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Palm Kernel Expeller (PKE), also widely known as Palm Kernel Cake (PKC) or, in Indonesia, Bungkil Sawit, is a significant by-product derived from the palm oil industry. It is the residual solid material remaining after the extraction of oil from the kernel of the palm fruit, harvested from the oil palm tree (Elaeis guineensis). Historically considered an agricultural waste product, PKE is now recognized as a valuable commodity in international trade, primarily utilized as an ingredient in compound animal feeds for livestock, particularly for ruminants. Its widespread availability, cost-effectiveness, and nutritional profile have made it a staple in the global feed market, with major producers like Indonesia and Malaysia exporting vast quantities worldwide.

Beyond its primary role in animal nutrition, ongoing research explores its potential in biofuel production, as a substrate for industrial enzyme manufacturing, and in the creation of biodegradable materials, highlighting its growing importance in a circular economy.

History and Global Production

The oil palm tree, native to West Africa, was introduced to Southeast Asia in the early 20th century as an ornamental plant. Its commercial potential was soon realized, leading to the establishment of vast plantations, first in Malaysia and later in Indonesia. The rapid expansion of the palm oil industry, particularly from the 1970s onwards, resulted in a corresponding surge in the production of its by-products, including PKE.

Initially, the disposal of PKE posed an environmental challenge for palm oil mills. However, as the demand for animal feed ingredients grew, its potential as a feedstuff was investigated and confirmed. Today, the PKE industry is intrinsically linked to the palm oil market. Indonesia and Malaysia are the world's undisputed leaders in palm oil and, consequently, PKE production, accounting for over 85% of the global supply. This production has significant economic implications, providing a secondary revenue stream for palm oil producers and supporting a global logistics and trade network.

The global trade of PKE is substantial, with the European Union (particularly the Netherlands and Germany), New Zealand, South Korea, and Vietnam being major importers. New Zealand, for instance, relies heavily on PKE as a supplementary feed for its large dairy industry to manage pasture shortfalls.

Production Process

The quality, nutritional composition, and physical characteristics of Palm Kernel Expeller are heavily influenced by the oil extraction method used on the palm kernel. The two predominant industrial methods are mechanical extraction (expeller pressing) and solvent extraction.

Preliminary Processing

Before oil can be extracted, the palm kernels themselves must be processed. After the palm fruit is pressed to extract crude palm oil (CPO) from the mesocarp, the remaining nuts are separated. These nuts are then dried and cracked to release the inner kernel. The kernels are cleaned to remove shell fragments and other foreign materials, as the amount of residual shell can significantly affect the final PKE's fiber content and overall quality. The cleaned kernels are then typically broken into smaller pieces and conditioned with heat (flaking) to rupture the cell walls and facilitate more efficient oil release.

Mechanical Extraction (Expeller Pressing)

Mechanical extraction is the most common method for processing palm kernels. It involves forcing the conditioned kernels through a barrel-like chamber at high pressure using a rotating screw (the "expeller" or "press"). The immense pressure physically squeezes the oil out through small openings in the barrel, where it is collected. The solid, de-oiled portion is extruded as a dense cake, which is then broken up and cooled to form Palm Kernel Expeller.

Characteristics of Expeller-Pressed PKE: This is the most common type found on the market. It typically has a higher residual oil (fat) content, usually ranging from 8% to 12%. This higher fat content makes it more energy-dense but can also make it more prone to rancidity if not stored correctly. The intense pressure and resulting friction also generate heat, which can sometimes lead to a darker color and a "toasted" aroma. This process does not involve any chemicals, which is often viewed as a positive attribute.

Solvent Extraction

Solvent extraction is a more chemically intensive but highly efficient process. After light pre-pressing to remove a portion of the oil, the kernel material is washed with a solvent, most commonly hexane. The solvent dissolves the remaining oil, and the resulting oil-solvent mixture (called the miscella) is drained away. The mixture is then heated to distill the solvent for reuse and isolate the crude palm kernel oil. The remaining solid flakes are toasted with steam to remove any residual solvent, a process known as desolventizing-toasting (DT). The final product is more accurately termed Palm Kernel Meal (PKM) to distinguish it from the expeller-pressed version.

Characteristics of Solvent-Extracted PKM: This method results in a product with a very low residual oil content, typically 0.5% to 3%. Consequently, it has a lower gross energy value but a proportionally higher concentration of other nutrients like protein and fiber. The product is often finer and paler in color compared to expeller-pressed PKE.

Chemical Composition and Nutritional Profile

Palm Kernel Expeller is classified as a medium-grade protein and energy source, but its defining characteristic is its high fiber content. The exact composition varies depending on the production method, the genetic variant of the oil palm, and the efficiency of the de-shelling process.

