Nut of the Week - Walnuts

Walnut seed shell inside its green husk

A walnut is an edible seed of any tree of the genus Juglans, especially the Persian walnut, Juglans regia. Broken nutmeats of the eastern black walnut, from the tree Juglans nigra, are also commercially available in small quantities, as are foods prepared with butternut nutmeats.

Walnut seeds are high density source of nutrients, particularly proteins and essential fatty acids. Walnut seeds, like other tree nuts, must be processed and stored properly. Poor storage makes walnut seeds susceptible to insect and fungal mold infestations; the latter produces aflatoxin - a potent carcinogen. Mold infested walnut seed batch should not be screened then consumed; the entire batch should be discarded.

Walnuts are rounded, single-seeded stone fruits of the walnut tree. The walnut fruit is enclosed in a green, leathery, fleshy husk. This husk is inedible. After harvest, the removal of the husk reveals the wrinkly walnut shell, which is in two halves. This shell is hard and encloses the kernel, which is also made up of two halves separated by a partition. The seed kernels - commonly available as shelled walnuts - are enclosed in a brown seed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen so preventing rancidity.

There are two major varieties of walnuts grown for its seeds — the English walnut and the Black walnut. The English Walnut originated in Persia, and the Black walnut is native to the United States. The Black walnut is of high flavor, but due to its hard shell and poor hulling characteristics it is not grown commercially for nut production. The commercially produced walnut varieties are nearly all hybrids of the English walnut.

Walnuts, like all seeds, are living organs in which respiration processes dominate. Once harvested, the seeds continually consume oxygen and release carbon dioxide. The storage life of seeds depends, in part, on the rate of this respiration.

The ideal temperature for longest possible storage of walnut seeds is in the -3 to 0 oC and low humidity - for industrial and home storage. However, such refrigeration technologies are unavailable in developing countries where walnuts are produced in large quantities; there, walnut seeds are best stored below 25 oC and low humidity. Temperatures above 30 oC, and humidities above 70 percent can lead to rapid and high spoilage losses. Above 75 percent humidity threshold, fungal molds that release dangerous aflatoxin can form.

Freshly harvested raw walnut seeds with water content between 2 to 8 percent offer the best color, flavor and nutrient density.

Walnuts are one of the several high nutrient density foods. 100 grams of walnuts contain 15.2 gram protein, 65.2 gram fat, and 6.7 gram dietary fiber. The protein in walnuts provides many essential amino acids.

While English walnut is the predominant commercially distributed nut because of the ease of its processing, its nutrient density and profile is significantly different than black walnut.

Unlike most nuts that are high in monounsaturated fatty acids, walnuts are composed largely of polyunsaturated fatty acids (47.2 grams), particularly alpha-linolenic acid (18:3n - 3; 9.1 gram) and linoleic acid (18:2n - 6; 38.1 gram). The beneficial effects of this unique fatty acid profile has been a subject of many studies and discussions. Banel and Hu conclude that while walnut-enhanced diets are promising in short term studies, longer term studies are needed to ascertain better insights.

Raw walnuts contain glyceryl triacylates of the n-3 fatty acid alpha-linolenic acid (ALA), which is not as effective in humans as long-chain n-3 fatty acids, and (mostly insoluble) antioxidants. Roasting reduces antioxidant quality. In 2010, a report published in the Journal of the American College of Nutrition said that walnuts and walnut oil improve reaction to stress.

A study has suggested that consumption of walnuts increases fat oxidation and reduces carbohydrate oxidation without affecting total consumption, suggesting that walnut consumption may improve the use of body fat in overweight adults. Walnuts have been shown to decrease the endothelial dysfunction associated with a high-fat meal. Aged rats fed diets containing 2% to 6% walnuts showed reversal of age-associated motor and cognitive function, but a 9% walnut diet impaired performance, suggesting a J curve.

On October 11, 2006, ScienceDaily published a report which stated "New research shows that consuming a handful of raw walnuts along with meals high in saturated fat appears to limit the ability of the harmful fat to damage arteries," and attributed the result to a 2006 article in the Journal of the American College of Cardiology. The lead researcher, Emilio Ros, MD, PhD, was quoted as saying "People would get the wrong message if they think that they can continue eating unhealthy fats provided they add walnuts to their meals." Funding for the study was provided by the California Walnut Commission, an industry marketing agency.

Scientists are not yet certain whether walnuts act as a cancer chemopreventive agent, an effect which may be a result of the fruit's high phenolic content, antioxidant activity, and potent in vitro antiproliferative activity.

Compared to certain other nuts, such as almonds, peanuts and hazelnuts, walnuts (especially in their raw form) contain the highest total level of antioxidants, including both free antioxidants and antioxidants bound to fiber.