PuroSole's solar roasting technology

PuroSole’s technique of roasting coffee by solar radiation stems from the almost obsessive research to replace the exploitation of non-renewable natural resources with solar energy and to reduce the resulting environmental pollution. Coffee roasting is a practice that dates back to the 15th century and which over time has evolved from perforated pans to today’s computerized roasters.

The idea behind this technique is to induce the roasting of the raw coffee bean, rather than from contact with hot air, as currently in use, from direct solar radiation, or more precisely not with the heat of the sun, but with the direct irradiation of its light rays.

This innovative method has also unexpectedly influenced the organoleptic characteristics of the coffee, so much so that we have to investigate the physical processes involved and thus compare this solar radiation technique with the classic technique based on hot air.


The reduction in energy consumption that solar roasting involves, partly due also to the higher energy efficiency of the system, derives from the complete elimination of the heating of the grains through the combustion of gas, which is replaced by direct heating through solar radiation.

Obviously, by not burning the gas necessary to obtain the necessary heat, but by using the heat induced by the sun, this system is decidedly more eco-sustainable than any other classic roasting method used to date. Suffice it to say that every 1000 kg of coffee roasted with the sun, the more than 200 cubic meters of methane of classic roasting must not be burned, thus avoiding the production and release into the atmosphere of over 400 kg of CO2.


The principles of the processing of products, according to the organic method, are set by art. 6 of EC regulation no. 834/2007 from which:

carefully transform food, preferably using biological, mechanical and physical methods.

It is therefore out of the question that replacing the coffee roasting phase by gas combustion with solar radiation roasting is a significant improvement in one of the stages of transformation and that this can greatly contribute to the consolidation of the organic supply chain.

The classic roasting technique

To better understand the process differences involved in the new roasting technique with the sun compared to the techniques used today, a brief examination of the state of the art of the latter is necessary.

From wikipedia:
“Roasting or roasting is a generic roasting process, which subjects a substance to a high temperature, in order to dehydrate it, oxidize it and in some cases even partially carbonized.”

The definition given by wikipedia is clear enough but, in the specific case of coffee roasting, it requires a mention of two insidious and negative aspects that are inevitable in this process. Gasification and self-combustion.
By gasification we refer to the rapid transformation of oils and sugars into gas and their consequent expulsion from the bean being roasted. This process begins when the temperature of the coffee grain exceeds 100°C, with the evaporation of the water.
Subsequently, as the temperature increases, other substances are involved in a complex combustion and this until the temperature of 200-220 ° C necessary for roasting is reached.
The time dilation of this process increases the loss by expulsion as gas of the various and complex components of the bean being roasted, which is why this roasting phase must be completed in the shortest possible time. The self-combustion event, on the other hand, affects the final stage of roasting, when the temperature approaches 200°C. In fact, at this temperature carbonification begins and a self-fueled combustion process starts inside the grain, which continues to raise its temperature even without the contribution of external heating.
For this reason, shortly after reaching this stage, a rapid cooling of the freshly roasted coffee is necessary to reduce the temperature inside and prevent it from being consumed by complete self-combustion.

After these clarifications we pass below to describe, again briefly, the two main roasting techniques today

State of the art: the two main roasting techniques today.

Also from Wikipedia:
“In coffee roasting, the grains are subjected to temperatures of 200-220°C while they are stirred. There are essentially two roasting methods:” fluidized bed “, in which the raw beans are hit with jets of hot air. temperatures between 300°C and 400°C for a few minutes, remaining in suspension in the roasting chamber (hence the name of the process); and “rotating drum”, in which a metal drum is used inside which there are augers or fins to continuously turn the product and uniform the roasting, in which a gas burner conveys the hot air necessary for the process, for a time of about 15-20 minutes depending on the type of coffee, the capacity of the drum and the taste of the roaster”

While in the first system the coffee is roasted much more externally than internally, leading to taste and preparation anomalies, the second method clearly improves the aromatic yield of the coffee by making roasting uniform. open a bean in half by acting on the clear groove and observe the inside to realize the goodness or otherwise of the roasting process undergone.

