Oxidative Stability and Sensory Properties of Pecan Nuts

*Adriana María Descalzo, Sergio Aníbal Rizzo, Carolina Daiana Pérez, Andrea Biolatto, Enrique Alberto Frusso, Gabriela María Grigioni and Luciana Rossetti*

## **Abstract**

Pecans are the nut with the higher oil content. In addition, they present a large number of polyunsaturated fatty acids, which are susceptible to oxidation. Oxidative damage in pecans is traduced in lower quality aspects, appearance of rancidity and acidity, loss of sweetness and firmness, darker kernels, and darker shells. The use of different strategies for the conservation of entire and shelled nuts is discussed in terms of oxidation and the consequences on nuts quality.

**Keywords:** pecan, *Carya illinoinensis*, oxidation, volatiles, sensory, postharvest, tocopherols, antioxidants

### **1. Introduction**

Pecans nuts are the seeds of *Carya illinoinensis* (Wangenh.) K. Koch. They are an important source of vitamins and minerals, such as vitamin E, folic acid, calcium, magnesium, phosphorus, potassium, several B vitamins, and zinc. They are also rich in fatty acids, having around 58.1 to 68.18 g oil/100 g of nut. Particularly, the unsaturated/saturated fatty acids ratio is around 13.54, with 93% of unsaturated fatty acids in the oil [1].

They also contain β-sitosterol and squalene ranging from 88.74 to 220.42 mg 100 g−1 and 30.98 to 115.59 mg 100 g−1, respectively [2]. This is a disadvantage for the conservation of the nuts and oils because unsaturated fatty acids are prone to oxidative damage. But on the other side, they contain a high level of γ-tocopherol (the main form of vitamin E in pecans) and polyphenols, especially epigallocatechin-3-gallate (EGCG), recognized as a health promoter compound [3] and ellagic acid associated with hepatoprotective activity [4]. Recently, the determination of the proximal composition in 11 cultivars of Brazilian pecans [2] showed total phenolic content variations ranging from 19.88 mg GAE g−1 ("Desirable" pecans) to 45.25 mg GAE g−1 ("Imperial").

Pecans are also a source of protein between 6.88 and 9.26 g .100 g−1 nut with low carbohydrate content (between 4.92 and 17.33 g 100 g−1 nut), and dietary fiber (5.55 to 15.94 g 100 g−1 nut).

In terms of dietary issues, pecans have the lowest net carbohydrates content of any nuts. In fact, one ounce of pecans contains just 1.1 grams of net carbohydrates. This means its consumption is possible, while following a low-carb diet.

#### **2. Lipid composition in terms of oxidative stability**

Pecan nuts are rich in fats, with a net yield of around 58.1–66.18 g oil/100 g of nut mass. Six classes of lipids were separated and identified as complex lipids, monoglycerides, α-β-diglycerides, α-α'-diglycerides, sterols, and triglycerides. Triglycerides were predominant with a mean concentration for the six cultivars equal to 71.25 g/ l00g of nutmeat [5]. The total content of unsaturated fatty acids in the oil is as high as 93%. The unsaturated fatty acids are a group of lipids containing one or more double bonds in the structure. Monounsaturated fatty acids (MUFAs) contain one double bond in the fatty acid chain. If two or more double bonds are present, they are referred to as polyunsaturated fatty acids (PUFAs).

The number of double bonds has an impact in terms of cell membrane mobility. The higher PUFAs, the higher lipid mobility within membranes. Saturated fatty acids (SFAs) do not contain double bonds in their structure and contribute to a more rigid cellular structure. The low ratio of saturated to unsaturated fatty acids in the cell membranes increases membrane fluidity and permeability [6].

Omega-6 linoleic acid and omega-3 alpha-linolenic are essential fatty acids that cannot be synthesized endogenously by most of the animals; therefore, their constant dietary intake is crucial [7].

The chromatographic profile of Stuart pecans [6] fatty acids is shown in **Figure 1**.

The largest peak corresponds to oleic acid (C18: 1 n-9), followed by linoleic acid (C18: 2 n-6). This picture is typical of the fatty acids profile of pecan products (Picture from the Food Science laboratory INTA). Indeed, pecan varieties show a

**Figure 1.** *Gas chromatography (GC-FID) typical profile of pecan nuts.*


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

**Table 1.**

 *Main fatty acids in pecan kernels from Brazil, US and Argentine varieties (g/100 g fat).*

*Different letters within column indicate significant differences (p < 0.05).*

#### **Figure 2.**

*Relationship between oleic acid, linoleic acid and Peroxidizability Index. Dots in blue indicate C18: 1 n-9 vs. PI; triangles in red indicate C18: 1 n-9 vs. C18: 2 n-6.*

conservative fatty acids profile with slightly different shapes attributable mainly to the genetic, environmental, and plant phenological status [8]. In **Table 1**, the means of major fatty acids are shown in different cultivars from different regions (US, Brazil, and Argentina).

As lipid composition is critical for oxidative stability, different fatty acids and their indexes are presented in **Table 1**. Oleic acid (C18: 3, n-9) is the main constituent of pecan fatty acids. It has one double bond at the carbon in position nine. Therefore, this FA is relatively stable in terms of oxidation. Its concentration in pecan kernels is inversely proportional to the linoleic acid (C18: 2, n-6) concentration (**Figure 2**) and consequently to the peroxidizability index (PI).

The peroxidizability index may indicate the susceptibility of pecans to oxidation. At higher levels of oleic acid, the lower PI in the samples. Mahan showed the lowest total PUFA level and consequently the lowest PI. The variation in the lipid content affected the pecan stability, as oxidative damage induces quality loss and correlates positively with rancidity [8].

Taking together all the data in **Table 1**, it is noticeable that the samples with higher oleic acid (C18: 1 n-9) presented lower levels of linolenic acid (C18: 2 n-6), that is, Mahan vs. Western with a negative and significant linear correlation (R2 = 0.89), as illustrated in **Figure 2**.

By virtue of the number of peroxidizable double bonds, samples with higher oleic acid are more stable and show less PI. Apparently, during the development, oleic acid is transformed into linoleic acid by the action of desaturates. As the proportion of oleic acid increased or decreased, it was in tandem with an opposite change in linoleic acid. The ratio oleic/linoleic coincided with the ratio oleic/PI, indicating the susceptibility of the cultivars [8]. Therefore, it can be considered for breeding programs or further postharvest handling in terms of oxidative stability.

Desirable pecans grown at two different locations (BW and CW) showed differences in their oleic-linoleic composition. The fatty acid profile may depend on environmental conditions, cultivar, maturity, and horticultural practices. Thus, the selection of appropriate cultivation conditions is an important factor to consider when selecting pecan cultivars [10].

For example, increasing the nitrogen fertilization rate increases the protein content and oleic acid levels in kernels. At the same time, it showed a tendency to lower tocopherol contents [11].


**Table 2.**

*Fatty acids profile in pecan oil from Argentine cultivars (g/100 g pressed oil).*
