J. Benton Storey
Professor of Horticulture
Texas A&M University
September 12, 1997
A high dietary intake of saturated fats has been linked to arteriosclerosis and coronary disease. Arteriosclerosis is a process in which fatty substances, especially cholesterol and triglycerides are deposited in the walls of medium-sized and large arteries. Cholesterol in blood is transported in combination with specific aggregates of lipids and proteins called lipoprotein. Normally, most cholesterol is carried in low density lipoprotein (LDL), and is a significant risk factor for coronary heart disease. Other plasma cholesterol is transported in high-density lipoprotein (HDL). Research indicates when HDL cholesterol is high, coronary heart disease risk is lowered.
Saturated fats have been shown to increase plasma cholesterol and LDL levels. Plasma cholesterol can be reduced by twenty mg/dl by reducing the consumption of saturated fats from 17 to 10 percent of the total calories in the diet.
One would think that the more unsaturated a fatty acid is, the healthier it would be because it would reduce the plasma cholesterol. However, this is not necessarily the case. Consuming mostly polyunsaturated fatty acids tend to reduce levels of both LDL and HDL, while research indicates that consuming mostly monounsaturated fats tend to reduce only LDL.
The definitive work by Grundy measured the plasma LDL and HDL after four weeks on a liquid diet containing 40% fats from palm oil (saturated fats), high oleic safflower oil (monounsaturated fats), or high linoleic safflower oil (polyunsaturated fats). The poly and monounsaturated fat diets had equal effects on lowering plasma LDL. However, the polyunsaturated diet lowered plasma HDL more frequently than did the monounsaturated diet.
Grundy also compared a high monounsaturated fat diet that contained 40% fat and 43% carbohydrates and a low fat diet that contained 20% fat and 63% carbohydrates. Both diets lowered total plasma cholesterol. The monounsaturated diet lowered LDL by 21% compared with the low fat diet which lowered LDL by 15%. The low fat diet also raised the plasma level of triglycerides and lowered HDL, while the monounsaturated diet had no effect on either of these. Thus, the monounsaturated diet may be as effective as a diet low in fats and high in carbohydrates.
Pecan nuts contain about 65 to 70 percent oils. Approximately 65% of the oil of fresh pecans consists of oleic and 26% linoleic fatty acid. Our laboratory has consistently found high quality ‘Desirable’ pecans to have as much as 74% oleic.
Preliminary work indicates that there are different oleic/linoleic ratios consistent with different native pecan populations in the Colorado, Brazos and Guadalupe River bottoms. Discovery of a population high in oleic acid would be a major break-through because such germplasm could become a vital part of pecan breeding.
The pecan research team at Texas A&M University has identified a native pecan in Freestone County, Texas that has an oleic content comparable to that of olives. This native from Freestone County has been placed in the USDA Germplasm Repository as breeding material. Work should be undertaken to learn the inheritance pattern of the oleic/linoleic ratio in pecans. Future breeding procedures could be developed that would lead to higher quality nuts.
Pecans contain oil that compares very favorably with oil from other oil seed crops. These oils were purchased from a local grocery store and run through a gas chromatograph. The expeller pressed pecan oil came from a retail supplier in California. The low oleic content, compared with our own cold pressed nuts, indicates that the expeller nuts had probably been out of storage so long that they had lost much of their quality through oxidation of the monounsaturated fatty acids to polyunsaturated fatty acids and smaller carbon fragments. If the pecan oil industry is to evolve into a viable industry much more consideration should be given to the quality of the oil stock.
Only olive and canola oils compare favorably with high quality cold pressed pecans. Pecan oil could be marketed competitively with olive oil, but canola oil is lower in price. Pecan oil is now consumed as a part of the nut and it is delicious. In the future it could be marketed as a salad oil because the pecan flavor in the oil could make it more popular than less flavorable oils. A long term objective would be to produce a cooking oil for food processors.
In the storage of pecans, the oliec/linoleic ratio decreases with time. However, the conversion from monounsaturated to polyunsaturated fatty acid was found to be delayed by drying the nuts to 4% moisture immediately after early harvest. Improper drying can lead to darker seed coats and a considerable increase in free fatty acids, both of which are characteristics of deteriorating quality. Research points to the best drying temperature to be 35 degrees C (95 degrees F) with an air volume of 21 m3/sec (45 CFM).
Pecans can become an important health food because of their high level of monounsaturated fats. Dietary research with pecans needs to be conducted to confirm their value in lowering LDL content while maintaining HDL content of the blood plasma. Clinical work is now underway by Scott Grundy of the Southwest Texas Medical Center in Dallas, using high quality pecans from Texas. It is hopefully anticipated that the same positive results can be found with pecans as with high-oleic safflower because the same monounsaturated fatty acid can be found in both .