As reported by wikipedia, in the two traditional roasting systems the heat is transmitted to the coffee grain by air convection, very forced in the case of the “fluid bed” and more moderate in the case of the “rotating drum”. Simplifying, “fluid bed” toasting is equivalent to cooking food in the home oven with thermo-ventilated mode, while “rotating drum” toasting is equivalent to cooking food in the home oven with normal mode: with thermoventilation cooking it will be more intense and faster, but it will cook the food even more on the outside than on the inside than with normal cooking in the oven. In both cases, however, the heat is transferred by the air heated to 300°C – 400°C to the entire coffee bean through contact with the outer layer of the bean itself.
Even if in modern coffee roasters the temperature and the air flow are controlled by automated systems that allow to accurately determine the roasting profile, that is the temperature curve of the coffee grain during the whole process, inevitably with the two techniques just described, every single coffee bean will be roasted more and more on the outside than on the inside, i.e. the bean will always be roasted too much on the outside and little inside. On the other hand, increasing the roasting time, albeit at a lower temperature, to improve uniformity, would involve a prolonged evaporation and combustion of the sugars and oils inside the bean with a consequent reduction in the organoleptic characteristics of a good coffee.
So summarizing we can summarize as follows:
fast roasting = non-homogeneous grain
slow roasting = degradation of the organoleptic characteristics

The complexity in the search for the best compromise in the modulation of the various factors that influence the delicate roasting process, which is specific to each different origin of coffee, is the main reason why this practice is still considered an art today.

Roasting by irradiation of direct sunlight.

1. The positive aspects of process physics

The peculiarity of roasting by irradiation with sunlight consists in a better uniformity of the degree of roasting between the external part and the internal part of the bean and a greater conservation of its natural properties with consequent enhancement of its organoleptic characteristics. This is because solar roasting makes it possible to obtain uniformly roasted beans in a shorter time than the traditional techniques described above, also eliminating the need for forced air convection. The roasting method by solar radiation, described in this document, is essentially based on heating the coffee beans through the electromagnetic energy of the light emanating from the sun.
The idea behind this new technique arises from the observation of the still raw coffee bean. In fact, what has been noticed, and which has started the study of this innovative roasting method, is that the raw coffee beans not completely opaque.

The raw coffee bean is light green and partially translucent, i.e. not completely opaque to light and when illuminated from the outside the light penetrates inside the bean from which it is absorbed and converted into heat at different depths depending on the length. wave. Sunlight has a special feature that is practically not reproducible by any type of artificial light-generating device: it has a very broad spectrum of colors. This means that it contains all the different wavelengths, from the ultraviolet to the infrared range.

This aspect means that the heat is “generated” inside the coffee bean simultaneously at different depths and not only propagated from the outside to the inside as is the case with traditional heating in contact with hot air. The photons that penetrate the surface of the coffee grain release energy during their journey in a different way depending on both their frequency and the color of the bean, which in turn changes with the degree of roasting.

Furthermore, in this technique, the function of air is even opposite to the traditional one. In fact, the air that surrounds the bean, not being heated by the sun’s rays by virtue of its transparency, will always be colder than the bean itself, thus counteracting the increase in temperature on the surface of the coffee grain and contributing to the reduction of temperature gradient between the outside and the inside of the grain itself.

Another visible effect of roasting by solar radiation is the rapid burning of the internal film, an effect that helps its detachment and subsequent elimination. The light energy that reaches the grain causes the temperature of the light film to rise very quickly.

2) The critical aspects of the physics of the process

The most delicate aspect of the physics of this process is given by the extreme directionality of the light.
To roast the coffee correctly, during the entire cooking phase, each bean must be directly illuminated from the different angles cyclically and at the same time without being exposed to the intense light from the same angle for too long.
In fact, the temperature of a coffee bean exposed to this intensity of light can rise by almost 10°C per second (*). Unlike the classic heating process with hot air, which penetrates and circulates in every free space between the beans during roasting, the light will only hit the coffee beans in front it meets, and which naturally cover the beans behind, leaving them in the shade. . It should be emphasized that the energy for roasting, in this technique, cannot reach the coffee in any other way: if a bean is not directly irradiated it can remain almost completely raw even if immersed together with the rest of the perfectly roasted coffee.

(*) For example, a grain of coffee of sieve 18 which weighs 170 mg when half roasted, has a maximum section of about 90 mm² of surface. Taking into account that the specific heat of the coffee is about 1.4 (J/g x °C), this bean, absorbing an energy of 2.25 Watt when illuminated with a solar radiation of intensity equal to 2.5 MegaLux, can undergo a temperature increase of 9.4°C per second.

