The canning industry plays an important role in the food sector, providing safe food regardless of the seasonality of the crops. Currently, companies recover processing waste for feed and biogas production, and they may turn it into a valuable source of macro and micronutrients in the future.
The preserved vegetable industry, and in particular that of tomato preserves, is one of the most flourishing in the world. World production exceeds 180 million tons per year, 20 million are produced in Europe, two thirds are destined for the fresh market, one third for the canning industry with a predominance of concentrate, sauces, ketchup, paste, pulp, peeled and many other specialities.
Production is subject to seasonality. Tomatoes for industry are grown in temperate climate zones, roughly corresponding to the 40th parallel North and South. 90% of the crops come from the Northern Hemisphere and the processing takes place between July and December, the remaining 10% comes from the Southern Hemisphere and the processing takes place between January and June. The main producing and processing countries are China, India, Turkey, the United States, Egypt, Italy, Iran, Spain, Mexico, and Brazil.
The interests of the sector are protected by two international organizations: AMITOM – International Mediterranean Association of processed Tomatoes) based in Avignon (France) and WPTC (World Processing Tomato Council). AMITOM is a group of leading manufacturers in Europe and the Middle East and in turn adheres to the World Processing Tomato Council which includes, among others, the North American Group, Argentina, Australia, Brazil, Canada, Chile, China Japan, Peru, South Africa.
Harvest and sorting
As with any other food processing, the quality of vegetable preserves is determined by the raw material quality and by the processing technologies used. Harvesting machines are used to harvest industrial tomatoes. The plant is cut at ground level and raised with elevators equipped with shaker chains that ensure a first removal of dirt and debris. Some rotating organs help to detach tomatoes from the vines. The tomatoes fall onto a conveyor system which carries them to the next processing stage, while the vines move along the machine and are discarded back into the field.
The sorting belts carry the tomatoes in the opposite direction toward the front of the machine. Sorting is carried out by an electronic sorting machine formed by a projector and photosensors that first of all recognize the colour of the tomatoes and the presence of foreign bodies. Pneumatic cylinders remove any foreign bodies and non-compliant tomatoes, while a conveyor system unloads the conforming product into the loading trailers that run alongside the harvester. The tomatoes thus harvested must reach the processing plant as quickly as possible and, in order to maintain their original organoleptic and nutritional characteristics, must be processed within a few hours.
Possible damage during transport due to impacts, infestations, or temperature changes can become source of rotting with consequent depreciation or loss of part of the crop. On arrival at the plant, the tomatoes are weighed and discharged into a collection channel where the continuously circulating water sends them to the washing section which removes mud, stones or other foreign bodies. The recirculated water is finely filtered to block even the smallest residues. Cleaned tomatoes are further sorted, and pieces that do not conform by colour or other defects of various kinds are discarded. The conforming product is subjected to different processes depending on the desired finished product.
Concentrate Processing
For the production of tomato concentrate fruits are rinsed with drinking water under pressure and passed through a blanching machine and a shredder. Heating activates the activity of pectinolytic enzymes and facilitates peel removal. The temperature applied during the chopping phase of the fruit influences the characteristics of the finished product. In the cold-break technique the operating temperature is between 60°C and 75°C, in the hot-break technique temperatures are between 85°C and 100°C. In the first case a relatively fluid juice is obtained, due to the highest activity of pectinolytic enzymes; in the second case, the product is thicker and viscous.
A refiner then separates the juice from skins and seeds; the crushed material forcibly passes through a series of sieves with ever smaller holes (1.2 to 0.5 mm). Sieves with larger holes (1.2 mm) block seeds, stalks, foliage residues and most of the skins. The subsequent sieves remove the smaller particles that have escaped the previous sections of the refiner. The juice, collected in a stainless-steel tank, is then fed into evaporators. The tank is sized to prevent long product stagnation inside it. Such stops can promote microbial growth and acidification of the finished product. The evaporators work at low pressure and remove excess water until the desired concentration is reached. Salt or other ingredients may be added to the concentrate prior to packaging.
Packaging
The concentrate intended for the consumer is usually packaged in tubes or tinplate cans. The concentrate for the professional sector is instead packaged in steel drums or in bag-in-box. Dosing is carried out with volumetric fillers. The product is hot-packaged, and cooling creates a vacuum that helps protect the product from oxidation while reducing possible corrosion in metal cans. The lids are crimped to the body of the can, while the tubes are closed by folding the bottom and sealing it with mechanical or ultrasonic techniques. In both types of packaging, the ratio between surface area of the container and product quantity does not guarantee sterilization of the inner walls of the container.
Hot filling is therefore followed by further heat treatment with the packages being placed in a boiling water bath for a few minutes. Next come the cooling by cold water mist and the drying with hot air. Tubes cooling takes place in cooling tunnels usually divided into three sections: The first involves a dry stop at the filling temperature; the second provides cooling with cold water sprays; the third is drying by air blades.
Aseptic packaging is preferred for drums and bags-in-boxes. The product is sterilized by an indirect method using concentric tube, triple tube or scraped surface heat exchangers. The concentrate stops for the time necessary to equalize the temperature of the product and to make the heat treatment effective. This is followed by cooling to 40 °C using exchangers similar to the previous ones, where the vapour is replaced by a cooling fluid. Filling is done with an aseptic filler using technologies and procedures that reduce contamination risks.
Other processes
Tomato varieties rich in fiber and pectin are used for the purée, giving preference to the hot-break technique. The purée can be obtained only by direct pressing, followed by centrifugation and mechanical refining of fresh, healthy and ripe tomatoes. The water is then partially removed, paying attention to the pH of the finished product, which must be lower than 4.5. Salt, spices or other ingredients can be added before packaging.
