I. Electronic Sorting Technology
To reduce the aflatoxin content in peanut products and minimize manual sorting in large-scale industrial production, skinned peanuts must pass through an electronic color sorter, often called an “electronic eye.” This device removes peanuts with skin still attached, partially skinned peanuts, and moldy kernels, thereby ensuring product quality. Large-scale peanut processing enterprises worldwide, especially peanut food factories, widely use electronic color sorters.
Developed from electrostatic separators, the color sorter is controlled by an invisible optical sensing device. The average capacity per sorting head is adjustable between 70–250 kg/h, maintaining a constant sorting rate 24 hours a day.
The working principle in peanut processing is as follows: when peanut kernels pass in front of the electronic eyes or photocells, the electronic eyes, spaced at 120-degree intervals, observe each kernel from three angles. The scanned color intensity is compared against a background value provided in the scanning chamber. Qualified products pass through, while a blast of compressed air ejects unqualified products (such as moldy kernels) from a nozzle.
The sensitive photocells detect peanut kernels with skin, skinned moldy peanuts, or spotted peanuts. This triggers the electronically controlled compressed air system of the sorter to blow the defective kernels into a waste chute for collection, thereby purifying the peanut kernels. The sorter’s ability to scan from several angles requires the kernels to pass the photocell in a single, specific orientation. The equipment structure allows kernels to pass through a constant-speed chute, flowing uniformly and unidirectionally past the detection electronic eyes for continuous sorting.
Peanuts that have passed through the color sorter still require manual auxiliary sorting to remove any remaining moldy or skin-on peanuts.
II. Salt Coating Technology for Salted Peanuts
Some have used edible oil to glaze peanut surfaces. The method involves adding about 0.5% salt to the hot peanuts immediately after they are removed from the roaster. When the peanuts are half-cooled, oil is added, followed by another 0.75% salt.
A recently developed method uses monoacetylated zein and diglycerides applied in peanut coating. Zein, a specific prolamin protein in corn, is very similar to shellac. It is a mild, edible, nutritious protein with no usage limit. It has been used for coating nuts and confectionery for years. Spraying it on peanuts can extend the shelf life of salted peanut products. Used alone, zein is very brittle and prone to cracking, so acetylated glyceride esters can be added as plasticizers.
The shelf life of coated salted peanuts can be 3–4 times longer than uncoated peanuts, and even 5 times longer for peanuts with an oleic to linoleic acid ratio greater than 1.2.
Methods for applying salt and coating are as follows:
1. **Rotating Drum/Pan Method (standard among confectioners):** Place 45 kg of peanuts in a rotating disk or drum. Spray or coat with 2.25 kg of liquid peanut coating. For dry peanut kernels, an equivalent amount of salt solution can be added. As the drum rotates, the coating is evenly distributed on the peanuts, producing salted peanuts.
2. **Conveyor Belt Spraying:** The solution is sprayed onto peanuts on a moving metal mesh belt. As the peanuts can tumble on the second or third belts, the salt or coating is wholly applied to the surface.
3. **Screw Mixer Application:** A mixing screw conveyor is used. During conveying, the coating is sprayed onto the tumbling nuts, achieving the coating purpose.
The salt-coating solution requires additives such as distilled monoglycerides, BHA, BHT, TBHQ, and citric acid. The coating is colorless, tasteless, and transparent. The additive dosage is 1%–5% based on the peanut surface area.
Coated peanuts have long-term stability and can be stored for 6 months at room temperature. They have low bacterial counts, contain very few impurities, and the coating protects the kernels from oxidation, retaining their lighter color.
III. Characteristics of Salted Peanuts
Salt loss from the surface of salted peanuts is often caused by careless handling and transportation. When peanuts are fried in liquid oil, even if salt powder is sprayed, the salt tends to fall off after cooling. However, if fried in solid shortening, as the peanuts cool, the solidifying fat sets the salt firmly on the surface. The salt adheres tightly, even during handling, regardless of vibration. Another method to prevent salt loss is to fry peanut kernels in coconut oil, which solidifies at about 24°C and becomes harder below this temperature.
A method to solve salt loss involves spreading hot peanuts from the fryer or oven on a table for slight cooling. Then, hot coconut oil (melting point 26.6°C) is applied to the surface. The peanuts are much hotter than the oil, so the oil does not penetrate deeply but remains on the surface. When the coconut oil is evenly distributed by mixing, salt is added. As the peanuts cool to room temperature, the salt naturally solidifies onto the surface. This method effectively adheres salt to the kernels, thereby overcoming salt loss.
