Dehulling is a removal of seed coat from which pulse fiber is produced as a byproduct. Hull of pulses especially peas and lentils contains high antinutrients, lipids which tend to cause off flavor and odor, and most of insoluble fiber (Wood and Malcolmson, 2011). It should be noted that there is few dehulled nor split beans commercially available, reflecting a difficulty of removing hulls of beans from cotyledons compared to pulses such as peas and lentils
Two dehulling and splitting processes are wet and dry milling. Wet process involves soaking for 3 to 14 hours depending on types of pulses, drying to an optimum moisture which is usually 7–11%, milling to dehull and separate large and small particles, and sieving. On the other hand, dry process involves tempering, which can be skipped if case of easy-to-dehull seeds. In contrast, for some seeds that are difficult to dehull, seeds are pitted to increase the moisture absorption and tempering process is repeated. Generally dry process produces better quality split peas and lentils than wet process.
Traditionally, attrition type mills are used for dehulling and splitting processes where two stones are oriented either horizontally or vertically and the gap between the stones are adjusted to the seed size (Wood and Malcolmson, 2011). A roller mill with a carborundum stone can be used. In this mill, the stone is tapered and rotates inside a perforated metal casing where the gap between the roller and casing decreases from the inlet toward the outlet. (Wood and Malcolmson, 2011). Similar to attrition type mills, the roller is mounted slightly downward to facilitate passage of the seed
Factors affecting splitting and dehulling include moisture content, seed size and uniformity of size, and seed hardness (Mangaraj and Singh, 2009; Wood and Malcomson, 2011). Too high or low moisture can increase the breakage and fine particles thus the drying process is critical after soaking/tempering. Uniform seed size is important to maximize the yield; thus, grading and sorting steps become critical. Similarly, the consistency of seed shape is important in order to maximize the yield after dehullling
Milling is defined as a particle size reduction step, which involves both dehulling and splitting in addition to flour milling. Flour milling is the last step of particle size reduction to produce flour, where the flour can be used in many applications. The pulse is stabilized by roasting and/or steam precooking prior to milling. Either process partially gelatinizes the starch, denatures the protein, and inactivates enzymes, thereby increasing product shelf life. Use of dry or wet milling produces different pea flour purities, each with applications suitable to specific food functions. The fiber alone of such products can be enough to capture consumer interest and loyalty. Offering more than 10 grams of natural dietary fiber per ¼ cup, whole pulse flour can be an easy and effective way to achieve the recommended daily intake value of fiber
Depending on the final product, which are usually raw flour or pre-gelatinized flour, either dry milling or wet milling are used to prepare pulse flour. Compared to wheat milling, few studies investigating the impact on milling quality on pulses have been published. Yet, milling is an important processing step to determine the end product quality especially in fractionation of pulses. Air classification is widely used technology to fractionate pulses into protein, starch and fiber. A high degree of cotyledon particle size reduction has to be achieved in order to maximize the protein and starch separation. Impact milling such as hammer milling is commonly used for flour production from dry pulses without pre-treatment. In addition, pin milling is the technique most utilized in achieving fine particle size in pulse milling. Pin milling is used in a combination with air classification to achieve higher degree of protein fractionation. Tyler (1984) evaluated the factors that affect impact milling quality of 8 legumes. He found that milling efficiency, determined as a means of protein separation efficiency, was affected by the amount of crude fiber, water-insoluble cell wall materials present in the seed, and seed hardness (Tyler, 1984).
In terms of different milling methods, Maskus et al. (2016) found that the stone-milling resulted in larger particle size flour in both whole and split yellow pea compared to roller milling and pin milling. In addition, the authors revealed that the particle size remained relatively large in whole pea flour compared to split pea flour due to presence of high dietary fiber content from hull; thus, grinding hulls and cotyledon separately prior to flour milling was preferable to achieve uniform, and finer particle size (Maskus et al. 2016). The exception was the pin milling that enabled the simultaneous reduction of both hull and cotyledon to similar particle size. It is important to note that compositional differences of flour caused by different milling procedures affect the physical and functional properties of pulse flour including water absorption, starch gelatinization properties, flour color, and flavor
Obtaining consistent and ideal particle size distribution has been a challenge for pulse milling industry. The biggest challenge on pulse milling is to obtain flour that uniform in size. Pulses tend to become sticky when milled into flour, which increases the tendency of the flour to stick to processing equipment. Presence of seed coat or hull greatly affects the milling properties such as particle size distribution, screening, flow rate, and milling yield. Impact mills such as a hammer mill is often used to produce whole flour. The mill employs a steel durum containing either a vertical or horizontal rotating shaft, which is fitted with hammer bars. The particle size is reduced until the particles are able to exit through a metal screen (Wood and Malcomson, 2011). Different mills produce different ranges in particle size. Pin mill and roller mill produce the most uniform and finest particle size flour while stone and hammer mill produce flour with the largest and least uniform particle size (Maskus et al, 2016). Particle size differences depend on the processing method as simultaneously milling hulls and cotyledons to the same particle size is difficult on most mills. Only pin mill enables the simultaneous particle size reduction to the same particle size (Maskus et al, 2016). Such differences cause functionality difference. For example, the fine fraction of pulse flour prepared by a hammer mill had a lower initial gelatinization temperature compared to more coarsely milled counterpart. Finer particles tend to contain more starch fractions than protein fractions (Wood and Malcomson, 2011). Furthermore, different fractions provide different sensory profile. Raw pulse flour tends to develop undesirable flavor during storage due to the enzyme lipoxygenase (Wood and Malcomson, 2011). The enzyme promotes degradation of unsaturated fatty acid resulting in hydroperoxide formation, and it is mostly present in the cotyledon of pulses. The enzyme is inactivated by the heat treatment; thus, negative flavors can be reduced by heat treatments such as pre-gelatinization.
The roasting process stabilizes pea flour, partially gelatinizing the starch, denatures the protein, and inactivates enzymes resulting in an increased product shelf life. Roasted pea flour serves as an effective flavor carrier and flavor improver, ideal for making more nutritious flatbreads, tortillas, pita breads, crackers, cookies, energy bars, and extruded snacks. In addition, pre-gelatinized flour has better color and flavor stability, fewer antinutrients, and enhanced functional properties, especially functionalities from denatured protein (Wood and Malcomson, 2011). Processing conditions can be varied depending on the ideal flour properties, but a heat treatment of flour with dry heat for 6–8 min at 104-105 °C have been proven to inactivate the enzyme, lipoxygenase (in case of soybean) (Wood and Malcomson, 2011)