The organoleptic characteristics

The rapid and uniform roasting facilitated by solar radiation is undoubtedly one of the points in favor of this roasting technique. But it is not the only one: in fact the other important aspect inherent in this innovative technique is the absence of forced air convection on the coffee bean. In traditional roasting systems both with fluid bed and with rotating drum, the air is forced to circulate between the coffee beans as the only heat carrier between the gas burner, which generates the thermal energy, and the coffee, which must absorb.

Once the high temperature air molecule (300-400°C) has been in direct contact with the coffee bean, it cools as it transfers its heat to the bean itself and must be replaced by a new hot air molecule. For this reason it is always transmitted into the air, exactly as it happens with a hairdryer. This forced circulation, however, also carries away with it the aerial products of the physical transformation that occurs in the grain.
In roasting by radiation, on the contrary, the bean is always surrounded by its own vapors produced by the transformation of oils and sugars. Transformation that is a consequence of the chemical reaction induced by the heat generated inside the grain itself by the light radiation. To make a comparison, it is a bit like the cooking difference between boiled meat and roast meat. In boiled meat most of the substances produced by cooking will end up in hot water (which will turn into broth), while in roast almost all of the cooking product will remain inside.

As we can therefore expect, this different roasting technique produces gods differentiate the organoleptic results. The objective chemical analysis of these results is complex, as is the one on written roasted coffee, due to the reactions of different reactions that take place in the roasting process. On the other hand, a judgment based on subjective analysis would clearly be biased. However, for this reason the tasting tests, which on double-blind comparisons, were carried out by a small group of volunteers are valid and just as a curiosity.
The same single-origin raw coffee (Santos – Brazil) was roasted both by classic roasting plants and by this innovative plant and, in 100% of the comparisons, the evaluation of solar roasting was always considered superior to the traditional one in reference to the parameters of: aroma, acidity, aftertaste, body and creaminess. Naturally, it is possible to carry out new tasting tests by roasting coffees of different origin and quality on the pilot plant.

The technique of direct irradiation with concentrated light

Solar energy, as it reaches us, does not have the sufficient intensity to be used directly for coffee roasting.
To obtain the heating of raw coffee with the increase in temperature over the time necessary for proper roasting, sunlight must be concentrated.
The intensity of sunlight for a roasting completed in a time between 5 and 20 minutes must have a value from 2 to 8 Megalux or from twenty-five to one hundred times the intensity of the sun on a day with clear skies. To obtain this luminous intensity, the sun is reflected by different mirrors, each of which with a surface size similar to that of the container to be heated, which all aim at the same objective. In this way, the intensity of the light obtained will be equal to the sum of all the light beams sent by the reflectors used. To keep the light beam thus produced a fixed point throughout the day, devices called concentration heliostats are used, which are motorized instruments that automatically change the inclination of the mirrors according to the change in the position of the sun. These heliostats that collect solar energy are placed outdoors in order to better capture the sun from every angle and direct the reflected light concentrating it towards a roasting chamber that contains the coffee to be roasted.


The coffee container receiver can be placed either outdoors in front of the heliostats or inside a building with a glass opening for the passage of reflected light from the mirrors. Since the sun’s rays arriving on the earth travel almost parallel, the distance between the heliostats that collect the sun and the receiver with the coffee to be roasted can be several tens of meters without any loss of energy. From the energy point of view for each kg. of raw coffee requires at least one kilowatt of radiant energy. In fact, to evaporate the 200 gr. of water on average contained in one kg., raw coffee absorbs 520 kilojoules which, with an irradiation energy of 1 Kw, correspond to about 9 minutes (520 seconds). This evaporation of water corresponds approximately to the weight loss that occurs in roasted coffee compared to raw coffee. After the evaporation phase, the remaining half of the energy supplied by the sun is used to bring the temperature of the bean from just above 100 ° C to the 200-220 ° C needed to complete the roasting process. To ensure this energy necessary for roasting, for each kg. of raw coffee serve about 2 square meters. of solar collector mirrors. So for ex. to toast 30 kg. serve 60 square meters. surface exposed to the sun which will direct the beam concentrating it on the container of the coffee to be roasted.

One aspect that must be taken into consideration with this roasting technique is that the energy transfer towards the roasted bean is directional. The light that falls on the coffee grain does so mainly from a preferential direction, as opposed to the hot air roasting system which instead completely surrounds the bean. For this reason, the coffee containment and mixing system must be designed with great care to allow each individual grain to be exposed to light with a short cycle and for a fair fraction of the total exposure time.