The type of salt used is also essential. Granular salt adheres more difficultly to the kernel surface than flake salt under the same conditions. The best salt is refined, consisting of 100% uniform crystals.
Because peanuts are high in oil, and because copper and iron accelerate fat/oil rancidity, salt actually inhibits the rancidity-promoting effect of trace copper and iron. In the best salted peanut products, the average contents of copper and iron should be kept below 1.5×10⁻⁶, and the calcium and magnesium content should also be controlled to minimize oil rancidity.
The salt content in salted peanuts is about 2% of the peanut weight. The specific amount can vary based on processor requirements, different regions, and customer taste preferences. Salt is often fed onto the hot, fried peanuts through a hopper.
IV. Salt and Hypertension
Salt is widely used in peanut products like salted peanuts, peanut butter, salted in-shell peanuts, and boiled salted in-shell peanuts. The salt content in these products is generally around 2%. Excessive salt intake is known to affect human health.
Salt-free peanut foods include raw peanuts with skin, roasted in-shell peanuts, dry-roasted unsalted peanuts, unsalted peanut meal powder in confectionery, unsalted peanut butter powder, natural peanut butter, peanut protein isolate, ice cream, and salad dressing.
Hypertension is an increasingly common condition directly related to blood sodium levels. Blood pressure decreases when dietary sodium chloride is very low. Therefore, the salt content of peanut butter must be strictly controlled.
V. Application of Antioxidants
Synthetic antioxidants, including propyl gallate and citric acid dissolved in propylene glycol, when mixed with salt, can extend the shelf life of fried peanuts by 50%–200%.
When antioxidants are added to salted peanuts, they are often combined with synergists or metal deactivators. In many cases, natural antioxidants are phenolic and can be sourced from nature or synthesized. In contrast, synergists or metal deactivators are acids at specific concentrations, such as ascorbic acid, citric acid, tartrates, or phosphoric acid. Antioxidants used in peanut butter include Vitamin E, lecithin, BHA, BHT, and TBHQ.
There are three primary methods for applying antioxidants to roasted peanuts:
1. **Adding Antioxidants to Frying Oil:** This is a suitable method for treating kernels. Adding 0.125%–0.25% of an antioxidant (e.g., 24.8% propyl gallate, 22.5% citric acid, 52.7% propylene glycol) to the frying oil significantly reduces rancidity. However, continuous frying at 162–190°C can reduce the antioxidant’s effectiveness.
2. **Adding Antioxidants to Salt:** This is a simple and effective method. Add the antioxidant mixture (e.g., 405 parts) to 100 parts of salt. This salt is then applied at a rate of 2% of the peanut weight. Using this process can more than double the shelf life of salted peanuts due to the antioxidants.
3. **Spraying Antioxidants:** This can be done continuously or intermittently. In continuous spraying, antioxidants are applied in proportion to peanuts on a conveyor belt before frying. In production, an air compressor is often used for spraying. The dosage can be adjusted based on production needs, conveyor speed, and nozzle flow rate. The batch process involves intermittently spraying antioxidants onto the kernels.
Peanuts coated with a mixture containing distilled monoglycerides and an antioxidant can have a shelf life extended by 12 months, with the antioxidant dosage controlled at 10 ppm.
VI. Packaging of Salted Peanuts
Early salted peanuts were all skin-on. With advances in technology, automatic packaging machines with or without vacuum functions emerged, changing packaging and retail methods. Salted peanuts are sold skinned or skin-on, alone or mixed, packaged in flexible cellophane or plastic bags, or vacuum-packed in various types and styles of tins or glass jars. Improved packaging reduces air and light exposure, prevents moisture absorption, controls mold growth, and thus extends shelf life. Using antioxidants in packaging materials also extends shelf life. Antioxidants can be added to wax (e.g., paraffin) used in waxed paper or cardboard, which also helps preserve salted peanuts. Beeswax can replace paraffin to prevent oxidation and off-flavors.
When materials like polyethylene, cellophane, or others are used for bags, they can be treated similarly to waxed paper for packaging salted peanuts. A challenge with incorporating antioxidants into packaging materials is their loss due to volatility and steam distillation, which can result in up to 90% loss during use.
New, improved processing and packaging methods for salted peanuts involve adding monosodium glutamate to fried peanuts to enhance their natural flavor, then vacuum-packaging. Alternatively, small pouches made from aluminum foil and polyethylene composite film can be used for packaging, often filled with inert gas.
