A Brief History of Olive Oil Milling
Producing olive oil photo credit depositphotos
To the great surprise of many, extra virgin olive oil is, in fact, a fresh fruit juice. However, it is not the same kind of juice we enjoy with our breakfast. Crushed olive fruit produces a highly perishable, pulpy paste that yields oil, water, and solids, which are mechanically separated to create a healthy, concentrated fat, i.e., olive oil. This explains its fastidious demeanor, ever thwarted by the foes of light, heat, air, and time. Its eventful journey from branch to bottle is practically as old as the ages, yet only a handful of decades have crafted the high-quality oils so readily available today. Among flashy designer brands and old-world kitchen stalwarts, each bottle requires the inclusion of a specific phrase to validate its extra virgin grade: “superior category olive oil obtained directly from olives and solely by mechanical means.” Those mechanics have evolved over thousands of years, telling a story of human history that has transformed civilization and revolutionized this humble food into a superstar of the kitchen.
Early Olive Cultivation
Olive trees
To start at the beginning is to go back to the domestication of the wild olive tree, which began in the Neolithic period, as nomadic homo sapiens around the Fertile Crescent developed agricultural techniques that allowed them to create sedentary communities and develop rudimentary technology. Archeologists have found evidence of milling tools, vessels, basins, and equipment used for oil extraction dating as far back as the 8th century BCE in what is now Palestine and Israel, as well as Bronze Age production in Jordan, Syria, Egypt, Tunisia, Turkey, and Crete, later spreading to coastal Southern Italy and Spain.
In the 1st century AD, Roman writers Columella and Pliny the Elder recognized the importance of best agricultural practices for olive oil production. Pliny went so far as to categorize the qualities of oil. Both writers emphasized the necessity of harvesting the olive at peak ripeness, picking the fruit before its green skin turned black, as opposed to using fruit left to ripen fully on the branch, or worse, fall to the ground. This description of what we now call ‘early harvest’ remains the current methodology for producing flavorful, highly phenolic extra virgin oils to this day.
Olive Oil Extraction
Crushing
Pressing
Separating
The process of olive oil extraction, from its inception until the mid-20th century, comprised three phases: crushing the olive fruit into a paste, pressing the paste to expel the liquid, and separating the oil. In its earliest form, the paste was made by simple means of hand crushing or stomping on a sackful of olives. Later methods used various forms of mortar and pestle or a horizontal stone roller set into a rectangular stone basin. The paste would be placed into a fibrous sack and twisted at both ends or weighed down with stones to expel the liquid, often doused with hot water to coax out as much oil as possible. The liquid would be collected in a ceramic or stone vessel and left to separate. Once the oil rose to the top, it was skimmed off.
Advances in Milling Technology
Cylindrical or conical millstones
Fibrous mats
Lever and tower presses
Hydraulic presses
Mats being used to press olives photo credit depositphotos
Developments in milling technology were necessitated by a growing demand for olive oil as production increased along Mediterranean trade routes expanded by the Phoenicians, Greeks, and Romans. Enter the use of millstones. The Roman Empire developed the mola olearia and trapetum to create a factory-style method for more efficient extraction. Powered by manpower or a mule, this advancement used cylindrical or conical grindstones that rotated around a central post fixed to a circular basin. The scale of this early machinery facilitated crushing increasingly larger batches of fruit, with the resulting paste spread over fibrous mats and stacked atop one another. The stacks would be positioned under large wooden levers attached to a fixed point in a wall. Using stone counterweights or torsion from a perpendicular wooden screw, the lever would apply pressure to the stack of mats, expressing as much liquid as the counterweight allowed.
Conical mill stones
Numerous versions of lever or tower presses remained in use until the introduction of the hydraulic press in the 19th century. In the history of milling technology, this was a short-lived advancement, yet there are still some places where mechanized millstones and hydraulic presses are used, although the number of these mills are few and far between. Most keep this 'traditional method' in use to preserve cultural heritage, or due to the vast expense of upgrading equipment. Avid supporters of heritage food ways may commend this, but is traditional always better? Considering the fragile chemical and organoleptic properties of olive oil, the answer is a resounding no.
Disadvantages of Traditional Olive Oil Extraction
Let’s return for a moment to the four foes of light, heat, air and time. Even in the best harvest conditions, the moment olives are separated from their branches the fruit immediately begins to degrade. It is then a race against time and temperature before fermentation sets in, creating defects that distort flavor and aroma and alter chemical composition, all of which can prevent oil from being classified as extra virgin grade. Even if the fruit reaches the mill in peak condition, millstones and hydraulic presses operate in the open air, highly susceptible to the four foes due to slow crushing times, increased exposure of the surface area of both paste and liquid, and spoiled residue on fibrous mats. Despite the valorization of old world foodways, the traditional method holds far greater risk in creating defective oil.