Il caffè per essere irraggiato dalla luce solare può anche essere è contenuto in un cilindro di vetro borosilicato che permette il passaggio di oltre 90% della luce. In questa immagine i grani di caffè vengono mantenuti in sospensione e continuamente miscelati da un flusso di aria dal basso verso l’alto. Sia il cilindro di vetro che l’aria in circolazione, in virtù della loro trasparenza non sono riscaldati dalla intensa luce incidente che invece viene assorbita dai chicchi di caffè. Quindi anche in questo caso sia l’aria che il contenitore non contribuiscono alla tostatura con il contatto diretto con i grani di caffè.

Energy saving

Energy saving is also one of the main positive aspects of this technique, the energy saving deriving from the elimination of the main source of thermal energy necessary for gas burners, replaced by an unlimited and free resource such as solar, determines a, albeit minimal , however, an appreciable reduction in production costs. Considering that the traditional roasting phase of one kg. of coffee requires approximately 6 MJ (methane has 37 MJ per cubic meter), it takes 162 cubic meters of methane to roast a ton of coffee. Even considering an industrial cost of methane of € 0.20 / mc, there is a saving of around € 32 per 1000 kg of coffee.

Energy efficiency

Solar radiation mainly heats the coffee beans only, and not the air they are surrounded by. Theoretically, the roasting by irradiation could take place safely even in cold air, and even if in reality the air temperature in contact with the beans rises, the energy released by the beans into the air is still minimal, so much so that, if for to roast raw coffee in the traditional way it is necessary to heat the air to 300-400°C with a power of at least 5 Kw per kg., in the case of roasting by solar radiation, a power of only one Kw is sufficient. per kg. , this is because the maximum temperature inside the container is that of the coffee and will only reach 200-220°C at the end of roasting.
The use of lower temperatures, with the consequent reduction of problems related to thermal insulation and the need for a thermal energy recovery system, make this technique much more efficient than traditional roasting systems. Another positive feature of direct radiation is the total absence of thermal inertia of the container. In fact, roasting begins instantly cold as soon as the coffee is illuminated by sunlight, without any need to preheat anything in the machine. This also contributes to the high overall energy efficiency of this system.


While considering all the positive aspects related to the use of solar energy in the coffee roasting process, a critical aspect that instead requires a special organization should be considered well, namely that linked to the discontinuity of the availability of solar energy itself. Due to its particularity, the concentration roasting system requires fairly constant solar lighting throughout the roasting phase. This aspect unfortunately limits the use of this technique to only days in which a clear sky is predictable, with good reliability, in which the sun is not covered by clouds too frequently. It is in fact very important that, once the roasting process has begun, it is not interrupted in the following 5-20 minutes. The process of raising the temperature of the roasting coffee must be kept under control and respect the maximum tolerances allowed by its roasting profile. This limitation, although in fact the only critical aspect of this roasting technique, significantly affects the times, days and places where it can be applied.

For example, the two graphs shown above show us the number of roasting cycles that could have been carried out in the years 2014 and 2015 in the Rome area. The green bars indicate, for each week of the year, the periods of at least 20 consecutive minutes of full insolation, that is, without any interruption by passing clouds.
Of course, depending on the place, the formation and movement of the clouds will follow different rules, as well as the number of hours of sunshine change depending on the latitude of the place itself. In any case, what we can see, looking at the two graphs relating to the Rome area, is that the probability distribution of having a clear sky for at least 20 minutes is relatively constant over the entire calendar year.
An important figure that can be seen from the graphs is the total number of possible cycles each year, which in the two cases shown is about 3000. To make a comparison with a classic roasting machine that works eight hours a day at full speed, a solar roasting plant installed in the Rome area would work the equivalent of about three hours a day.
Fortunately, this critical aspect can be contained by the ability of the coffee, both raw and roasted, to be stored for long periods in silos. Careful seasonal planning based on the place where the system is installed will allow for good productivity. Finally, remember that, contrary to what it may seem, the intensity of solar radiation in winter is not very different from that in summer. The difference is only in the different inclination with which the radiation reaches the ground and in the number of hours per day in which it is usable. This is just to emphasize that, apart from the different hourly productivity between the months of the year, there are no substantial differences in the quality of the final result

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