1. Introduction
Agriculture is one of the most ancient and important sectors in the world. According to Eurostat [
1], in 2015, just in Europe (EU-28), more than 178 million hectares were utilized as agricultural areas, including arable land (60%), permanent grassland (33%), permanent crops (7%), and other agricultural land, such as kitchen gardens (<1%). The fresh vegetable production area occupied around 18% of the total permanent crops area, and specifically, the tomato production area accounted for over 10% of the aforementioned fresh vegetable production area. In terms of global production, around 130 million tons of tomatoes are produced each year, of which around 42 million tons/year are eventually processed. In the EU-28, 16.6 million tons of tomato fruits are produced per year, representing 12% of the total global production [
2]. Despite these statistics, the geographical distribution is very heterogeneous because Italy and Spain are the largest tomato producers, representing two-thirds of the total European production.
Millions of tons of tomato are processed every year to produce products for which the manufacturing requires peel removal, such as peeled tomatoes (whole, diced, or sliced), juices, sauces, and ketchup. Peeling is, therefore, the first unit operation performed during the industrial transformation of tomatoes prior to further processing, and as such, its performance is crucial for maximizing the efficiency of the overall process as well as for preserving the quality of the fresh product [
3].
Hot lye peeling is one of the most popular industrial methods of peeling tomato fruits. It involves the chemical pre-treatment of tomatoes by immersing the fruits in a hot lye (NaOH) solution at a high concentration (8–25%), which depolymerizes the external layer of tomato skin, facilitating its splitting and removal by peel eliminators (washing, core scrubber, and pinch bed/rollers) [
3,
4,
5,
6]. Although this peeling method is reported to be highly effective in producing high-quality products with high peelability [
4], its usage presents several problems, such as high-water and energy consumption and especially the disposal of large amounts of peeling effluent discharge characterized by high salinity and high organic content [
4,
7].
In order to reduce or avoid chemical contamination in wastewater and other negative environmental impacts, food processors have adopted pressurized steam peeling coupled with cold water or vacuum cooling and pinch rollers, as an alternative peeling technique [
3,
6,
7]. During such processing, tomatoes are exposed to low- or high-pressure steam (50–200 kPa) for a few seconds (10–60 s), which causes the tomato skin to weaken (biochemical mechanism), vapor to form under the skin with the consequent increase of internalized pressure, and the peel to crack (thermal and mechanical mechanisms), all of which are required for effective peeling [
3,
7]. Although this method does not cause the serious environmental problems that lye peeling causes, it may lead to lower product quality, and it requires a lot of water, high pressure, and energy, which increases the cost of the final product [
4,
6].
For these reasons, current research is focusing on new sustainable peeling alternatives that can effectively peel tomatoes with minimum losses and a higher-quality end product, while also causing fewer environmental problems and reducing water and energy consumption. In recent years, the application of unconventional technologies as a pre-treatment for the peeling process—such as infrared radiation heating, ohmic heating, ultrasounds, and enzyme use—has been investigated intensively [
4,
5,
7].
Among these technologies, manufacturers are showing a growing interest in the application of pulsed electric fields (PEF) as a tool in food processing. The effect of PEF pre-treatment is the permeabilization of the cell membranes, which can improve the mass transport of intracellular compounds (e.g., water, juices, and solutes) in several processes of food industry (e.g., drying and extraction) [
8] upon the application of an electric field of moderate intensity (E < 10 kV/cm) and relatively low energy (W
T < 10 kJ/kg) [
9]. Many investigations have proven that PEF can enable energy-efficient dehydration of plant food matrices [
8], as well as enhance the extraction yield of juice and bioactive compounds from a wide range of plant food materials and food processing by-products [
9,
10].
From an environmental point of view, the agri-food sector is responsible for one-quarter of the impact in different environmental categories [
11,
12]. In light of the relevance of the agri-food sector in human feeding, the economy, and the environmental, a lot of effort and research have been developed in recent years to increase the sustainability of production not only at the cultivation stage, but also at the food processing stage.
