Refining Vegetable Oils: Chemical And Physical Refining

Have you ever paused to think about the cooking oil you use so easily in your kitchen? It's an important ingredient in the preparation of meals. But have you ever thought about the origins of these culinary ingredients?

The process is quite interesting. Vegetable oil comes from various sources like soybeans, rapeseed, and even palm and olive fruit. In these oils, there's a mixture of components, with triglycerides taking center stage, making up about 98% of the composition.

In the remaining 2%, you receive a mixture of other components, some of which are very healthy and beneficial like vitamins and antioxidants. But that 2% also hosts some troublemakers that can affect the taste and quality of the oil, like impurities and unwanted substances. These products are not health hazards, but they are like uninvited guests in the oil.

Sometimes there are even chemicals like pesticides, metals, and other undesirable substances. Their origin? Well, it can be traced back to the environment where the crops are grown, how the seeds are transported, and even how the crude oils are processed and stored.

Refining crude oils should strike a balance between getting rid of the bad stuff and preserving the good while retaining desired qualities, maintaining safety and quality standards

In this article, we will explore the two methods of oil extraction in the vegetable oil refining industry: chemical refining and physical refining.

But first let's go through the history of vegetable oil.

What is the history of vegetable oil?

The history of vegetable oil is a remarkable journey that goes back thousands of years. It all started when our resourceful forefathers realized the potential hidden inside the plants they had at hand, making cooking oil a reality.

Imagine a world, where people learned how to use fire and homemade stoves to heat oily substances from plants. The outcome? The plants produced precious oil, which could be extracted. This was the beginning of the rich history of vegetable oil.

Ancient China and Japan were in the forefront of this culinary innovation, producing soybean oil as early as 2000 B.C. Meanwhile, in the Mediterranean, southern Europeans were creating their own culinary wonder, olive oil, as early as 3000 B.C.

However, the story was not the same everywhere. In Mexico and North America, peanuts and sunflower seeds took centre stage. People roasted and beat them into a paste, then boiled this resultant mixture in water. They were roasted, pounded into a paste, followed by this mixture cooked in water.

Africans, on the other hand, have their own unique method. They shredded and beat palm kernels and coconut meat before heating the end product pulp to a boil, skimming hot oil from the water. Interestingly, palm oil had the highest volume of production, reaching 73.83 million metric tons in 2021/2022.

What's more, before the 1800s, the oil was extracted by using a stamper press. Fast forward a bit, an English engineer named John Smeaton had invented the "roll mill" to make vegetable crushing easier. The stamper press was modified by Joseph Bramah, giving life to the "hydraulic press."

But the real game-changer emerged in the end of the 19th century in the United States. V.D. Anderson introduced the "Expeller." Using this mechanism, oil seeped out of the press's slots, and they increased the pressure to force even more oil through the cage.

As time passed, people improved their oil extraction techniques. They began using solvents, which turn-out to be a game changer, and methods significantly improved over the years. And guess what? We're now getting up to a stunning 90% oil output, an enormous improvement from the modest 10% they used to get back in the 19th century. In the 2021/22 crop year, vegetable oil production reached over 200 million metric tons worldwide.

As technology evolved, the possibilities expanded. Some seeds that were once considered waste have now been ingeniously transformed into oil sources. This ongoing culinary adventure keeps evolving.

So, the next time you reach for that bottle of vegetable oil, remember the incredible journey that brought it to your kitchen, spanning millennia and continents. It's a testament to human ingenuity, showcasing how we've harnessed nature's gift to enhance our culinary experiences, making the ordinary extraordinary.

How is vegetable oil processed in the Chemical Refining Method?

Chemical refining has been a traditional method since ancient times. The process removes Free Fatty Acids (FFA) through acid-base neutralisation. Alkali substances are used to neutralise the acidic components in crude oil. However, during the edible oil production process, soap can be produced. To deal with this you have to use either a washing method or a centrifugal separation method to get rid of the soap. In the bleaching phase, you have to apply activated clay under vacuum conditions to eliminate colour and capture any undesirable metal elements.