Typical Nutritional Composition of Palm Kernel Expeller (Dry Matter Basis):

Nutrient	Typical Range (%)	Key Considerations
Crude Protein	14 - 20%	Deficient in essential amino acids like lysine, methionine, and tryptophan. Protein digestibility can be low.
Crude Fiber	13 - 20%	High concentration of Non-Starch Polysaccharides (NSPs), primarily mannan. Gritty texture due to high fiber.
Ether Extract (Fat)	8 - 12% (Expeller)	A good source of energy. High in palmitic and lauric acid. Prone to rancidity if not stored properly.
Ash	3 - 6%	A good source of key macrominerals.
Shell Content	5 - 15%	A key determinant of quality. Higher shell content increases crude fiber and reduces digestibility.

Protein and Amino Acid Profile

While the crude protein content appears moderate, its biological value for monogastric animals (like poultry and pigs) is limited by a poor amino acid profile. It is notably deficient in the most critical essential amino acids—lysine, methionine, and tryptophan—which are crucial for muscle growth and egg production. Furthermore, a portion of the protein can be bound within the fibrous cell wall structures, making it indigestible.

Fiber Components

The fiber in PKE is unique. Unlike the cellulose-rich fiber found in many other grain by-products, PKE fiber is dominated by mannan, a non-starch polysaccharide (NSP). This high mannan content (PKE can contain up to 35% β-mannan) is a primary anti-nutritional factor for poultry, as they lack the endogenous enzymes to break it down. In the gut, undigested mannan increases viscosity, hinders the digestion of other nutrients, and can promote the growth of unfavorable bacteria. However, for ruminants, these mannans can be fermented by rumen microbes.

Mineral Content and Anti-Nutritional Factors

PKE is a rich source of several essential minerals, including Magnesium (Mg), Phosphorus (P), Iron (Fe), Copper (Cu), and Zinc (Zn). The high copper (Cu) content, often exceeding 30 mg/kg, poses a significant risk for sheep, which are highly susceptible to copper toxicity. Diets for sheep must therefore limit PKE inclusion to avoid accumulation of copper in the liver. Another anti-nutritional factor is the "grittiness" from residual palm kernel shells, which can reduce palatability and feed intake if present in high amounts.

Applications in Animal Nutrition

The primary global use of Palm Kernel Expeller is in animal feed formulations, where its cost-effectiveness makes it an attractive ingredient. However, its application varies significantly depending on the species.

Ruminant Feed (Cattle, Sheep, Goats)

Ruminants are the best-suited animals to utilize PKE effectively. The microorganisms in the rumen can ferment the high fiber content, including the mannans, to produce volatile fatty acids (VFAs), which are a primary energy source for the animal.

Dairy Cattle: PKE is widely used in total mixed rations (TMR) for dairy cows. It provides a good balance of energy and protein and its high fiber content helps maintain healthy rumen function and butterfat levels. Its high phosphorus and magnesium content are also beneficial. Inclusion rates typically range from 15% to 30% of the total diet, though rates up to 50% have been used without adverse effects on milk production in some studies. Beef Cattle: In feedlot operations, PKE serves as a cost-effective bulk ingredient to support growth and fattening. Its balanced energy profile contributes to weight gain, and it is often mixed with other energy sources like molasses to improve palatability. Sheep and Goats: PKE can be used for sheep and goats, but with caution due to the high copper content. Formulations must be carefully balanced to avoid toxicity, and inclusion levels are generally kept lower than for cattle.

Monogastric Feed (Poultry and Swine)

The use of PKE in poultry and swine diets is more challenging due to their simple digestive systems, which cannot handle high levels of fiber well.

Poultry (Broilers and Layers): High levels of PKE in poultry diets can depress growth, reduce feed intake, and lower egg production. The high mannan content leads to poor nutrient absorption. Consequently, PKE inclusion is typically limited to less than 10% in broiler diets and slightly higher in layer diets. Swine: Pigs are slightly better at digesting fiber than poultry, but high PKE levels still pose challenges. For young piglets, inclusion is generally avoided. In grower-finisher pigs, it can be included at levels of 10-20% as a partial replacement for more expensive ingredients like corn or soybean meal, but performance may be impacted.

Environmental and Economic Considerations

Palm Kernel Expeller plays a dual role in the context of the palm oil industry. On one hand, its utilization is a prime example of valorization, turning a high-volume waste stream into a valuable product. This improves the overall efficiency and sustainability of palm oil production and reduces the environmental burden of waste disposal.

On the other hand, the PKE industry is inextricably linked to the controversies surrounding palm oil production, including issues of deforestation, biodiversity loss, and carbon emissions from land-use change. The sustainability of PKE is therefore dependent on the sustainability of its parent industry. Certification schemes like the Roundtable on Sustainable Palm Oil (RSPO) are working to address these issues, and sourcing PKE from certified sustainable sources is becoming increasingly important for buyers in environmentally conscious markets.

Economically, PKE is a vital commodity. It provides a stable, low-cost feed ingredient that supports the economic viability of livestock industries globally. For producing nations like Indonesia, the export of PKE is a significant source of foreign revenue. The industry supports jobs not only in production but also in logistics, shipping, and trade.