2.1. Panela (unrefined non-centrifugal brown sugar)

Panela is a natural sweetener obtained from the extraction and evaporation of sugarcane juice (Saccharum officinarum), also called "guarapos" in Spanish. In the world, it is known by the names described in Table 1. Traditionally, this food is manufactured in artisanal facilities called mills, or "trapiches paneleros" in Spanish ^10,11.

Panela is valued for its energy contribution and its contribution to food security and sovereignty. The main component of panela is sucrose, although its composition includes glucose and fructose, vitamins A, C, D, E and B, and phosphorus and zinc. This food product contains five times more minerals than brown sugar and 50 times more than white sugar ^13,14. The sugarcane derivatives are attributed health properties, as an antioxidant effect by the action of polyphenols ¹⁵, anti-carcinogenic ^16,17, and others uses in the prevention of pulmonary diseases ^18,19.

2.2. Cane honey

Cane Honey is the product resulting from the evaporation of panela cane juice, reaching a solids concentration of 70° Brix and temperatures between 97 - 98° C ²⁰. It is a viscous liquid, with a bittersweet, translucent flavor, highly soluble in water and of a light brown or amber color. It consists mainly of fructose and glucose. In this case, the use of immature or green canes is preferred, due to the fact that they avoid the rapid crystallization of the product once it has been bottled ⁷.

Cane honey is used as a substitute for table sugar, since its high solubility allows it to be incorporated into any preparation. Its nutritional properties are similar to those of panela; however, it is a very desirable food for people with caloric requirements, due to its high content of reducing sugars ¹².

2.3. Powdered panela

The panela powder is a brownish yellow solid highly soluble in water, composed almost entirely by sucrose. The product is obtained by means of heating, concentration and exhaustive churning of cane honeys, until reaching 96° Brix and temperatures of 125-128° C ¹⁴. The agitation of the dehydrated juices favors the aeration and the breaking of the particles at the moment of the solidification, until obtaining granules of diverse sizes, like this: fine grain (Tyler sieve No. 16), coarse grain (Tyler sieve No. 3) and clods or "boronas"⁵.

In the production of powdered unrefined raw sugar there are two critical points: first, the reducing sugars must be maintained between 0.5 and 1.0 %, since they disadvantage the crystallization of the material; second, given the hygroscopic nature of panela, the time between production and packaging should be minimal and should be packed in waterproof materials and in conditions of low humidity ²¹.

In the literature there is information on bromatological characterization of cane derivatives; however, it should be considered that its composition is affected by the variety of cane and by the conditions of processing and storage. An example of such variations is the one reported in the work of Guerra and Mujica ¹⁶, who found significant differences in the composition of two handcrafted powdered panelas and one standardized, due to the transformation practices and the standardization of the process.

The physicochemical requirements of non-centrifugal brown sugar (solid block), honey and granulated panela depend strongly on the production conditions of each of these foods (Table 2). They are much more rigorous in the case of granulated panela, due to its high hygroscopicity and loss of stability and caking. In addition, knowledge of the required quality parameters is a fundamental factor to achieve the standardization and improvement of the processes of transformation and conservation of any cane derivative.

3.1. Cane bagasse properties (as fuel)

Bagasse is a fibrous and heterogeneous material that is obtained by compressing the canes in the extraction stage. The bagasse is used for combustion and the generation of the heat necessary in the evaporation of the water contain in the sugar cane juice ²⁹.

The bagasse elemental analysis in dry base changes according to the variety of cane, the maturity state and the crop conditions ³⁰. An average composition of this material was reported by Arteaga et al. ³¹: 48.0 % of Carbon, 43.1 % of Oxygen, 6.2 % of Hydrogen, 0.4% of Nitrogen and 2.3 % of Ashes. According to Marcelo et al. ³², the empirical formula for bagasse corresponds to C_3.73H_6.3O_2.98N_0.00129.

