*Oxidative Stability and Sensory Properties of Pecan Nuts DOI: http://dx.doi.org/10.5772/intechopen.106175*

As expected, the fatty acids profile in pecan oil resembles those from kernels, as fat is the major constituent of pecan nuts (**Table 2**).

As shown in **Table 2**, results in extracted pecan oil, the desirable variety had the higher oleic/linoleic relationship, whereas starking showed the lowest, as demonstrated for the pecan oil extracts [12].

Oil is highly susceptible to oxidation, because it does not have the protective structure of the nut proteins and polyphenols, with a shell containing antioxidants. The presence of a large number of phenolic compounds in pecan nut shells may explain the protective activity.

#### **3. Antioxidants in pecan shell, oil, and nuts**

#### **3.1 Shells**

Recently, the presence of 29 and 27 phenolic compounds in nut shells in aqueous and hydro-alcoholic phases has been reported, respectively, from samples submitted to ultrasonic-assisted extraction. The major compounds resulted in catechin (>260 mg/g dry sample); gallic acid (>120 mg/g dry sample); epicatechin (>24 mg/g dry sample); myricetin (>12 mg/g dry sample); ellagic acid (>11 mg/g dry sample); and vanillin (> 6 mg/g dry sample). Other minor compounds are chlorogenic acid, vanillic acid, siryngic acid, epicatechingallate, fustin, P-coumaric acid, taxifolin, ferulic acid, rosmarinic acid, quercetin, salicylic acid, myricetin, eriodictyol, naringenin, and galangin [13].

#### **3.2 Nuts and oil**

The main antioxidant in pecan oil is γ-tocopherol 24.97 ± 0.90 (mg/100 g), followed by α-tocopherol 1.65 ± 0.02 (mg/100 g). The presence of γ-tocopherol and a high concentration of oleic acid are associated with the high oxidative stability of pecan nut oil [14]. In previous work, we described that in Stuart pecans, tocopherols ranged from 5.8 to 142 mg kg−1 nut of α- and γ- tocopherol, respectively. Minor quantities of β- + δ- tocopherols were also detected (less than 0.5 mg.kg−1) [6].

Phytosterols are bioactive compounds that act on oxidative stability in plant cell membranes. They have in their chemical structure, a steroid nucleus with hydroxyl groups (3-β-hydroxyl group), which could be related to a mild antioxidant activity exerted in the lipid phase of biological membranes [15]. β-sitosterol is the main component of pecan sterols (approximately 75%), and its concentration is variable and depends on their ripening state. In Tunisian pecans, the changes in 4-desmethylsterol, 4-monomethylsterol, 4, 4-dimethylsterol, and phytostanol composition were quantitatively and qualitatively investigated during the ripening of three varieties (Mahan, Moore, and Burkett). Fifteen phytosterols and one phytostanol were quantified. The greatest amount of phytosterols (2852.5 mg/100 g of oil) was detected in the Mahan variety, 20 weeks after the flowering date (WAFD). Moore had the highest level of phytostanols (7.3 mg/100 g of oil) at 20 WAFD. Phytosterol and phytostanol contents showed a steep decrease during pecan nut development. Results from the quantitative characterization of pecan nut oils revealed that β-sitosterol, Δ5-avenasterol, and campesterol were the most abundant phytosterol compounds at all ripening stages [16]. Also, they protected oil and fruits from oxidative processing. It has been shown that in fried pecans, tocopherols and phytosterols were the main functional components in the oil-soluble part, to avoid oxidation [17].

The values for total carotenoids in cultivars varied from 0.897 to 1.403 μg/g of oil without any significant difference among cultivars for total carotenoid content [18]. Some antioxidants reported in the literature are extracted in **Table 3**.


#### **Table 3.**

*Main antioxidants in pecans: γ-tocopherol, phytosterols and total phenols.*

#### **4. Oxidative stability**

The conservation of pecan nuts is important in terms of their quality because the sensory defects are undesired for commercial purposes and further consumption.

Many authors described the oxidative stability of nuts with accelerated assays. Indeed, the results may be useful to understand the relationship between biochemical and sensory properties, but they do not predict properly the mechanisms under real storage conditions, because higher temperatures may accelerate lipid oxidation/degradation and Maillard reactions with the concomitant formation of Strecker products that are not expected or are less produced under low-temperature storage [22].

Sensory defects can be predicted in pecans by multiple biochemical indicators. Rancid flavor, moisture, and conjugated dienes were representative of quality deterioration, whereas secondary oxidation products related to higher thiobarbituric acid reactive substances (TBARS) [23]. Other authors indirectly associated the odor of treated pecans with oxidative deterioration and rancid flavor using an e-nose with a sensor array [24]. Recently, a report indicated that storage temperatures below 10°C were more effective to preserve Barton pecans than low oxygen atmospheres with 20, 3 and 1.5 Kpa O2 [25]. In addition, the breakdown of flavonoids and reaction products from Maillard browning could be responsible for the formation of the reddish-brown

color observed in degraded nutmeats. Browning can be predicted using a mathematical model with a first-order decay equation [26].

