Oil from an oil press has an unpleasant odor. What could be the cause?

In our daily life, cooking oil is an essential ingredient in our cooking. Its quality is directly related to the health and taste of our food. However, sometimes we discover that oil on the hydraulic press has a nasty odor. Not only does this affect the texture and taste of the oil, it can also indicate quality issues. For consumers, smelly cooking oil can degrade the dining experience and even raise food safety concerns. For producers, this can mean that their products may struggle to gain market recognition, leading to economic losses. Therefore, it is of great practical significance to study the causes of the odor of oil presses.

Unpleasant Odors caused by the deterioration of raw materials.
Unpleasant raw material deterioration is a common cause of oil odors. Take molds. When raw materials such as peanuts and corn are stored in a humid environment at the suitable temperature, mold thrives. During growth, mold secretes enzymes that break down nutrients in raw materials, triggering a complex series of chemical reactions. Aspergillus flavus, for example, is a potent carcinogen. During deterioration process, the ingredients give off a pungent, musty odor. The odor will be incorporated into the oil pressure process, giving a distinctly musty and unpleasant odor to extract the oil.
Randomness is also a common form of raw material deterioration. Oxidative rancidity occurs when oil in the feedstock comes into contact with oxygen in the air and is affected by factors such as light, heat and humidity. Oxidation produces peroxides, which are further broken down into small molecule compounds such as aldehydes and ketones. These compounds have unique odors, such as acidity. Rapeseed, for example, can easily stink if stored for too long or under inappropriate conditions. The resulting oil will have a strong sour taste, seriously affecting the quality of the oil.
Odors Caused by Contaminated Raw Materials
Raw materials can be contaminated in various forms, and pesticide residue are a common contamination. Pesticides are often used to control pests and diseases during crop cultivation. Improper use, such as overuse, prohibition, highly toxic pesticides or application near harvest season, can lead to excess pesticide residues in raw materials. Some organophosphorus pesticides have a pungent odor. When ingredients containing pesticide residues enter the oil press, the odor is mixed with the oil press, creating a foul odor.
Heavy metal pollution cannot be ignored. Industrial effluent and exhaust gases emissions, as well as the use of fertiliser containing heavy metals, can lead to soil and water contaminated with heavy metals, which are then absorbed by crops. For example, the accumulation of heavy metals such as lead, mercury and cadmium in raw materials not only poses a serious threat to human health but also affects the odor of oils. Chemical pollutants such as polycyclic aromatic hydrocarbons can also enter raw materials through environmental pollution, producing acrid or unpleasant odours.
In the real world, a small oil mill produces oil with a distinct chemical odor by extracting raw materials from contaminated farmland. Excessive levels of heavy metals and organic pollutants in the raw material were detected, which not only affected oil sales but also posed a potential threat to consumer health.

Improper Cleaning of hydraulic press, Unpleasant Odors
After a long period of use, a large amount of oil, dirt and impurities will accumulate in the hydraulic press. At suitable temperature and humidity, these residues undergo a series of chemical reactions. For example, oil in oily dirt is oxidized and decomposed to produce small molecules with unpleasant odors, such as aldehydes and ketones. In addition, leftover impurities can become breeding ground for microbes. As microbes grow and multiply, they break down organic matter, producing volatile substances such as hydrogen sulfide and ammonia. The substances have a pungent odor that can be mixed with freshly squeezed oil to create a foul odor.
For example, a long period of time in an oil press factory without a thorough cleaning of the unit resulted in the accumulation of a thick layer of oil, dirt and impurities in the chamber of the unit. During a pressing cycle, the oil produced had a distinct sour odor. Inspection found that improper cleaning could led to microbial growth, oxidation and grease acidity.
Smell of Aging Components
Key components of an oil press, such as the chamber and screw of an oil press, will gradually age after long use. For example, due to chronic pressure and friction, the materials in the press chamber may wear off, causing the surface to become rough or even crack. These changes increase friction between the combustion chamber and the raw material, producing more heat and accelerating the oxidation and decomposition of the oil. In addition, ageing ingredients release metal ions such as iron and copper, which catalyse the oxidation reaction of oil and produce unpleasant odors.
Plastic components like screws can smell plastic as they age. Some low-quality plastic components break down at high temperatures and pressures to produce volatile organic compounds. These compounds have a pungent odor and can contaminate oil. For example, after running for some time, a hydraulic press found a slight odor of burnt plastic in the oil. Inspections found this

Improper temperature control during oil pressure can produce odors.
Temperature plays an important role in the oxidation and decomposition of oil. Excessively high temperatures greatly speed up the the oxidation rate of oil. Unsaturated fatty acids in oil react easily with oxygen at high temperature to form peroxides. These are further broken down into odorous small-molecule compounds such as aldehydes and ketones, leading to the odor of burning and odour in the oil. For example, in the oil extraction process, if the heating temperature exceeds a certain limit, the oil extracted will have a distinct burning odor, affecting the taste and quality of the oil.
Excessively low temperatures can also adversely affect oil extraction efficiency and product quality. Too low a temperature will lead to poor oil mobility in the feedstock, difficult to separate from the feedstock, resulting in reduced extraction efficiency. In addition, low temperature will lead to condensation of moisture in raw materials, affect oil extraction and product stability, and indirectly lead to oil odor.
Influence of Other Process Parameters (Optional Extension)
In addition to temperature, pressure, time and other process parameters will also have a related effect on oil odor. Excessive pressure will cause impurities and odorous substances in the raw materials to mix more into the oil, affecting the oil's purity and odor. Too long a pressing time will lead to oxidation of oil and produce a bad smell, while too short a pressing time will prevent oil from being fully extracted from the raw materials and affect oil yield and quality.
Source: professional oil engineering book, ``oil extraction technology '', systematically describes the effect of temperature on oil quality during oil pressure process, and provides theoretical support for the analysis of oil deodorization caused by improper temperature control. In this paper, the quality of oil under different temperature conditions is compared and analyzed, and the relationship between oil temperature and flavor is illustrated intuitively.
Comprehensive analysis and Solution of recommendations
Raw materials, equipment and processes are often interrelated and can lead to the unpleasant smell of petroleum products. For example, deterioration of raw materials can accelerate corrosion and contamination of equipment, while improper cleaning of equipment can lead to the breeding of microorganisms, further affecting the quality of raw materials and oils. Improper control of process parameters will aggravate problems of raw material deterioration and equipment wear.
Different solutions can be adopted for different reasons. With regard to raw materials, strict quality control is essential, with the selection of reputable suppliers and rigorous inspection and screening of raw materials to ensure fresh and pollution-free conditions. In the area of equipment, cleaning and maintenance should be strengthened. Detailed cleaning plans and maintenance cycles should be developed. Hydraulic presses should be regularly thoroughly cleaned and inspected, aging parts should be replaced in a timely manner. It is very important to optimize the hydraulic parameters for the hydraulic process. Temperature, pressure and time should be appropriately adjusted to specific raw materials and equipment to ensure optimum condition.
Conclusion:
In summary, the main causes of the odor of oil presses are: raw material spoilage or pollution, improper cleaning of equipment or components aging, the oil press in the processing process temperature control is not in place. These factors are interrelated and together affect the quality of oil. In order to protect the health of consumers and improve the quality of oil products, we must thoroughly investigate and solve these problems. By strictly controlling the quality of raw materials, strengthening the cleaning and maintenance of equipment, and optimizing the parameters of the oil pressing process, we can effectively reduce the odour of oil products and provide consumers with safe and delicious edible oil.