Packaging is carried out in glass bottles, polycoupled containers or tinplate cans. Peeled tomatoes are produced using pear-shaped varieties with an easily detachable peel. The intact tomato is quickly scalded at 130/140°C in a pressurized water vapour environment and then placed in a second vacuum environment where, following flash evaporation, the peel is efficiently loosened and removed. The loosened skin is then removed together with any remaining fragments, and the fruits are packaged in tinplate jars.
Sterilization occurs at temperatures around 95°C in static baths at atmospheric pressure or by passages in water and pressurized steam, followed by water cooling. For pulp production, the washed, sorted and scalded tomatoes are sent to a pulper where pneumatic rollers push them against a mesh which transforms the product into a more or less finely extruded pulp, depending on the characteristics of the mesh. The pulp is then mixed with sauce and packaged in jars for subsequent pasteurization.
The exploitation of by-products
The by-products of the tomato industry are many and varied: There are peels, seeds, vascular tissues and pulp residues. They may exceed 10% of the weight of the processed product. Their incorrect disposal can have a considerable environmental impact. Today they are mainly used in agriculture as fertilizers, biochar or feeds, for the production of biodiesel and electricity, for bio-solarization (a soil disinfection technique that leads to the reduction of pathogens by raising the temperature of the soil, by covering it with transparent plastic tarps). These by-products could be better exploited if reused in the food sector.
They can in fact prolong the storage and improve the rheological and organoleptic properties of the products to which they are added or become ingredients in functional and nutraceutical foods. They can contain on average 38% fiber, 21% crude protein, 13% fat and 4% ash. They are rich in micronutrients, antioxidants like carotenoids and polyphenols, pectin, vitamins (C, A and K), and minerals (potassium and iron). Over the years, the European Commission has considered it appropriate to finance various research projects aimed at exploiting them.
Carotenoids are natural fat-soluble pigments that have long been considered functional ingredients for use in novel food, pharmaceutical and cosmetic formulations. In particular, tomato by-products could be excellent sources of lycopene, β-carotene, α-carotene, lutein, phytoene, phytoluene. The phenol family includes simple phenols, phenolic acids, coumarins, stilbenes, flavonoids, lignans, condensed tannins. There is a scientific belief that polyphenols can intercept reactive oxygen species and free radicals, reducing the oxidation rate of certain molecules, protecting cell membranes from lipid peroxidation. They would also be able to chelate metals that can catalyse some oxidation reactions and could intervene in the regeneration of some vitamins.
In theory, tomato industry waste can become a discrete source of polyphenols without replacing the current main sources for industrial applications, namely grape seeds and green tea. Pectin has always been used as a gelling agent, stabilizer, emulsifier and thickener. It is a structural plant fiber found in the cell walls and intracellular layer of plant cells, particularly in fruits such as citrus fruits and apples. Dried tomatoes can contain up to 15% pectin. Although present in significant quantities in different plant species, only pectin extracted from some of them has the functional properties required in industrial applications. Its primary use is as a gelling agent; this functionality is conditioned by the size of the molecule and its degree of methoxylation which in turn varies depending on the source and extraction conditions.
The pectins used in industrial production are the low methoxyl category (content of methyl ester groups less than 50%) which gels in the presence of calcium ions, and high methoxyl pectin (with more than 50% methyl ester groups), which gels in the presence of sugar and acid. The strength of the gel is related to the molecular weight of pectin. As the molecular weight increases, the gel becomes more viscous and stronger. In the world of nutraceuticals, the idea is gaining ground that multicomponent extracts can be more effective and have broader functions than the extracts of isolated compounds. Their concentrations and functionality also depend on the processing undergone by the tomato, any waste pre-treatment, and the extraction method.
Several studies attest that the heat treatments applied during tomato processing influence the carotenoid content in by-products and that this variability is also influenced by the type of carotenoids present. The availability of lycopene increases as a result of the heat treatment which makes it easier to extract. Heat breaks down cell walls and weakens the bonds between lycopene and fruit tissue, improving its extraction, but excessive heating degrades the active substance following isomerization and oxidation. Products processed with the cold-break technique have an average lycopene content of 40 mg/100 g dry weight, while samples processed with the hot-break technique have an average lycopene content of 9.5 mg/100 g dry weight.
The thermal degradation also varies depending on the compound, with losses of up to 28% during processing to obtain tomato paste, while other carotenoids such as beta-carotene, phytoene and phytofluene show no significant change. Among the main obstacles that prevent a massive use of these by-products by the food industry there are problems related to their health and hygiene safety, the complex and unstable chemical composition of useful micronutrients, the variable quality of raw materials, the seasonality of processing, the lack of legislation, the economic feasibility, and other environmental and sustainability assessments in general. The main health and hygiene issue is related to possible contamination with pesticide residues or the presence of other contaminants such as mycotoxins, heavy metals, persistent organic pollutants.
The instability and rapid degradability of stored by-products may compromise their safety as a result of the formation of toxic compounds. One of the techniques identified for better preservation of biomass before processing is fermentation, which can, however, affect the composition of some bioactive compounds. Transferring on an industrial scale advanced extraction techniques successfully developed in the laboratory is not easy. The main obstacles are the difficult optimization of the process, the current low economic viability, and a poor regulatory framework, which is still focused solely on the feed sector.
Most studies concerning the development of sustainable and ecological extraction processes for bioactive compounds have so far been performed only in the laboratory, basing the optimization of the extraction parameters on mathematical-statistical models. In theory, an easy scalability of the processes could be hypothesized, but this step seems to be much more complicated than expected and influenced by factors not relevant in the laboratory, first of all the decision to choose between batch or continuous extraction, from the solvent/source material ratio and many others.
