Rice Processing

Rice Processing

In this post, we’ll discuss rice processing, including its types and varieties, the influence of processing variables, and the different treatments rice can undergo to achieve special characteristics.

Rice, alongside wheat and corn, is one of the most consumed cereals worldwide. However, unlike the latter two, rice is usually consumed with minimal processing, either as whole grain or with the bran and germ removed while retaining its original form, typically cooked in abundant water.

Regarding the significance of rice cultivation and consumption, it’s evident that major producers and consumers are concentrated in East and Southeast Asia. Countries like China, India, Japan, Myanmar, and Cambodia often appear as leading rice producers. Consequently, the FAO established its rice study centre in this region, specifically in the Philippines. However, other countries with significant rice cultures, such as Brazil, also rank among major producers, along with smaller ones like Cuba or some Central American countries. In Europe, principal producers like Spain, Portugal, or Italy are situated in the southern region. Moreover, these countries not only excel in production but also boast rich rice cultures in certain regions.

Rice Quality

Firstly, it’s essential to differentiate rice varieties by their shape. There are elongated varieties, classified as indica, and shorter, rounded ones, classified as japonica. This is typically the primary distinction among rice grains and the main criterion for commercialization, labelled as long or short (indica or japonica). However, highly valued varieties, such as Bomba in Spain, renowned for paella, or Arborio and Carnaroli in Italy, ideal for risotto, can be marketed separately. Among indica rice, Thai and Basmati varieties stand out, prized for their organoleptic aspects like flavour, cooking retention, and final texture after cooking.

Regarding rice texture, the ratio of amylose to amylopectin is a determining factor. Starch comprises amylose and amylopectin, with amylose retrograding after starch gelatinization. During cooking, moisture and heat cause starch gelatinization, followed by retrogradation, leading to rice texture firming and a looser final product. Generally, long-grain rice varieties contain higher amylose levels, hence tend to be fluffier after cooking. This is a simplified explanation, as factors like amylose chain length or amylopectin branching and chain length also influence retrogradation tendency, albeit these complexities are less commonly used for rice quality control. However, traditional retrogradation capacity analysis, such as that conducted with the RVA (Rapid Visco Analyser) after grinding rice to a specific particle size, can be employed. The higher the retrogradation tendency, the looser the rice after cooking.

The loose or sticky nature of rice after cooking isn’t inherently good or bad but a characteristic to consider. Some cultures prefer loose grains, while others prefer stickier ones, and the same applies to certain dishes. Western culture typically favours loose texture in most preparations, such as Spanish paella. However, dishes like sushi benefit from stickier varieties, aiding in rice roll formation. In Eastern cultures, eating with chopsticks is common, where stickier texture facilitates picking rice blocks without grains detaching and falling, hence favouring stickier varieties.

In China and certain Asian regions, glutinous rice cultivation and consumption are common, where the starch is entirely amylopectin (without amylose), resulting in significantly lower retrogradation tendency and a much stickier final rice texture. Despite being called glutinous, these rice varieties are gluten-free and suitable for celiac individuals.

It’s also worth noting that freshly processed rice tends to be stickier after cooking, while stored rice tends to become drier over time. In Western countries, it’s common to store rice for some time before commercialization to enhance its less sticky character.

In addition to organoleptic characteristics, rice quality is significantly influenced by its hardness or fragility. During processing, some rice grains may break, necessitating their removal. More fragile rice varieties tend to break more, causing significant losses in rice industries, hence the need to reduce rice fragility. Both variety and pre-processing treatments influence fragility, with drying being particularly critical. Rice is typically cultivated in high humidity conditions, and when harvested, it contains high moisture content. If not removed, this moisture could lead to microbial growth in the grain, resulting in negative consequences like mycotoxin formation. Various drying methods, either slow at ambient temperature or rapid with increased air temperature, can be employed. The latter, though effective, tends to increase grain fragility. Poor drying conditions often translate to severe losses for producers. The most effective rice drying usually involves a combination of rapid drying when grain moisture content is high, followed by resting to equalize internal and external moisture, and then slower drying when initial moisture has partly reduced and drying becomes critical.

