Based on the research studies that we have reviewed about different ways of roasting nuts and seeds, we would start out by saying that there does not appear to be any magical best way of roasting the foods in this amazing food group. Even though nuts and seeds tend to be similar in terms of their total protein, total fiber, and total fat content, their overall nutrient composition is actually quite diverse, and so they can be impacted quite differently by varying roasting temperatures and times.
As a general rule, the most nutrient-rich form for nuts and seeds is their raw form. Provided that your nuts and seeds are high-quality and correctly stored, you are going to get the greatest concentration of nutrients from this raw form. And in keeping with this principle, you will notice that we used nutritional data for raw nuts and seeds whenever possible on our website. (In some cases where good data on raw nuts and seeds was lacking, we did use the dried form for these foods.)
However, we also recognize that raw nuts and seeds may lack many of the delicious flavors and aromas that can be created through roasting. For this reason, we realize that many people will be more likely to take advantage of the nutrient richness found in this food group if their nuts and seeds have been roasted.
Unless you are confident in the roasting methods used with a commercially roasted product, we recommend the home roasting of raw nuts and seeds if you prefer not to consume them in raw form. We make this very general recommendation for two reasons. First, it can be difficult to find dry roasted versions of some nuts and seeds, and the added oil required for oil-roasting can be more susceptible to heat damage than fats that are naturally present within the nuts and seeds. As whole natural foods, nuts and seeds have their own natural protective factors and these factors help protect their fats from damage by heat and other factors. By contrast, processed added oils may not have any natural sources of protection from heat. Second, even when dry roasted nuts and seeds are available, it can be difficult or impossible to determine the temperature used in roasting.
Commercial roasters of nuts and seeds typically follow HACCP (Hazard Analysis Critical Control Points) safety guidelines for roasting as established through the combined efforts of the, U.S. Department of Agriculture, U.S. Food and Drug Administration and Centers for Disease Control and Prevention, and Department of Health and Human Services. Minimum roasting temperature guidelines are generally based on Salmonella heat resistance values and range from 248°F-284°F. However, we recognize that home roasting is different, and we haven't come across any comparable government recommendations for home roasting temperatures. Nor have we come across any evidence to show that different levels of safety are associated with home roasting at different temperatures.
As a general rule, research studies do not show large nutrient changes in association with roasting temperatures 300°F and below. Most of the nutrient changes that we have seen with roasting temperatures between 160°F and 300°F involve—at most—10-20% loss of select nutrients. There are some mixed findings involving certain phytonutrient changes within this roasting range. Also, in some cases, nutrient loss appears to fall into this same range even when roasting takes place at higher temperatures. Based on this general research review, we believe that you can get strong nutritional value from nuts and seeds when using home roasting methods based on this general temperature range of 160°F-300°F.
One aspect of nut and seed roasting of special interest to many people is possible damage to their fat quality when roasting. As a general rule, nuts and seeds are more stable to heat than might be expected given their high fat content. (About 75-80% of the calories found in nuts and seeds come from fat.) This greater-than-expected stability is most likely related to their relatively high percentage of monounsaturated versus polyunsaturated fats. Nuts like cashews and almonds contain 2-4 times as much monounsaturated as polyunsaturated fat, and of these two fat types, monounsaturated fat is less susceptible to damage by heat and other factors. Less susceptible still is saturated fat, whose amount is roughly equivalent to the amount of polyunsaturated fat in cashews and about 33-50% of the polyunsaturated fat level found in peanuts, pumpkin seeds, and sesame seeds. These relatively high levels of monounsaturated and saturated fat help support the overall stability of fat content in nuts and seeds.
Two clear exceptions to this rule would be flaxseeds and walnuts. Both of these foods contain more polyunsaturated fat than any other fat type, as well as high levels of omega-3 polyunsaturated fat. Within all of our eight nuts and seeds, we would single out these two foods as being especially important to protect from heat damage. If not being consumed in raw form, you may want to consider roasting these two particular foods at lower temperatures and/or for shorter periods of time.
Almost all of the studies that we reviewed analyzed roasting times between 10-60 minutes. Based on our own experience with flavors, aromas, texture, and taste, we cannot find any reason to roast at the maximum end of this time period. In keeping with our general WHFoods approach of minimal cooking times with delicious results, we recommend roasting times of approximately 10-20 minutes rather than longer time periods.
We recommend the use of an ungreased stainless steel baking sheet or cast iron cookware for nut and seed roasting. In either case, you will want to create a single layer of nuts or seeds to allow for more uniform air flow around each nut or seed. It is fine to open the oven a few times and shake the nuts or seeds a little bit in order to reposition them slightly and promote more uniform roasting. Just make sure to wear an oven mitt to protect your hand and forearm.
While raw nuts and seeds can usually be stored in tightly-sealed containers in your refrigerator for months-long periods of time, we do not recommend this same approach to home-roasted nuts and seeds. Instead, we recommend the roasting of small amounts of nuts and seeds that you plan to consume over the next couple of days. This recommendation is based on our desire to err on the side of caution with the more delicate nutrient composition of roasted nuts and seeds. Continue to keep your roasted nuts and seeds tightly sealed in the refrigerator, even when planning to enjoy them within a relatively short period of time.
To ensure you are actually cooking at the temperature you have chosen, we suggest that you check the accuracy of the temperature gauge on your oven. This check can easily be done with an inexpensive oven thermometer. You can find an oven thermometer at any hardware or kitchen store, or even the kitchen equipment aisle of your grocery store. Look for a stainless steel, spring-operated dial type that has both a flat bottom for standing and a hook for hanging on the rung of an oven shelf. The cost is typically $5 to $12.
To test your oven, place the thermometer on a center rack, preheat the oven for 15 minutes and then compare the reading on the thermometer to your oven gauge. If it is higher or lower than what your oven gauge indicates, then you know that your oven tends to run either hot or cold and can compensate. For instance, if your gauge reads 170°F, but the thermometer reads 200°F, you know that your oven tends to run 30 degrees hot, and you can adjust the temperature downward to 140°F to compensate.