(adapted from Olive Production Manual for Oil, UC ANR):
Virgin olive oil is essentially a minimally processed fresh fruit juice that retains many of the nutritional qualities of fresh olive fruit. The processor uses mechanical means to crush olives to form a paste, then separates the oil from the paste without using heat or solvents.
Processing facility
Siting considerations for olive processing facilities include convenient road access, proximity to production areas, access to water, electricity, and propane or natural gas, and availability of staff. A typical processing line to produce virgin olive oil includes a crusher, malaxer, decanter, separator, and oil storage.
Receiving
Processors should inspect the fruit upon receiving it to assess its condition and maturity index. Processors also should analyze oil and moisture content to decide on the parameters for paste preparation in the crusher and malaxer.
Growers generally achieve better oil extraction by delivering fruit with moisture levels in the 50 to 56 percent range. Fruit with a moisture level below 50 percent produces a dry paste, which poses a significant operational challenge to the processor and may affect the oil’s sensory quality by giving it “dry” flavor notes. Fruit with a moisture level above 56 percent produces a paste prone to emulsions, and the oil is more difficult to extract. To maximize oil yield at the processing facility, it’s important to carefully implement a preharvest irrigation plan in the orchard.
Low free fatty acidity, which is an indicator of initial fruit fermentation; oils from sound, healthy fruit that is processed in a timely manner should not exceed 0.35 percent free fatty acidity. Low peroxide value, indicating low fruit oxidation; oil from sound, healthy fruit that is properly processed should not exceed 10 meq O2 per kilogram of oil. Bin temperature minimally elevated above air temperature, which is an indicator of minimal fruit fermentation; optimal is less than 9°F (5°C), and good is less than 18°F (10°C).
Fruit storage bins should be ventilated or perforated to limit fermentation during storage. It is critical that fruit completely exit the hopper before the hopper is refilled. Olives that lodge in the hopper for an extended period ferment and cause a “fusty” sensory defect, increasing oxidation and reducing shelf life.
It is important that harvest crews maximize the removal of mummified fruit, olive knot, dead wood, and other MOO ("materials other than olives"). Keeping MOO in the delivered fruit to a minimum improves the ability of deleafing and washing equipment to eliminate it, which yields higher oil extraction and quality and reduces potential damage to processing equipment and downtime.
The washing unit traps heavier objects, such as rocks and metals; magnets on the incoming fruit belts supplement the removal of metals, although they do not attract nonferrous metals. To remove soil and residues, a line of nozzles sprays water over the olives as they are conveyed toward the crusher.
Some processors augment the deleafing and washing equipment with optical sorting technology: cameras detect and reject MOO and olives that are damaged by fermentation, pests, or disease.
If processors do not have the capacity to dry the olives with an air knives system, they may bypass the washing step to avoid increasing fruit moisture, which decreases oil extractability, phenols, fruitiness, and shelf life. Washing may be necessary only for fruit that is carrying field soil and impurities. However, processors should weigh the value of bypassing the washing system with the risk that rocks or metals may enter and damage processing equipment.
Crushing
The objective in crushing is to break the cells of the olive and release the oil for extraction. Crushing tools include stones, discs, knives, rollers, and depitters, but the most common crusher is the hammer mill, due to its high output, simplicity, easy maintenance, and adjustability.
The processor adjusts the grid size based on the variety, moisture content, size, and maturity of the fruit to obtain the processor’s desired outcome. Generally, olives should be crushed so that the paste is as fine as possible. A small grid size is typically appropriate for small, low-maturity, and dry olives (less than 56% moisture), and a large grid size is appropriate for large ripe olives, and olives with high moisture.
Fruit with high moisture may create emulsions, which are gel formations of oil and fat that can be spotted in the malaxer. Emulsions significantly reduce oil extraction. To avoid the risk of emulsions, growers should prepare for harvest by testing the fruit for moisture and adjust field irrigation schedules accordingly. The processor can reduce the risk of emulsions by increasing the crusher grid size and/or slowing the rotation speed of the crusher.
For firm, low-maturity fruit, the processor can add enzymes to the crusher to help release the oil. Enzymes (produced from the fungus Aspergillus aculeatus or A. niger) are biologically active proteins used extensively in wine, beer, and bread production. In oil extraction, they degrade the pectin and cellulose of the olive cell walls and vacuoles, helping to release the oil. Enzymes have no impact on oil quality and are not in the final product—their higher density makes them easy to remove during centrifugation. Processors often find that the cost of adding enzymes is more than offset by higher oil yields.