Moreover, the conservation of pecan nuts for 10 months postharvest is interesting because it allows producers to commercialize their product until the next harvest period. They may be stored in clean burlap bags, at a maximal atmosphere humidity of 70%, and in a dark, clean, and ventilated chamber.

In previous work, we have described that the TBARS value (thiobarbituric acid reactive substances), hexanal, and pentanal were the main predictors of pecan oxidative stability [6]. The degradation of linolenic acid produced hexanal and pentanal as major volatile aldehydes [23, 27] as a result of the cleavage and oxidation of the double bonds. Hexanal and pentanal correlated positively between them, indicating a similar occurrence. Both volatiles were produced in the samples but at different concentrations.

Interestingly, both volatiles showed a positive and significant correlation with TBARS and rancid score, as determined by a sensory panel [6]. Some correlations are shown in **Figure 3**.

This feature may indicate that a TBARS value of 1 corresponds to a rancid score of 5, using a 10-cm nonstructured scale where the lower extreme meant "extremely weak" and the upper extreme meant "extremely strong." Therefore, a TBARS value equal to 1 (mg.kg-1) can be signaled as the threshold value to perceive pecan kernels as rancid.

Oxidation progress within the storage of pecans is independent of the concentration of tocopherols content in the samples. Both α- and γ-tocopherols are preserved during unshelled pecans storage at either refrigerated or room temperature, whereas oxidation triggering measured by TBARS started at seven months postharvest, regardless of the storage temperature [6].

#### **5. Conservation of pecans and sensory attributes**

Many authors describe different strategies for the conservation of nuts. These strategies comprise shelled nuts with different packaging and atmosphere environments, as well as in shell conservation, mainly for in-bulk storage in pecan facilities under different temperatures and oxygen conditions. The compromise between energy cost and pecan quality maintenance will depend on producers' and retailers' possibilities.

#### **5.1 Shelled nuts under vacuum storage at room temperature**

In a previous work, shelled nuts were submitted to room temperature storage, either within nylon-polyethylene bags, or in polypropylene containers. They were conserved for 150 days. Peroxides were raised in both treatments as well along with the time of storage. They described a significant linear reduction (p < 0.0001) in all the sensory characteristics (visual color, typical flavor, odor, and texture) during the storage period. The acceptability scores were similar for both types of packaging, indicating that vacuum treatment did not contribute to prolonging the pecan's shelflife [28]. The shelf life for both treatments was determined in 120 days of storage. After that time, sensory scores dropped below six (scale 1 to 10).

#### **5.2 Raw and roasted shelled pecans stored at room temperature**

Roasting promoted the oxidation of the lipids, with higher TBARS and peroxides levels compared with raw nuts. However, the nonsignificant differences in rancid

aroma and rancid flavor between raw and roasted pecans implied that the sensory response was not sensitive enough to differentiate any increase in oxidative products at an early stage of storage. In this study, panelists did not differentiate the lightness of the raw and roasted products, whereas darkening was indicated in roasted pecans using instrumental measurements.

Storage affected the crunchiness of raw and roasted pecans, with those stored at 65% relative humidity (RH), having lower scores than those stored at 55% RH. During storage, rancid aroma and flavor developed in both raw and roasted pecans with higher records for roasted (p < 0.05). Varying the relative humidity, 55% and 65% RH, during storage, did not affect flavor or aroma scores significantly for either raw or roasted pecans [23].

#### **5.3 Shelled coated pecans**

Shelled "Desirable" pecans were coated with different mixtures of (MC = methylcellulose, CMC = carboxymethyl cellulose, HPC = hydroxypropyl cellulose, PG = propylene glycol, BHA = butylated hydroxyanisole, BHT = butylated hydroxytoluenecaboxy methyl cellulose). All types of coating preserved pecan kernels from oxidative damage compared with uncoated pecans. Coated kernels initially had slightly higher off-flavor, perhaps due to the coating itself, but had less off-flavor and better overall flavor after nine months of storage. Also, the hexanal levels after five months of storage were twofold less in coated than in uncoated kernels. For that reason, the coating could reduce lipid oxidation (i.e., rancidity) and preserve the color during marketing at ambient temperature by limiting oxygen contact with the kernel lipids. This strategy would reduce costs to the pecan industry and improve quality for the consumers [29].

