Abstract
The aim of this doctoral thesis is to evaluate the addition of bioactive compounds with antioxidant and antimicrobial activity, via the method of molecular complexation in amylose, to biodegradable starch films and, more specifically, the effect on their barrier, physicochemical, mechanical, antioxidant, antimicrobial and optical properties. Also, the aim of the study was to evaluate the effect of the combination of nanoclay and molecularly complexed bioactive compounds in amylose, in the preservation and the enhancement of the quality of food products under normal as well as accelerated aging conditions. Finally, an attempt was made to evaluate the durability of starch films with nanoclay under accelerated elongation conditions and to find the statistical model that best predicts the probability of film failure at various combinations of elongation level and duration of elongation. In addition, the effect of rheological memory of starch nanoclay films on the strain hardening phenomeno ...
The aim of this doctoral thesis is to evaluate the addition of bioactive compounds with antioxidant and antimicrobial activity, via the method of molecular complexation in amylose, to biodegradable starch films and, more specifically, the effect on their barrier, physicochemical, mechanical, antioxidant, antimicrobial and optical properties. Also, the aim of the study was to evaluate the effect of the combination of nanoclay and molecularly complexed bioactive compounds in amylose, in the preservation and the enhancement of the quality of food products under normal as well as accelerated aging conditions. Finally, an attempt was made to evaluate the durability of starch films with nanoclay under accelerated elongation conditions and to find the statistical model that best predicts the probability of film failure at various combinations of elongation level and duration of elongation. In addition, the effect of rheological memory of starch nanoclay films on the strain hardening phenomenon after successive elongations within the elastic deformation was studied.As for the methodology used, it followed the following steps: first, a parametric analysis was designed to evaluate which factors play an important role and how the productionand properties of biodegradable starch films with nanoclay are affected by these factors. More specifically, the following factors were selected as factors for the parametric analysis: starch, glycerol and nanoclay concentrations, drying temperature, type of drying container and starch type. The responses studied were the moisture of films, the mechanical properties (ultimate tensile strength and elongation at break), the barrier properties (water vapor permeability), the thickness of the films, as well as the optical properties of the films (opacity and colorimetric parameters). Through the results of this process, it was also possible to extract the combination of factors leading to the films with optimal properties, which is very important, before proceeding to the production of films by molecular complexation of bioactive substances in amylose.Secondly, the optimal combination of the parametric analysis factors was used to produce starch films with molecular complexation of bioactive substances in amylose via themethod of gelatinization complexation and the compounds used were carvacrol, ascorbic acid and hydroxycinnamic acid, at concentrations between 2 and 22 % w/w on amylose. These films were studied in terms of ultimate tensile strength and elongation at break, water vapor permeability, film thickness, opacity and chromatometric factors. In addition, the antimicrobial activity against total aerobic mesophilic and psychrotrophic microbiota of fresh minced beef and the effect of packaging on the preservation of colour, oxymyoglobin levels and the physico-chemical quality of minced beef during refrigeration for 4 days were also studied.Finally, in order to formulate the final films containing the combination of bioactive substances complexed in amylose and nanoclay, linalool and ascorbic acid were selected at concentrations of 5 and 15 % w/w on the amylose and the same analysis as in the previous case was carried out. In addition, an attempt was made to evaluate the quality of basmati rice and brown rice packaged in starch films, with a combination of 5% w/w ascorbic acid complexed in amylose and nanoclay, which were subjected to anaccelerated aging test for 20 days at 70 °C. The assays made on the powdered rice were the measurement of acidity of aqueous and alcoholic extract and the measurement of ricecolour.In addition, in order to evaluate the actual fatigue of starch films as food packaging, an experimental design was organized to simulate the continuous degradation that the packages underwent, using the method of accelerated shelf-life tests, with the elongation as an acceleration factor, monitoring the logarithmic tensile ratio (the quotient of the tensile strength divided by the break cycle). Also, the ability of starch nanoclay films to have rheological memory, which is reflected by the increase in tensile strength, during the application of tensile cyclic loads, which were applied either consecutively or with time intervals, was evaluated. Thus, starch and nanoclay films were used, which were elongated for different times and percentages, and the tensile strength after consecutive elongations with 1-week intervals was compared with the tensile strength observed after consecutive elongations with no time interval between them.Regarding the parametric analysis, it was shown that the thickness increased along with the increase of starch, glycerol and nanoclay concentration. The moisture of films increased with the increase of glycerol concentration. The ultimate tensile strength decreased with the increase