Peanut butter

A tractor being used to complete the first stage of the peanut harvesting process

Due to weather conditions, peanuts are usually planted in spring. The peanut comes from a yellow flower that bends over and penetrates the soil after blooming and wilting, and the peanut starts to grow in the soil. Peanuts are harvested from late August to October, while the weather is clear.[25] This weather allows for dry soil so that when picked, the soil does not stick to the stems and pods. The peanuts are then removed from vines and transported to a peanut shelling machine for mechanical drying. After cropping, the peanuts are delivered to warehouses for cleaning, where they are stored unshelled in silos.[25]

Shelling must be conducted carefully lest the seeds be damaged during the removal of the shell. The moisture of the unshelled peanuts is controlled to avoid excessive frangibility of the shells and kernels, which in turn, reduces the amount of dust present in the plant.[25] After, the peanuts are sent to a series of rollers set specifically for the batch of peanuts, where they are cracked. After cracking, the peanuts go through a screening process where they are inspected for contaminants.[25]

The dry roasting process employs either the batch or continuous method. In the batch method, peanuts are heated in large quantities in a revolving oven at about 800 °F (430 °C).[26] Next, the peanuts in each batch are uniformly held and roasted in the oven at 320 °F (160 °C) for about 40 to 60 minutes.[25] This method is good to use when the peanuts differ in moisture content. In the continuous method, a hot air roaster is employed. The peanuts pass through the roaster whilst being rocked to permit even roasting. A photometer indicates the completion of dry roasting.[27] This method is favored by large manufacturers since it can lower the rate of spoilage and requires less labor.[25]

After dry roasting, peanuts are removed from the oven as quickly as possible and directly placed in a blower-cooler cylinder.[25] There are suction fans in the metal cylinder that can pull a large volume of air through,[27] so the peanuts can be cooled more efficiently. The peanuts will not be dried out because cooling can help retain some oil and moisture.[27] The cooling process is completed when the temperature in the cylinder reaches 86 °F (30 °C).[25]

After the kernels have been cooled down, the peanuts will undergo either heat blanching or water blanching to remove the remaining seed coats. Compared to heat blanching, water blanching is a new process. Water blanching first appeared in 1949.[25]

Peanuts are heated by hot air at 280 °F (138 °C) for not more than 20 minutes in order to soften and split the skins. After that, the peanuts are exposed to continuous steam in a blanching machine. The skins are then removed using either bristles or soft rubber belts. After that, these skins are separated and blown into waste bags. Meanwhile, the hearts of peanuts are segregated through inspection.[25]

After the kernels are arranged in troughs, the skin of the kernel is cracked on opposite sides by rolling it through sharp stationary blades. While the skins are removed, the kernels are brought through a one-minute hot water bath and placed on a swinging pad with canvas on top. The swinging action of the pad rubs off the skins. Afterward, the blanched kernels are dried for at least six hours by hot air at 120 °F (49 °C).[25]

After blanching, the peanuts are screened and inspected to eliminate the burnt and rotten peanuts. A blower is also used to remove light peanuts and discolored peanuts are removed using a color sorting machine.[25]

After blanching the peanuts are sent to grinding to be manufactured into peanut butter. The peanuts are then sent through two sizes of grinders. The first grinder produces a medium grind, and the second produces a fine grind.[25] At this point, salt, sugar and a vegetable oil stabilizer may be added to the fine grind; this adds flavor and allows the peanut butter to stay as a homogeneous mixture.[28] Chopped peanuts may also be added at this stage to produce "chunky" peanut butter.[25]

Before packaging, the peanut butter must first be cooled in order to be sealed in jars.[25] The mixture is pumped into a heat exchanger in order to cool it to about 120 °F (49 °C).[28] Once cool, the peanut butter is pumped into jars and vacuum-sealed, a process which removes air and deoxygenates the peanut butter to inhibit its oxidation. The jars are then labeled and set aside until crystallization occurs. The peanut butter jars are then packaged into cartons distributed to retailers, where they are stored at room temperature and sold to consumers.[25]

A 2012 article stated that "China and India are the first and second-largest producers, respectively", of peanuts.[13] The United States of America "...is the third-largest producer of peanuts (Georgia and Texas are the two major peanut-producing states)"[13] and "more than half of the American peanut crop goes into making peanut butter."[13]

In a 100 gram amount, smooth peanut butter supplies 597 calories and is composed of 51% fat, 22% protein, 22% carbohydrates (including 5% dietary fiber), and 1% water (table). Both crunchy and smooth peanut butter are sources of saturated and monounsaturated fats (mainly oleic acid) as 25% of total serving amount, and polyunsaturated fat (12% of total), primarily as linoleic acid).[1]

Peanut butter is a rich source (20% or more of the Daily Value, DV) of dietary fiber, vitamin E, pantothenic acid, folate, niacin, and vitamin B₆ (table, USDA FoodData Central). Also high in content are the dietary minerals manganese, magnesium, phosphorus, zinc, copper, and sodium (added as salt during manufacturing). Peanut butter is a moderate source (10–19% DV) of thiamin, riboflavin, iron, and potassium (table).