An important thermal property for the design and energy assessment of the combustion process in panela furnaces is the net calorific value (VCN). Montoya and Giraldo ²⁸ cite Eq. 7 to theoretically estimate this parameter:

V𝐶N = 18,309 - 31.09 𝑚_𝑟 - 196.05𝑚 _z -207.6𝑚 _w (7)

VCN in kJ/kg, y m mass fraction of: z, ash; 𝑤, water; 𝑟 residues. And the other hand, Sardeshpande et al. ²⁹ propose:

V𝐶N = 18,260 - 207.1 - 182.06𝑎 -37.1B (8)

where VCN is expressed in kJ/kg; and 𝑥 humidity fraction, 𝑎 ash content and B sugar concentration in residual bagasse.

4.1. Sugar cane harvest ("Apronte" in Spanish)

The name corresponds to the operations of cutting, transport and cane conservation in the productive installation. It is desirable to store the raw material in an isolated, paved and covered area indoors ^5,10. According to Osorio ⁶, the time elapsed during this operation must be the maximum of three days, to avoid the destruction of the stem and the accumulation of sucrose inversion, which caused a decrease in the production of panela and affects its quality.

4.2. Grinding or extraction

The cane juices are obtaining in mills that work by compression and rotation of the vegetal material, in a system of three or four horizontal rollers. To have a good extraction in motorized mills, without sacrificing equipment or increasing power, some manufacturers recommend operating under specific conditions of capacity and speed (Table 5). According to Diaz and Iglesias ¹¹, with speeds of 20 rpm or more, the extraction is reduced, the energy consumption is increased and the gears and parts of the equipment wear out.

The main disadvantages in traditional mills of three horizontal rollers are: the contamination of the cane juice with oils and lubricants of the equipment, to the wear of the cylinders, to the deficient maintenance, to the inadequate evacuation of the gases of the engine inside the productive unit ³⁵. Some equipment manufacturers have proposed novel models that mitigate this problem, technological schemes with four horizontal rollers and / or systems to prevent cross-contamination of juices. However, its acquisition cost is considerably higher.

Figure 1 Production process for panela and derivatives ⁷.  

The extraction of sugar cane juices is the indicator that has more influence on the overall performance of the process and is defined as the ratio between the amount of juice recovered in the mill and the weight of the compressed cane. An extraction between 58-63 % is considered like a good mill performance, in facilities for panela production the mill extraction fluctuates between 40-65 % ⁶.

As a result of the milling process, a solid by-product called bagasse is obtained, with humidity between 50-60 %. This material should be dried to a maximum humidity of 30 %, to be used efficiently as a fuel in the burner, and thus generate the gases that transfer heat to the juices that are concentrated ³⁸.

4.3. Cleaning and clarification

The cane juice comes out of grinding with a large amount of impurities, which must be eliminated in their entirety to avoid the turbidity of the honey and the final product, and to guarantee the safety criteria from the physical, chemical and biological point of view. The removal of foreign material is of special importance in the manufacture of granulated panela, since it constitutes a barrier to crystallization and affects the appearance of packaged food ³⁹.

4.3.2. Clarification of sugarcane juices

The content of flavonoids and organic acids in the juices contributes to the darkening of their natural brown color, so they must be removed in order to produce a lighter and brighter color in product ^39,41.

Figure 2 Pre cleaner - type CIMPA. 

Clarification is the elimination of dissolved and suspended solids in the juice, through the combined effect of the heating supplied by the burner and the action of cellulose polymers with binding properties. The solids in suspension aggregate with each other and form a heterogeneous mass that is known as mud, or "cachaza" in spanish, which floats on the juice and is extracted manually ⁵.

Temperature is an important factor in the addition of flocculant material, so a heating rate between 1.0 - 1.5° C / min on the clarifying pan is recommended. A high temperature favors the reincorporation and dissolution of the polluting material (tempering), while a low temperature prevents the agglomeration and separation of the undesirable solids from occurring. The flocculating agent must be added in two parts and in equal proportions: the first, between 60 - 70° C, and the second, when reaching 75 - 92° C ³⁸. The mud removed from clarification in the first instance is known as black cachaza, while the foam removed in the final stage is called white cachaza ⁴².