It is suitable for all fats and oils, even if they are slightly deteriorated. Below, we have provided a detailed procedure for the chemical refining method:

Degumming

Degumming is the first step to remove gums from the crude oil. These gums are primarily composed of phosphatides, entrained oil, and meal particles. Phosphatides can absorb water and turn oil-insoluble. In the refining world, we use various degumming techniques: chemical, enzymatic, and membrane degumming.

Chemical Degumming

Chemical degumming essentially treats crude oil with various substances. Here's a quick rundown of the categories:

• Water Degumming: Soft water, about 2–3% of the total phospholipids content in crude oil, is added to hot oil and intensely mixed. Then settle or centrifuge to remove gums and lower phosphorus content to around 12–170 ppm.
• Acid Degumming: This method is utilized to convert non-hydratable phospholipids into phosphatidic acid and calcium/magnesium biphosphate salts. Crude oil is treated with phosphoric acid at around 90°C to settle the non-hydratable phospholipids. There are variations like super acid degumming carried out at lower temperatures for even lower phosphorus content.
• Dry Degumming: Concentrated acid (70–80%) is added to hot crude oil to decompose the metal salts of acidic phospholipids.
• Acid Refining: In this process, acid-pretreated oil is neutralized with caustic acid. The resulting caustic soaps are efficient in reducing non-hydratable phospholipids and achieving a low phosphorus level (10 ppm).
• Organic Degumming: Add an aqueous solution of organic acid (usually citric acid) to create three phases: a heavy phase with citric acid (which can be recycled), a lighter phase with oil, and an intermediate phase with gums. It's capable of producing oil with less than 10 ppm phosphorus content.
• EDTA Degumming: Introduce a chelating agent like EDTA in water to the oil, and sometimes use a detergent like SLS for better interaction between the non-hydratable phospholipids in the oil phase and the chelating agent in the water phase. However, emulsion stability can be challenging.

Enzymatic Degumming

Phospholipids in crude oil can have emulsification properties that affect the oil's oxidative stability. Enzymatic degumming uses enzymes to selectively cleave the polar and nonpolar parts of phospholipids. Various types of phospholipases are utilised, each targeting specific aspects of these molecules.

For example, the EnzyMax process employs phospholipase A2 for degumming, resulting in very low residual phosphorus levels in the oil. The process involves maintaining specific pH, temperature, and mixing conditions. Enzyme recycling can be challenging and may introduce side products that affect oil quality.

Membrane Degumming

Phospholipids can be removed via ultrafiltration with a polymeric membrane. This process allows degumming without the use of solvents. However, using membranes at an industrial scale has challenges such as in maintaining membrane stability in organic solvents. Membranes used in organic solvents often have shorter lifespans. Also, efficient cleaning protocols are still under development.

Neutralisation

Neutralization tackles free fatty acids to keep the oil fresh.

Here's how it works: We add caustic soda (NaOH) to the oil. Caustic soda reacts with FFA to form soap, and this reaction looks like this:

R-COOH (acid) + NaOH (base) ⟶ R-COONa (soap) + H2O water But, if we're not careful, caustic soda might also react with the good oil itself.

Triglyceride + Caustic Soda ⟶ Soap + Glycerol

To avoid this, we have to be precise with the amount of caustic soda we use, especially when the oil isn't very acidic.

Here's a tip: Caustic soda doesn't work well if there are more than 15 grams of FFA for every 100 grams of oil because it can create too much soap, and we lose good oil in the process.

Neutralization not only gets rid of FFA but also cleans up the oil by removing other impurities. However, we need to be careful because sometimes, we might lose some good stuff too.

The soap formed during this process is heavier than the oil, so they naturally separate. We then wash the oil with water to remove the soap and any remaining impurities. The leftovers, which we call soapstocks, can be used in various ways, like making biodiesel or feeding animals.