Modern Techniques
Continuous production lines
Metal crushers
Malaxers
Centrifuges
Filters
Storage tanks
Bottling machines
Modern olive oil factory with storage tanks
The second half of the 20th century is where oil extraction developed the best defenses against the four foes. The continuous cycle system created a production line that moves smoothly through each phase of the milling process from fruit to bottle. The advent of metal crushers, malaxers, decanting centrifuge, polishing centrifuge, cellulose filtration, storage tanks, and bottling machines became a watershed for oil quality. They serve to crush the fruit into a paste, stir the paste to prepare the oil for separation, spin the paste at high speeds to separate the oil from water and solids, remove any residual impurities, filter the oil for maximum shelf life, and deposit the finished oil into bottles or bulk containers. This technology makes the popular term 'cold pressed' or ‘fresh pressed’ a misnomer, as no pressing of any kind is used for extraction, and 'cold' indicates temperatures are kept below 28 degrees Celsius throughout the milling process.
To better envision the function of each phase we can look to the kitchen. Think of the crusher as a food processor - an enclosed machine that rapidly shreds the fruit fed into it (pit included). The malaxer can be likened to a mixmaster, churning the paste at a steady speed. The decanting and polishing centrifuges are akin to industrial salad spinners, separating the oil from water and impurities. Cellulose filtration can be imagined as uber dense piles of paper towels that strain out any final residue. Storage tanks are not unlike enormous versions of coffee urns used in conference rooms (minus the heating function), that hold the oil until it’s deposited into a container. If you could connect all of these pieces of equipment together with a plumbing system so one flows seamlessly into the other, we can begin to get an idea of the synergy of the production line.
Benefits of Modern Methods of Oil Extraction
A majority of this magic is done in a closed system that sees little oxygen. Many aspects of the mechanics are tightly controlled depending on the variety of olive being used, as some cultivars create a paste that more easily allows for oil extraction than others. Temperature is of the utmost importance and numerous variables of the machinery can be adjusted to minimize friction, heat, and duration of each phase. During harvest season, mills run 24 hours as producers arrive around the clock to process their olives. The duration from when the fruit is harvested to when it is milled is crucial to oil quality, ideally taking place within less than 24 hours (producer-owned mills are able to cut this time down significantly). It is from this point that the delicate crafting of olive oil is handed from grower to miller.
As batches of olives are trucked into the mill’s receiving area, the first step is to weigh the fruit to keep track of how much a grower has brought in and the resulting oil yield. Depending on the quantity of olives, size of the mill, and desired type of oil (artisanal vs bulk, for example), olives may be sorted by quality before being whisked on their conveyor belts to the blower, where remaining leaves, twigs, stones, and other loose debris are removed by fan power. Depending on the school of thought, the olives may be washed to remove any remaining dirt, however this is sometimes eschewed as the application of water can cause quality issues further down the line.
While every phase imparts its own fingerprint on the oil, malaxation is of primary focus, as it serves an important function for phenols and flavor. The carefully calibrated mixing of the paste activates enzymatic compounds released from the flesh and pit during crushing, imparting these molecules into the oil to give it a bouquet of aroma and taste. This is where the pillars of sensory attributes are developed; the fruitiness, bitterness, and pungency that will be evaluated by professional tasters at a later stage, measured against any defects found in the oil in order to determine if it qualifies as extra virgin grade. Any detectable defect lowers the oil from extra virgin to virgin grade.
Filtration
The last step of extraction can be controversial, but is widely considered best practice. Filtration creates stability for organoleptic and nutritional properties by eliminating residual water and sediments that cause degradation. In short, longer shelf life. Some unfiltered oils are prized, such as Olio Nuovo, made from the first harvest of the season in Italy. While it is traditionally left unfiltered, the oil doesn’t need a significant shelf life as it is generally consumed quickly, before fermentation or other defects can develop an impact on quality. Read more about filtration.
Waste & Sustainability
Olive waste matter
The milling process from beginning to end creates a huge amount of waste, namely leaves, waste water, and solid matter called pomace. The pomace oil industry is huge, using industrial chemical refinement to eke out the last 5% or so of oil this byproduct contains (often combined with a splash of about 10% extra virgin oil to be deemed suitable for human consumption). Incredible amounts of research are currently being done to recuperate and recycle the innumerable tons of refuse left over from olive oil production, with some exciting findings that may soon be applied to agricultural, medicinal, and material use.
Olive oil mills come in all shapes and sizes, from simple communal mills to enormous industrial complexes. Seeing them function at full throttle is quite a startling process to witness. An assault on the senses, it’s often hot, incredibly noisy, and in constant motion with people running to and fro, fretting over each stage of production. Seeing a mill in person has become easier with the advent of oleotourism, and guided tours are often available through local producers. Every mill, no matter the scale, provides a definitive moment that brings it all down to size - a small spout pouring the liquid gold that has been treasured throughout centuries and civilizations. To taste it is to experience the history of human evolution on your tongue, ever changing, one harvest at a time.