The agro-industry of sugar cane and panela in Colombia allows the use of anionic polyacrylamide, which is added in two fractions of 2 ppm each, at 50° C and 85° C respectively ^10,21. However, reducing the concentration of acrylamides to acceptable levels is an important challenge, fundamental for the national and international positioning of panela in the region. This guarantee of quality demands greater control in the variables of the process, avoiding the high temperatures generated by the undesirable compounds. Thus, the use of steam and vacuum evaporators is presented as a prominent alternative; in addition to the fact that the use of extracts and natural clarification agents helps to considerably reduce the presence of acrylamides in the final product.

In Colombia, natural flocculants stand out as balso (Heliocarpus americanus), cadillo (Triumfetta láppulal) and guácimo (Guazuma ulmifoliase) ^40,42. Ortiz et al. ⁴³ evaluated the removal of contaminating substances in sugarcane juice and recommend using mucilages in a concentration of 0.3 % weight - volume. Likewise, they were able to identify the cadillo as the best clarification agent in comparison with the balso and guácimo, since it had a higher yield in extraction and presented a lower solids retention in panela.

4.4. Evaporation, and concentration

After clarification the evaporation begins, which partially remove the water from the juices and increase them the solids content from 17° Brix to 70° Brix. Temperatures close to 97° C are reached, at which time the concentration begins and the product is called honey ²⁰. During the concentration, water is removed at a temperature higher than 100° C. When sugarcane juices reach 70 - 75° Brix, it is recommended to add a non-stick and anti-foam agent such as laurel wax or castor oil or coconut oil, to homogenize the honey and avoid burning of panela ³⁸.

The operation that allows determining the final point of panela is called picking, and takes place when the honey has reached 90 - 96° Brix and temperatures close to 120° C ^23,38. The plucking is carried out in a traditional way and based on experience, observing the color, density, runoff of honeys and formation of air bubbles; and in very few cases, with the use of portable refractometers.

4.5. Beaten, molding and packaging

Once the final sugar concentration point is reached, the evaporated honeys are transferred to a tray where they are cooled by an intensive and intermittent beaten and lose their adhesion capacity and acquire the ideal texture (grain) to be molded in panela form or any other derivative. The use of stainless steel is recommended as a material for the manufacture of the tray, although aluminum surfaces and molds with Teflon non-stick polymer coating are also commonly used ¹⁸.

The molding operation takes place when the honeys have already crystallized, so they are arranged in molds or boxes where the panela reaches its final shape. Packaging and storage represents the last stage of manufacture. Heat shrinkable plastics and laminated aluminum sheets are ideal for storing panela for long periods of time, without altering their organoleptic characteristics ⁶.

5.1. Combustion chamber

In this zone, the reactions between the fuel (bagasse) and the oxidizer (air) are carried out, to generate a stream of hot gases that will transfer heat to the cane juices contained in the pans ⁴⁶. The combustion chamber is built in rectangular shape, with cast iron doors and with internal and external walls in refractory brick. It is recommended to make an internal isolation with ash or with an air vacuum, to consider the system as adiabatic ⁴⁵.

The combustion chamber consists of a feed mouth, a grate or support and an ashtray. Traditional combustion chambers only admit bagasses with humidities less than 30 %, and work with excess air between 40 - 60 % to avoid the cooling of gases ^29,33,47. In addition, the direct contact of the pans with the flame, and the work temperatures too high, can damage the pans and the fins that increase the heat exchange ³⁷. According to Osorio et al. ⁴⁵, the ideal working temperatures in this area are between 1,000 - 1,300° C.

The CIMPA experimental station has implemented the WARD type chamber (CWC), which exhibits a reaction volume three times higher than a traditional furnace (Table 6). It also has a pre-drying ramp, allows wet bagasses and works with excess air, facilitating air circulation and preventing the formation of large amounts of CO. For its part, the PLANA camera is a modification of common burners, with fewer advantages than the CWC prototype CWC^37,46.