Washing and Drying

Washing and drying is a crucial operation that removes any remaining alkaline substances, like caustic soda and excess soap, as well as tiny traces of metals phospholipids, and other impurities from the oil.

• Preparation: To make sure we get things right, the crude oil needs to be well prepared. This helps prevent troublesome emulsions. We'll use hot water, ideally at 85-90°C, making up 5-15% of the treated oil.
• The Washing Process: We heat the oil with steam in a plate heat exchanger and then mix it with water in a centrifugal mixer. Next, it undergoes centrifugation to get a thorough water wash.
• Drying Out: The water-washed oil is then dried using a vacuum dryer until its moisture level falls below 0.1%. This step is essential before we move on to bleaching

Remember, moisture in water-washed oil can clog filters, especially when there's soap in the mix. So, proper drying is a must.

Bleaching

To bleach your oil followthe below steps:

• Prepare the oil by removing impurities like soap, phosphatides, and metals through neutralization and washing.
• Add special bleaching agents (neutral earth, activated earth, or activated carbon) to the oil, typically 0.15 to 3% of the oil's weight.
• Heat the mixture under a vacuum for 15 to 20 minutes at a temperature between 70 to 110°C to activate the bleaching agents.
• Consider using silica hydrogel for efficient absorption of impurities.
• If the oil has low phosphorus levels, use silica hydrogel initially and then add bleaching earth in the second stage to remove any remaining color.
• Finish by filtering the oil to remove the bleaching agents and any captured impurities.

Dewaxing or Winterization

In the winterization process, you're dealing with vegetable oils that have waxes in them, like sunflower, maize, or rice bran. When it gets cold, these waxes turn into tiny crystals, making the oil cloudy.

Here's what you do:

• Cool the oil to 10-15°C.
• Let it sit for a few hours so the waxes solidify.
• Pass the chilled oil through a filter.
• The filter separates waxes from clear oil.
• Result: clear, waxy-free oil.

Deodorization

The final step of edible oil refining, deodorization removes unwanted odors and flavors.

How Does It Work?

Your oil undergoes the deodorization process with these steps:

• Vacuum-steam distillation at high temperatures.
• Elimination of free fatty acids and volatile components responsible for undesirable flavors and odors.
• Removal of pigments, pesticides, and specific fatty acids.

The method of effective deodorization is like a recipe, with

• Vacuum levels of 2 to 4 mbar
• Temperatures between 200°C to 260°C
• A waiting period of 15 to 150 minutes in a continuous deodorizer.

While deodorization works wonders for removing unwanted odors and flavours, it has some trade-offs. You might lose some beneficial compounds like tocopherols, polyphenols, and sterols. High temperatures can also lead to less desirable reactions, such as trans-fat formation and unwanted changes in the oil's composition.

How is vegetable oil processed in a Physical Refining Method?

First Step - No More Soap: So, when you're physically refining vegetable oil, you're refining oil without the soap part. In chemical refining, they use caustic soda to remove free fatty acids. But in this process, we skip that step.

Three Main Steps: In this process, there are three main steps:

Degumming

First, you get rid of those phosphatides. It's crucial in physical refining and has to be done with care.

To check how well your refined oil sample has been degummed, you can use a test called "Degumming Efficiency." It's pretty simple, just follow this formula:

“Degumming Efficiency” (g100 g)=(P0−Pd)P0∗100,

Here, P0 is the initial phospholipids (in ppm) in your crude oil, and Pd is the remaining phospholipids (in ppm) after degumming.

Bleaching and Filtration

Just like in chemical refining of edible oil, this process makes your oil pristine. It will reduce levels of some colored pigments like carotenoids and chlorophylls. But that's not all – it also helps get rid of residues of phosphatides, phospholipids, contaminants, lipid peroxidation products, and other impurities.

Here's what happens:

• Mix your oil with acid-activated bleaching earth or another adsorbent.
• Maintain a standard bleaching temperature of 368–378°K (95–108°C).
• Filter out spent adsorbent, along with unwanted carotenoids and impurities.

