume, observing optical absorptions corresponding to antioxidant elements, reported such as beneficial for health. These properties also could be important in the food industry for the commercialization of functional foods.

In this work, the optical absorption spectrum of peppers was monitored by phase-resolved photoacoustic spectroscopy during a dehydration process based on hot-air drying, yielding simultaneous information about changes in the exocarp and mesocarp. Our results show that between all of the dehydration processes of green Capsicum annuum L. variety pasilla peppers, only very small changes occur in the different phase angles, which has been correlated with the small changes in the exocarp thickness. The phase-resolved spectra of mesocarp show more clearly the evolution of the carotenoid compounds with respect to the optical absorption spectrum without phase resolving, due to the last spectrum having a band broadening in that region with more signals convolved. We have shown that not only do the ripened chili peppers produce new carotenoid compounds, but also we are probing that the dehydration process, beginning with the green stage, preserves the nutrimental content, similar to changes that occur in the natural ripening process.