5.2. Heat transfer zone

In this section, the hot gases coming from the combustion chamber circulate through a tunnel and transfer part of their sensible heat to the cane juices, removing the water from the product by boiling at atmospheric pressure. The transfer zone consists of the duct of hot gases and heat exchangers, also called pans, bottoms or batteries. Figure 4 presents the most common geometries for panela pans. The heat transfer between the gases and the bottoms is presented by conduction, radiation and convection, while the honeys gain temperature by convection and conduction mechanisms ^6,44.

Hernández ⁴⁹ determined the global transfer coefficients for different pans, commonly used by the Colombian panela agro-chain. The pyro-tubular pan can be seen as the most efficient, since it shows a considerable increase in the area of heat exchange without sacrificing the overall transfer coefficient, by generating a convective turbulence inside the steps or pipes (Table 7). Other studies, such as of Tiwari et al. ⁵⁰ and Aldana and La Madrid ⁵¹, propose a useful methodology for the estimation of the convective coefficient (h) and the global coefficient (U) for the heat transfer in traditional furnaces.

The pans concentration line can be manufactured from metals such as copper, aluminum, stainless steel or iron, and can be designed in different ways depending on the size of the installation, the production capacity and the energy efficiency desired for the process.

Figure 4 Heat exchangers used by the panela agroindustry. From left to right: traditional semi-cylindrical, improved semi-cylindrical, finned and pyro tubular ⁴⁴. 

5.3. Chimney

The chimney is the vertical duct that connects with the end of the combustion chamber. It can have a cylindrical, cone or pyramidal trunk shape, and can be built in a refractory brick or iron sheet ⁶. This channel produces the air aspiration necessary to burn the fuel, and generates the draft required to transport the gases with the appropriate speed. A simple equation to estimate the drop pressure and the draft generated in the gas pipeline is presented in the work by Shiralkar et al ⁴⁷.

When the hot gases come out too quickly the heat transfer efficiency down. While the hot gases late to out reduces the temperature (300° C or less) and does not evaporate the water contained in the cane juices. According to Osorio et al. ⁴⁵, the temperature in the chimney should remain between 400 - 500° C to maintain high energy efficiency of the traditional furnace.

6.1. Energetic assessment of traditional furnace

Energy efficiency is defined as the ratio between the energy used and the energy generated by the fuel. According to Velásquez et al. ⁹, Velásquez et al. ⁵² and Marcelo et al. ³², the most considerable losses are due to the combustion gases cooling, the high chimney temperatures and the Poor insulation of the furnace walls. Other important variables on energy efficiency are bagasse: humidity, air excess during the combustion, heat transfer between gases and bottoms, chimney draft and location of the facility ⁴⁵.

The furnace thermal efficiency without modification is between 15 - 30 %, while the furnace CIMPA have indicators between 35 - 45 %. The energy losses per chimney constitute 70-90 % and the wall losses are 7 % ⁹.

6.2. Production indices

The production indices allow to estimate the yields of panela or any other by-products; and show together the adequate disposition and use of raw materials and inputs (Table 8 and Table 9).

6.3. Mass and energy balance in traditional furnace

Sardeshpande et al. ²⁹ and Shiralkar et al. ⁴⁷ developed a mass and energy balances for a traditional furnace with single and multiple batteries. Velásquez et al. ⁹ and Osorio et al. ⁴⁵ propose their balances models, focusing on combustion gases as a fundamental basis for the quantification of the production process energy efficiency. Velásquez et al. ⁵² proposed a guide for the exergetic balances and energy integration in a traditional facility for panela production.

The outcomes of mass and energy balance depend to on the climatic conditions and the production protocols of each facility. These variables have a direct influence on the heat generation, the fuel-combustion reaction, the boiling temperature and the specific by-products obtained. Between different furnaces can vary the fire gases flow and the percentage of mud formation; However, relationships such as bagasse consumption and air demand have recommended values that can be achieved through the standardization of the production process (Table 10).