After this step, your oil is ready for deodorization, which uses the same conditions as chemical refining.

Deodorization

Let's dive into the deodorization process. Here's what it aims to achieve and how it works:

Objectives of Deodorization:

• Remove Unwanted Stuff: It gets rid of volatile components like free fatty acids, off-flavors, and contaminants (pesticides, light polycyclic aromatic hydrocarbons), but it might also partially reduce some healthy compounds like tocopherols and sterols.
• Bleach and Purify: Deodorization thermally bleaches coloured pigments and peroxides.
• Enhance Quality: The main goal is to make the oil better in quality, appearance, and flavor stability for a longer shelf life.

Key Process Parameters:

• Stripping Steam: The amount of steam used.
• Time: How long the process lasts.
• Pressure: Done under low vacuum pressure.
• Temperature: Deodorization happens at high temperatures (>473°K) (>200°C), but this can lead to the formation of unwanted by-products.

Dewaxing

Sometimes, if your oil is loaded with waxes (like corn, rice bran, canola, or sunflower oils), you may have to do a dewaxing process. It's all about tailoring the process to your oil's needs.

In summary, the physical refining of edible oil has several advantages over chemical refining for vegetable oils. The physical refining method is about improving your oil's quality and being kind to your wallet and the planet.

Why Choose Physical Refining: Why, you ask? Well, it's because physical refining is a bit of a green hero. It's kinder to the environment, creates less waste, and it's lighter on your wallet. You'll use fewer chemicals, and less energy, and get more oil in the end. In a nutshell, it's a cleaner, greener, and cost-effective way to get top-quality oil.

However, keep in mind, that it's not for all oils. It shines when you've got oils with high acidity. Depending on how much phospholipids your crude oil has, you might need to tweak the edible oil refining process.

Refining Methods: Physical vs. Chemical

Components	Physical Refining	Chemical Refining
FFA(Free Fatty Acid) Removal	Done during deodorization	Removed in deacidification or neutralization steps
Main Application	Used when FFA removal occurs during deodorization	Suitable for oils with high phospholipid levels (e.g., cottonseed oil)
Degumming Requirement	Often requires thorough degumming	May not require extensive degumming, except for specific oils
Chemical Usage	Requires fewer chemicals, leading to cost-efffectiveness	May require more chemicals due to neutralisation or alkali treatment
Waste Production	Generates less waste makes it eco-friendly	Can produce more waste due to chemical processes
Oil Yield	Provides higher oil yields	May result in slightly lower oil yields

Which Refining Method is Better for Vegetable Oil?

When it comes to deciding the right method for refining vegetable oils, several aspects come into play. The decision should be based on the crude oil's particular quality, financial considerations, and environmental impact.

• Chemical refining is suitable for a wider range of oils. It comes with higher costs, potential side product formation, and slightly lower oil yields. This process must be selected for oil that requires extensive purification.
• Physically refined cooking oil is preferable for oils that don't require extensive degumming. This method is especially suited to particular types of oils. This process is economical and environmentally beneficial.

Making the Right Decision

The choice between chemical and physical refining should align with the oil's characteristics and the goals of cost-effectiveness, waste reduction, and environmental responsibility.

Both methods aim to remove impurities in vegetable oil, including phospholipids, pigments, off-flavors, free fatty acids, and other impurities. The choice between these methods depends on the characteristics of the specific crude edible oil being processed.

In a nutshell, the selection of the refining process plays an important role in the production of high-quality edible oils. Therefore, it should be made thoughtfully, considering the oil's properties and the desired outcomes in terms of cost-efficiency and sustainability.

Conclusion

You need to grasp the chemistry of edible oils and their processing to ensure a top-notch product. The purpose of refining is straightforward: make the oil higher quality, clearer, lighter in smell and colour, more stable, and safer by eliminating contaminants and oxidation products from free fatty acids (FFA). Hence, consider the characteristics of different vegetable oils, then choose the best refining method.