3.2. Physical and physicochemical properties
Table 3 shows the results for acidity, peroxide, iodine, and saponification values, unsaponifiable matter, OSI and water contents of the samples. The acidity and peroxide values are reference parameters to determine the conservation quality of fats and oils. The Codex Alimentarius (1999) report set the limits for maximum acid and peroxide values of cold-pressed and unrefined fats and oils as 4.0 mg KOH/g and 15 meq/kg, respectively. Considering the Codex requirements for acidity and peroxide index, only pracaxi oil was in agreement with both specifications, while all of the others samples were in agreement only with the peroxide index specification. The adopted methodology (AOCS Cd 8b-90) did not allow the peroxide value for bacuri fat and Brazil nut oil to be determined, due to the strong color of these oils. It is important to highlight the fact that the fats and oils studied here are not explicitly included in the Codex Alimentarius report. As reported by Rodrigues, Silva, Marsaioli, and Meirelles(2005), high temperature and high humidity, characteristics of the Amazon region climate, can affect quality and increase the free acidity of the oils. Thus, high values for acidity and peroxide index of the samples are directly related to the handling, processing and storage of the nuts and seeds. In general, to avoid losses of neutral oil, fats and oils with high acidity values should be submitted to the physical refining, while those with low acidity can follow the chemical refining.
Iodine values are related to the number of FA presenting unsaturation and high carbon chains, therefore the greater the amount of unsaturated FA, the higher these values. The highest values were observed in Brazil nut and patawa oils (91.1 and 75.9, respectively), since they are the samples with the highest PUFA contents.
Saponification is a measure of the average chain length of all FA present in the lipid. Since it is inversely proportional to the fatty acids average molecular weight, glycerides containing short-chain FA have higher saponification values than those with longe chain FA (Walia, Rawat, Bhushan, Padwad, & Singh, 2014). This index ranged from 246.4 mg KOH/g (tucuma kernel oil) to 164.4 mg KOH/g (pracaxi oil). Brazil nut oil and bacuri fat reached intermediate saponification values (187.5 and 189.1 mgKOH/g, respectively) close to those of olive oil and buriti oil (O’Brien, 2009; Silva et al., 2009).
Unsaponifiable matter includes substances dissolved in fats and oils that cannot be saponified, such as higher aliphatic alcohols, tocopherols, sterols, phenols and pigments. The unsaponifiable matter found in bacuri fat (2.8%) is comparable to corn oil’s, while all of the other samples have values close to those of palm, peanut and other refined oils (O’Brien, 2009; Pereira Lima et al., 2017).
OSI depends on the number and positions of double bonds (Knothe & Dunn, 2003). Even though bacuri fat had a significant MUFA content (Table 1), this fat presented the highest induction time (49.60 h). Such high stability is, probably, related to bacuri fat’s high unsaponifiable matter (2.8%) and its saturated/unsaturated ratio (Table 1). Pracaxi and patawa oils had oxidative stabilities of 4.73 and 4.97 h OSI, respectively; these values are comparable to palm oil’s. Murumuru had an oxidative stability of 18.0 h OSI, comparable to that of olive and buriti oils (Anwar, Bhanger, & Kazi, 2003; Silva et al., 2009). Brazil nut oil, the sample with the highest PUFA contents, had the lowest OSI.
Moisture can induce hydrolysis, increase free FA content and generate off-flavors, causing problems during the extraction and refining process (O’Brien, 2009). Water contents in the assayed samples were lower than 0.2%, a low value, especially for crude oils.