For people with a peanut allergy, peanut butter can cause a variety of possible allergic reactions, including life-threatening anaphylaxis.[35] This potential effect has led to banning peanut butter, among other common foods, in some schools.[36][37]

Symptoms[38]

• Shortness of breath
• Wheezing
• Tightening of the throat
• Itching
• Skin reactions such as hives and swelling
• Digestive problems

Peanut butter is included as an ingredient in many recipes: peanut butter and jelly sandwiches, peanut butter cookies, and candies where peanut is the main flavor, such as Reese's Pieces, or various peanut butter and chocolate treats, such as Reese's Peanut Butter Cups and the Crispy Crunch candy bar.

Peanut butter's flavor combines well with other flavors, such as oatmeal, cheese, cured meats, savory sauces, and various types of breads and crackers. The creamy or crunchy, fatty, salty taste pairs very well with complementary soft and sweet ingredients like fruit preserves, bananas, apples, and honey. The taste can also be enhanced by similarly salty things like bacon (see Peanut butter, banana and bacon sandwich), especially if the peanut butter has added sweetness.

One snack for children is called "ants on a log", with a celery stick acting as the "log". The groove in the celery stick is filled with peanut butter and raisins arranged in a row along the top are "ants".[39]

Plumpy'nut is a peanut butter-based food used to fight malnutrition in famine-stricken countries. A single pack contains 500 calories, can be stored unrefrigerated for two years, and requires no cooking or preparation.[40]

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  Peanut butter cookies, a popular type of cookie made from peanut butter and other ingredients
• 
  Buckeyes, a type of peanut-butter-based confectionery product

Peanut butter inside a hollow chew toy is a method to occupy a dog with a favored treat.[41] A common outdoor bird feeder is a coating of peanut butter on a pine cone with an overlying layer of birdseed.[42]

In a soft matter context, peanut butter can be considered as a colloidal dispersion, where solid, insoluble peanut particles are suspended in liquid oil. There are two types of peanut butter, and at room temperature, these two types of peanut butter behave differently. Non-stabilized peanut butter, also known as "natural" or "100%" peanut butter consists only of ground peanuts and peanut oil and may contain seasonings, such as salt. In natural peanut butter at room temperature, the insoluble peanut particles separate from peanut oil, and the difference in density causes the peanut oil to float upwards.[43] Stabilized peanut butter contains additional ingredients, such as vegetable oil, to prevent the grounded peanuts and peanut oil from separating into two layers.[43]

During the grinding process, the peanuts release oils, forming a peanut paste consisting of peanut oil and peanut grounds. The grinding process also causes an increase in the overall product temperature, and at this point a stabilizer might be added, such as hydrogenated vegetable oils.[44] At this temperature, the stabilizer melts, uniformly dispersing into the peanut paste. This oil then crystallizes once the product returns to ambient temperatures, and the formed crystalline lattices trap the stabilizer particles within the paste. This prevents the final peanut butter from separating into two separate phases.[45]

Without the stabilizer, the peanut oil alone is not enough, as it is unable to crystallize at room temperature.[46] The melting point of peanut oil is 3 °C (37 °F).[47] At room temperature, the oils in natural peanut butter remain liquid, causing a phase separation. Within the stabilized peanut butter, the microstructural features are able to remain well-dispersed in a matrix of stabilized oil due to crystallization, while in the unstabilized peanut butter, the features are not able to retain the same uniformity.[48]

For most viscous semi-liquid foods, rheological characteristics are determined in shear flow using a coaxial viscometer.[49] However, as peanut butter is not only a highly viscous material, it is also self-lubricating, meaning it releases oils under shear. If placed in a typical coaxial viscometer, the resulting flow pattern a distorted shear flow or plug flow.[49] For accurate data, rheometers typically require no-slip, and the properties of peanut butter do not satisfy this condition. This causes it to be particularly difficult to study its rheology. There have been a few methods devised to overcome this.

