Roasters used for manufacturing peanut butter can be either continuous or batch-type equipment. Batch roasters are suitable for small-scale peanut butter production, whereas large-scale operations require continuous, industrial-sized roasting systems.
After cleaning, removal of impurities, and ferrous materials, the peanuts can be sorted by variety and size before being routed for roasting. Since peanuts contain significant moisture, specialized equipment is necessary for this process.
In batch roasting, based on the equipment’s capacity, a typical charge is around 180 kg per batch. Hot air from a furnace, with temperatures as high as 426°C, is used. Within the roaster, the peanuts are heated by this hot air to about 160°C and held at that temperature for 20 to 60 minutes.
Each batch must be roasted uniformly in the batch oven. This ensures consistent color from the center to the surface of every kernel, with no scorched spots, no oil exudation, and no fat breakdown.
Peanut butter made from brown-roasted peanuts has a superior taste, which is why the most popular peanut butters exhibit a deep brown color. Regarding nutritional content, the roasting and skin removal processes reduce the Vitamin B1 (thiamine) content from approximately 9.6 ppm to 0.7 ppm, while the niacin content remains stable at around 17.2 ppm.
Continuous roasting equipment reduces both raw material loss and processing costs. In industrial production, excessively high temperatures are avoided as they can damage the oils, scorch the peanut surfaces, char the skins black, and cause oil smoke and smoky flavors to accumulate in the oven. Burnt cellulose can also contaminate the peanut surfaces, imparting a smoky taste to the final product.
Many continuous roasters, which have replaced batch ovens, utilize hot air to roast. In this method, peanuts are fed into a conveyor-elevator system. During roasting, electricity or natural gas heats the air, which drives the moisture from the kernels. The peanuts move continuously on a conveyor belt, allowing hot air to pass through the bed of kernels. The entire roasting process takes place on this conveyor.
Regardless of whether the hot air is heated indirectly via heat exchangers or directly by burning natural gas/LPG, the roasting temperature typically ranges from 130°C to 170°C. This range is adjusted based on the peanut variety, initial moisture content, and the desired end-product application. After roasting, the peanuts must be cooled as quickly as possible.
Roasting time plays a critical role in determining the final color of the kernels. Peanuts with lower initial moisture content will develop a darker color. The effectiveness of skin removal is not directly related to the roasting temperature.
Fresh peanuts are rich in Vitamin B1. The Vitamin B1 content in raw kernels changes when roasted at different temperatures. Analysis of the finished peanut butter reveals that the Vitamin B1 content decreases as the roasting temperature increases. In peanut butter made from low-temperature roasted kernels, the Vitamin B1 content is only about 20% of the original. It drops to about 14% in medium-temperature roasted kernels, and falls to 10% or less in high-temperature roasted kernels. Kernels roasted at very high temperatures can retain less than 3% of the original Vitamin B1. Superficially, it appears that the darker the color, the greater the loss of Vitamin B1. However, peanut butter made from medium-roasted kernels generally offers a better flavor and taste profile compared to those made from either lightly roasted or heavily roasted peanuts.
The oil content of roasted peanut kernels is slightly higher (47.4% to 47.9%) than that of unroasted kernels, primarily due to the loss of moisture and volatile matter. The skins of raw peanuts contain very little oil. However, the skins separated after roasting can have an oil content as high as 27%, because the seed coats absorb oil from the cotyledons during the roasting process. In roasted peanuts, the peanut oil within the cotyledons and subsequently in the peanut butter has similarly low levels of free fatty acids.