Rice Milling

Once rice is dried, it can be stored before entering the milling process. It’s important to clarify here that while the process of removing the outer layers of rice is termed milling, similar to obtaining flour from wheat, the aim here is not to break the endosperm’s integrity but to obtain whole, white grains.

As with all cereal primary processing industries, the first step involves grain cleaning to remove stones, dust, other plant and animal matter, and other impurities using specific equipment like magnets, sieves, aspirators, de-stoners, etc.

Rice is a husked cereal, so the initial step is husk removal. We discussed husks in detail earlier in this post. Historically, husks were removed using disc huskers, where grains passed between two plates, one fixed and the other rotating, set at a specific height, breaking and separating the husk. Nowadays, these disc huskers have been largely replaced by rubber roll huskers. Although these rollers wear out and need regular replacement, resulting in additional costs, they cause much fewer grain breakages. This underscores the importance of minimizing grain breakage.

The worldwide percentage of husk generated is significant, especially in poor rice-producing countries. Hence, it’s crucial to utilize husks properly. Due to its composition, husk cannot be used in human food. However, extensive research worldwide aims to maximize the utilization of this product. Uses range from livestock bedding to construction material and extraction of certain substances of interest. Burning husks for energy production is the most common application.

Once husked, the grain can be marketed as brown rice, though broken grains generated during processing need to be removed. However, since the consumption of brown rice is much lower than that of white rice, processing continues. Thus, after husking, the grain undergoes pearling, or bran removal. The grain is introduced into special cylindrical equipment where it rubs against the cylinder walls, facing central devices, and between them. This process resembles grain sanding and not only separates bran but also the germ. The grain can be slightly moistened during this process to soften the bran or even use abrasive materials, subsequently removed along with the bran, to aid in separation.

Following this process, the grain exits with separated bran and germ but as a single unit. Achieving proper bran and germ separation requires passing through aspiration equipment, where they are separated by their different densities. The resulting bran and germ mixture contains a noticeable amount of oil (mostly from the germ). Therefore, unless adequately treated, it will quickly become rancid due to exposure of fats to oxygen and enzymes that accelerate this process. To utilize this bran in human food, it’s suggested to undergo a thermal treatment to deactivate oxidative enzymes. Treated bran can be used alone as a fiber source or mixed with white flour to obtain whole rice flour.

It’s also possible to extract oil from the mixture to produce rice bran oil, although it’s commonly referred to as such, it primarily comes from the germ. This nutritionally valuable oil, like that of all cereals, is highly valued in certain regions but is produced in much smaller quantities compared to other oils, including corn oil.

The rice obtained thus far could be marketed as white rice after removing broken grains. However, the greasy nature of the bran and germ mixture may cause some to adhere to the grains. Therefore, it’s customary to pass them through polishers, large brush-like equipment, to separate these impurities from the grain. After this step, the grain goes through aspiration systems to definitively remove these impurities and through separator discs to separate grains broken throughout the process.

In some countries and historical periods, efforts were made to enhance the white and shiny appearance of rice, preferred by consumers. In Japan, a process known as “glazing” was used, where rice was coated with a glucose syrup and magnesium silicate (talc) to increase shine. However, contamination issues with asbestos in magnesium silicate production led to higher stomach cancer incidence. Although viable alternatives exist, this process is rarely used today.

The final product may still contain grains with image issues, such as black spots at the ends, greenish areas, or chalky spots. The only way to separate these unattractive grains is through colour sorter equipment. In fact, these machines were initially used in the cereal industry for this purpose and have now been adopted in wheat milling industry. The process’s previous bottleneck due to its slow speed has been resolved, and today, incorporating these machines into processing is highly feasible.

After all these steps, we have marketable white rice, ready to be packaged in suitable materials.

Broken rice grains constitute a significant by-product that must be effectively utilized. The main uses of this by-product are twofold. Firstly, it’s destined for animal feed, primarily for pets, typically sufficient to package it in appropriate materials. Secondly, it’s milled to obtain flours. In fact, most rice flour mills start with broken grains generated in rice mills. The main challenge here is that these broken grains aren’t properly sorted based on variety or grain hardness, resulting in irregularities in the obtained flours.

In a subsequent post, we’ll delve into special rice varieties, which I’m sure will surprise you.

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