## **5.4 Shelled pecans stored at different temperatures and different packaging conditions**

"Stuart" shelled pecans were stored for 12 months at 20°C, 2°C, and − 15°C packaged in cellophane bags or bi-laminated polythene-aluminum bags flushed with nitrogen. The rise in oxidation (TBARS) between the fourth and tenth month was 10-fold, 4-fold; 4.6-fold, and 4.6-fold for 20°C; 2°C, and − 15°C, respectively. Under nitrogen, oxidation showed no differences between temperatures. TBARS increase was 5.0-fold, 4.6-fold, and 3.7-fold for 20°C; 2°C, and − 15°C, respectively (**Figure 4**). Differences in lipid oxidation were also noticeable in color (**Figure 5**). Nuts stored in cellophane bags were less conserved than under nitrogen. This assay demonstrated that the use of bilaminated bags and nitrogen atmosphere conserved the quality of the nuts, without the necessity of using low-temperature storage. Without nitrogen, refrigeration or freezing were the suitable options. Unshelled pecans showed higher oxidation and darker color after 10 months of storage (Experimental data from INTA).

#### **5.5 Shelled and unshelled pecans stored at 20°C and −15°C temperatures under modified atmosphere conditions**

Dried shelled and unshelled "Barton" pecans were separated into two parts, one submitted to 1-MCP (SmartFresh®, 0.14% of active ingredient as maturation *Oxidative Stability and Sensory Properties of Pecan Nuts DOI: http://dx.doi.org/10.5772/intechopen.106175*

#### **Figure 4.**

*TBARS levels of shelled pecans stored under different packaging and temperature conditions. TBARS assay at the laboratory of Food Science and Technology, INTA.*

#### **Figure 5.**

*Aspect of pecans stored in different bags, with or without nitrogen and different temperatures. Color assay at the laboratory of Food Science and Technology, INTA.*

inhibitor) at a concentration of 1.0 μL L−1. Four groups (shelled and unshelled, with or without MCP-1 treatment) were stored under ambient temperature (17 ± 5.1°C) and relative air humidity (78.3 ± 11.2%). Each treatment was identified and placed separately into raffia bags until evaluation.

Since pecans respire and continue to have enzymatic activity throughout storage unless deactivated using heat or irradiation, it is essential to maintain the packaging environment that can slow down the rate of deterioration.

Results showed that 1-MCP application inhibited excessive acidity and lipid peroxidation in both shelled and unshelled nuts. Luminosity was better conserved in unshelled pecans. Also, some unwanted aldehydes were produced at lower levels, indicating some protective action of the shell. Nonetheless, 1-MCP treatment in this condition reduced the abundance of these volatile compounds.

The conservation of the shells prevented oxidative damage in the nuts. In summary, unshelled pecans stored under ambient conditions and with a 1-MCP application showed the best quality [30].

#### **5.6 Unshelled pecans stored at room temperature and 2°C**

Unshelled "Stuart" pecans were dried to a moisture content of 3–4% and placed in individual clean mesh bags, conserved either at 2 ± 1°C or at 20 ± 1°C at 65% of relative humidity in the dark for 10 months. These conditions resemble postharvest practices in pecan facilities.

Refrigeration did not avoid the trigger of oxidation, measured by TBARS, hexanal, and pentanal. But it restricted the final levels of oxidation compounds. Refrigerated storage of in-shell pecans resulted in differences detected at both biochemical and sensory scores, with significantly higher signs of oxidative deterioration at 20°C compared with storage temperature at 2°C.

Oxidative damage showed an exponential evolution that triggered from day 210 at 20°C and 30 days later at 2°C. A similar behavior was observed for the presence of rancid taste and typical flavor loss during postharvest storage.

The conservation of unshelled nuts at 2°C prevented the excessive formation of these compounds, compared with the conservation at 20°C, but it did not prevent the initiation of the oxidative process. Moreover, the results of sensory trials showed that pecans lost their typical flavor and sweetness, whereas augmented the bitter and rancid taste along with the storage with enhanced deterioration at 20°C compared with 2°C [6].

#### **5.7 In shell pecans are stored at room temperature, refrigerated storage, and different oxygen partial pressure (pO2)**

"Barton" unshelled pecans were stored in containers and placed in three rooms at 20, 10, and 1.5°C and under three different pO2, 20 kPa (ambient condition), 3 kPa, and 1 kPa. The ambient temperature (20°C) allows for avoiding refrigeration. Refrigerated storage (8–10°C) is currently adopted by companies. The lowest temperature used as a control was 1.5°C. After 12-month storage, pecans kept at room temperature (20°C) showed increased acidity and color change.

Luminosity decreased at 20°C with respect to other refrigeration conditions. However, samples kept at 20°C and with lower pO2 (3 and 1 kPa) maintained higher luminosity and less oxidation indicators throughout storage.

Therefore, adopting lower temperatures (1.5 and 10°C) resulted more effective at maintaining quality regardless of the atmosphere condition, without any significant differences in the luminosity and the presence of volatile aldehydes and acids production.

The use of low pO2 in storage facilities has shown positive results, especially at higher temperatures (20°C). There was little difference in quality between 1 and 3 kPa; thus, pO2 near to 3 kPa can be recommended, especially when there are required lower energy costs with refrigeration [24].