Squeezing flow viscosimetery uses two parallel plates to compress a fluid uniaxially[49] This method can be used to better understand the viscoelastic properties of peanut butter. Peanut butter samples can be placed between two lubricated plates, and samples can be subjected to either uniaxial deformation at various constant displacement rates, or to uniaxial creep deformation under various constant loads. As the plates compressed the sample, if the sample retained a cylindrical shape without bulging, this is evident that there is a lack of shear flow.

Using this method, peanut butter has been determined to be a power-law fluid with shear thinning properties.[49] In other words, under high shear rates, there is a lower apparent viscosity. This is likely due to the size difference in peanut and oil particles. The larger peanut particles likely form loosely bound aggregates that break down as shear rate increases (e.g. mixing), which allow the oil to better disperse between peanut particles, resulting in a reduced viscosity.

Another way to overcome the wall-slip effects, is to rough up the contact surface of parallel plate rheometers using a material such as sandpaper.[50] In order to determine if this method sufficiently reduces the wall-slip effects, stress growth experiments can be conducted. If the stress over time is independent of gap size, then wall slip has been successfully reduced.

Apparent yield stress

The apparent yield stress for the stabilized suspension (374 Pa) was significantly larger than the unstabilized sample (27 Pa) under the Bingham model.[50] This is likely due to the effects of the stabilizing agent.[50] During the grinding stage, the stabilizer dispersed around the peanut particles. At room temperature, the stabilizer crystallized around the particles, creating a strong network of particles within the suspension that can resist the onset of flow.[50] In unstabilized peanut butter, the peanut oil remains in a liquid state.[50] Even when the peanut particles are mixed in homogeneously, the peanut butter remains more liquid-like.[50]

Viscosity

Previously conducted creep (stress vs. strain) experiments were conducted to determine the viscosity of peanut butter. In the stabilized peanut butter, under stresses of 250 MPa, the viscosity increases rapidly with increasing strain, exemplifying solid-like behavior.[50] With stresses greater than 250 MPa, stabilized peanut butter displays liquid-like behavior.[50] In an unstabilized sample, the same viscoelastic transitional behavior was found at 10 MPa.[50]

Non-linearity

Both stabilized and unstabilized peanut butter displayed highly non-linear behavior,[50] and the storage (G’) and loss (G’’) modulus was determined. Both peanut butter types have a decrease in G’ and G’’ until critical strain amplitude is reached. Beyond this critical point, both moduli start to increase.[50] The initial observed decrease was likely due to a structure breakdown under strain.[50] Mentioned previously, the increase in strain causes loosely aggregated peanut particles to break, allowing a more homogeneous oil-peanut mixture to form. However, the increase in moduli at a critical strain implies a less homogenous structure is being formed, causing a greater resistance to flow.[50] This might mean at some critical strain, the particles start to behave in a shear thickening manner. A possible reason could be that the maximum volume packing fraction changes with strain amplitude.[50] Meaning at a critical strain, the flow would cause particles to create a less ordered structure resulting in an increase in viscosity.

Complex viscosity

Complex viscosity is a measure of the total resistance to flow as a function of angular frequency.[51] For peanut butter, it was found that the initial complex viscosity as angular frequency increased was very high. However, if the angular frequency was decreased and increased again, a different behavior emerged, and the peanut butter was unable to retain the same initial complex viscosity.[50] This shows that once the existing structure of the sample was broken, the sample's thixotropic effects, or the rheological properties dependent on flow history, are less pronounced.[50]

Grinding time

By varying the grinding time of peanuts, the resulting rheology and texture of natural peanut butter (with no stabilizer) can be affected.[52] More specifically, as grinding time increases, the apparent viscosity decreases. This is likely due to an increase in peanut oil produced by a higher grinding time, causing a lubricating effect to decrease viscosity.[52]

Particle size distribution

Increasing the grinding time also produced peanut butter with a narrower particle size distribution with high densities. As smaller particles can compact better with less void space than larger particles, density would increase as grinding time increased.[53] For shorter grinding times, there is a wider particle size distribution, meaning the overall peanut particle size is less uniform. This results in a wider linear viscoelastic region, and allows unstabilized peanut butter to behave more similarly to stabilized peanut butter.[53] This is because in stabilized peanut butter, the peanuts' protein bodies and cell wall fragments are able to be more uniformly distributed throughout the peanut butter, rather than clumping.[54] If the particle size is more widely distributed, it mimics the particle size distribution of stabilized peanut butter, resulting in a more stable natural peanut butter.[53]

The rheology of peanut butter may affect its best texture, flavor, storage stability, and overall quality. This understanding can be applied when determining better or alternative stabilizers for peanut butter or better grinding manufacturing processes for unstabilized peanut butter to prevent oil separation more effectively.[55][56]