Publications
Some of our research findings are listed below:
Polari, J. J.; Wang, S. C. ACS Omega, 2020, 5(11), 6074-6081: “Comparative effect of hammer mill screen size and cell wall degrading enzymes during olive oil extraction”
Polari, J. J.; Wang, S. C. Eur. J. Lipid Sci. Technol., 2019, 121, 1900168: “Hammer Mill Sieve Design Impacts Olive Oil Minor Component Composition”
Polari, J. J.; Garci-Aguirre, D.; Olmo-Garcia L.; Carrasco-Pancorbo, A. Wang, S. C. Food Chem., 2017, 242, 362-368: “Impact of Industrial Hammer Mill Rotor Speed on Extraction Efficiency and Quality of Extra Virgin Olive Oil”
Malaxation
Malaxation is a critical stage of paste preparation. The processor’s decisions in managing malaxation will have a significant impact on the quality and yield of oil. The impact of the decisions is conditioned by choices already made during crushing; our research shows smaller grid sizes combined with longer malaxation times result in higher yields. Many of the volatile compounds that compose the positive aroma of the oil are formed during malaxation.
The processor may consider adding talc (hydrated magnesium silicate) for high-moisture fruit (> 56% moisture) to break down emulsions. The cost of adding talc likely will be offset by increased oil extraction and improved centrifugation throughput in the next stage of the process. Talc does not negatively impact oil quality and is not in the final product—its higher density makes it easy to remove during centrifugation.
The most critical variables in malaxation are temperature and time. High-quality olive oils require a temperature that is as low as possible but not less than 72°F (22°C) to facilitate adequate extraction efficiency and ensure the solubility of phenols and chlorophyll. Malaxation time significantly affects the oil’s sensory profile and generally should be as short as possible. Typically, it is not less than 45 minutes, and up to 90 minutes or even longer may be necessary to achieve adequate separation of emulsified pastes.
Publication
Some of our research findings are listed below:
Polari, J. J.; Garci-Aguirre, D.; Olmo-Garcia L.; Carrasco-Pancorbo, A. Wang, S. C. Eur. J. Lipid Sci. Technol., 2018, 120, 180097: “Interactions Between Hammer Mill Crushing Variables and Malaxation Time During Continuous Olive Oil Extraction”
Oil separation
Decanters generally are categorized as two-phase or three-phase, which describes how each type of decanter separates the phases. A two-phase decanter has two outlets: one for oil and one for wet pomace (55% to 75% moisture). A three-phase decanter has three outlets: one for oil, one for water, and one for a drier pomace (45% to 55% moisture). A recently developed three-phase system with water-saving technology, called a multiphase decanter, if properly adjusted, can deliver oil, a pâté-textured pomace, and dry pomace.
Although the extraction results are similar for two-phase and three-phase decanters, the lower water injection rate is a key factor in why two-phase decanters are more common in modern processing facilities. Lower water injection rates result in less energy use and less wastewater to manage, while producing oils of greater intensity and phenolic content.
Processors should measure the oil content in the pomace to check on the facility’s oil extraction efficiency, aiming for a benchmark of 85 percent or higher.
Oil leaving a horizontal decanter has a significant amount of water and sediment due to the intermixing that occurs inside the decanter. If not sufficiently removed, the sediment and water produce anaerobic fermentations that quickly result in defects in the oil. To ensure good shelf life, processors should remove sediments and lower the moisture content of the oil to less than 0.2 percent. This process usually begins with the vertical separator (although some processors forego this stage of processing); the oil is further clarified using settling, filtration, or racking, or some combination of these methods.
The oil emerging from the vertical separator can be further clarified by 24 to 48 hours of storage, ideally in a stainless steel settling tank that allows for drainage from a steep conical bottom. Avoid having tank open to air and light, and maintain the temperature above 65°F (18°C) to aid settling. Drain settlings every 2 hours and remove any foam floating on the surface every 6 hours or as needed.
Filtration removes suspended solids and moisture in the olive oil before storage, eliminating any possibility of further fermentation. Our review (PDF 342 KB) showed that the type of filtration method can have positive, negative, or neutral impacts on the oil’s stability, phenolics, volatiles, appearance, pigments, sensory profile, and shelf life, with these impacts further influenced by the oil’s initial chemistry, sensory profile, the variety, and storage conditions.