of glycerol concentration and increased along with the increase of the concentration of nanoclay. The elongation at break increased with higher glycerol concentration, while, increasing the nanoclay concentration, did not significantly affect, within certain limits, this parameter. The water vapor permeability increased with higher concentrations of starch and glycerol, whereas, increasing the percentage of nanoclay, decreased this parameter. The opacity of films decreased with higher glycerol concentration and increased with higher nanoclay concentration. Τhe colorimetric parameter L* decreased with higher starch and nanoclay concentrations. Τhe colorimetric parameter b* increased with higher starch and nanoclay concentrations. Finally, the film with optimal properties consisted of: starch concentration 5.5% w/w, glycerolconcentration 30% w/w on dry starch, nanoclay concentration 10.5% w/w on dry starch, drying temperature 45 °C, chickpea as the starch type and plexiglass as the containertype.Regarding the production of films with molecularly complexed bioactive compounds, it appeared that their opacity and the colorimetric parameters L*, a*, b* were not affected, the only exception being hydroxycinnamic acid, while, regarding the mechanical properties, it appeared that the complexation of bioactive compounds led to an increase in the ultimate tensile strength of the films, without having a significant effect on the elongation at break. The water vapor permeability of the films with complexed bioactive substances was higher than the controls with the exception of ascorbic acid complexation. On the other hand, the addition of bioactive compounds via the method of molecular complexation in amylose led to a reduction of total aerobic mesophilic and psychrotrophic bacteria in minced meat, particularly in the case of carvacrol, and also helped to maintainthe bright red colour of meat throughout its preservation by maintaining high levels ofoxymyoglobin, particularly in the case of ascorbic acid. This was possible because it appeared that the antioxidant activity of the meat, due to the bioactive compounds, was kept high throughout the preservation of the minced beef.Regarding the production of films with complexed bioactive compounds and nanoclay, it was shown that in terms of film opacity, there was an increase within certain limits by the addition of nanoclay, while regarding the colorimetric parameters L*, a*, b*, it was found that, for the parameters L* and a* there were no major changes compared to the films containing only bioactive compounds and, on the other hand, it was found that the addition of nanoclay led to an increase in the values of the parameter b* in the films. With respect to mechanical properties and, more specifically, the ultimate tensile strength, the addition of the combination of bioactive compounds and nanoclay led to an increase in the ultimate tensile strength of the films, while, at the same time, the elongation at break decreased. However, in terms of water vapor permeability, the combination of bioactive compounds and nanoclay led to an increase in the values, while the addition of nanoclay together with complexed bioactive compounds did not affect the high antioxidant activity of the films. On the other hand, the addition of bioactive compounds via molecular complexation in amylose and nanoclay led to a decrease in the total aerobic mesophilic and psychrotrophic bacteria of minced meat, particularly in the case of linalool and nanoclay, and, also, led to an increase in the bright red colour of meat throughout its preservation, through the retention of high levels of oxymyoglobin, particularly in the caseof ascorbic acid and nanoclay. Regarding the study of the quality of rice packaged in starch films with ascorbic acid and nanoclay and subjected to accelerated ageing, it was apparent from the colour measurements that all starch films had higher values of the L* parameter than the commercial sample, as well as lower values of the b* parameter than the commercial sample. In the case of acidity, the rice samples packed in starch films with complexed ascorbic acid 5% w/w on amylose and nanoclay had, in most cases, the lowest values compared to all other samples, indicating the superiority of the combination of bioactive compounds complexation and nanoclay on the functional properties of the starch films.Regarding the accelerated lifetime tests on the films, using the elongation as an acceleration factor, a decrease in logarithmic tensile ratio was observed as the logarithmic exact break time increased, which was steeper at the lowest or reference elongation level, while an increase in films withstanding high break cycles was also observed, specifically in high time and low logarithmic tensile ratio regions. On the other hand, in terms of testing the different treatments, it appeared that the sample containing the highest percentage of starch and nanoclay and the lowest percentage of glycerol seemed to have the highest resistance to the different tensile conditions, since a large number of units broke at higher levels of logarithmic tensile ratio (2 to 3) making it the most resistant sample. Regarding the rheological memory evaluation measurements, the elongation-time combination protocols applied were 5% for 2', 5% for 5' and 7% for 2' in order to evaluate both the effect of elongation and time on the rheological memory of the polymer and thus the effect on the tensile strength. It was observed that, in the films containing nanoclay, the values of the tensile strength when the elongation was applied weekly versus sequentially increased with the passage of weeks, in many cases even in a linear manner, something that was not observed in films without nanoclay.
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