With this purpose, the life cycle assessment (LCA) has been applied as a powerful tool to evaluate the environmental impact of a food product along its life cycle. There are several valuable studies available in the literature that use this methodology to assess the environmental implications of conventional fruit or vegetable products’ value chains. In this sense, Longo et al. [
13] used the LCA to assess the energy and environmental performance impacts of organic and conventional apples. Baudino et al. [
14] applied the LCA method and also analyzed the strengths, weaknesses, opportunities, and threats by means of SWOT and TOWS analysis, in order to improve the production chain of kiwi fruit and baby kiwis in Italy. Accorsi et al. [
15] focused their study on applying the LCA to glass and plastic bottles of extra-virgin olive oil, and Mouron et al. [
16] compared the environmental impacts of losses of fresh potatoes with those of French fries. More focused on cultivation modes, other studies have also assessed the environmental implications of both open-field and greenhouse crops, considering a variety of fruits and vegetables, tomatoes among them [
11,
17]. Specifically with regard to tomato production, there are also studies that have focused on the environmental burdens associated with the cultivation of tomatoes, in greenhouses [
18,
19,
20,
21], open-fields [
22], or both [
23,
24]. Generally, the environmental burdens associated with greenhouse cultivation are much greater than the impacts associated with the open-field approach. Furthermore, the LCA of different tomato-based products has been previously studied by other authors. For example, Manfredi et al. [
25] analyzed tomato puree production, and De Marco et al. [
26] focused their research on mashed tomato package manufacturing. In both cases, the results strongly depended on the boundaries set to carry out the analysis as well as the life cycle stages considered.
Nevertheless, there are a lack of studies on tomato production combining the application of PEF technology and the evaluation of its environmental impacts from the LCA perspective. Pardo and Zufia [
27] evaluated the environmental impacts of some traditional and novel food preservation technologies from a LCA perspective. These techniques were autoclave pasteurization, microwaves, high hydrostatic pressure (HPP), and modified atmosphere packaging, and they were applied to different ready-to-eat meals based on fish and vegetables. More recently, Aganovic et al. [
28] used the LCA methodology, as well as energy balances, to compare a conventional thermal preservation technology with two innovative alternatives—PEF and HPP. This study was carried out in a tomato and watermelon juice processing line, and it concluded that no huge differences in environmental impact were found over the three aforementioned technologies, considering “gate to gate” system boundaries.
Within these premises, one of the purposes of this paper, which was carried out in the frame of the European project “FieldFOOD”, is to assess the potential of PEF technology at the industrial level to facilitate the steam peeling of tomato fruits. Furthermore, the study aims to perform a LCA study to estimate how the environmental impacts generated by peeled tomato production are influenced by applying PEF technology. Potential environmental benefits and bottlenecks are identified in order to address the main technological challenges.
4. Conclusions
Currently, pressurized steam is one of the technologies most frequently used to carry out tomato peeling on industrial scale. However, a lot of water and energy is consumed by using such technology. But when a PEF pre-treatment is applied, a tomato product of the same peelability and quality can be obtained by using a much lower pressure, thus using less steam and natural gas.
In order to analyze the environmental improvements associated with the experimental incorporation of this PEF treatment at industrial level, LCA methodology was applied to the processing line of an Italian company that manufactures peeled and canned tomatoes. Firstly, the total life cycle of the product (from cradle to grave) was analyzed, and it was found that all of the variation due to PEF incorporation was located at a few particular stages of the processing line—tomatoes washing and thermo-physical peeling. Thus, the detailed LCA was mainly focused on those stages.
In the case study analyzed in this work, when the PEF technology was applied, the amount of steam required for the thermo-physical peeling stage decreased by 20%. Moreover, is the PEF modifications did not required air blowing during the washing stage. Thus, both effects reduced the energy consumption of the process and its associated impact on several environmental indicators (climate change, ozone depletion, terrestrial acidification, etc.).
The tomato peeling and the canning stage had the highest impact on most of the environmental categories from a gate-to-gate perspective. When the total processing line is considered, the global environmental improvements are specially reflected in the benefits obtained by the ozone depletion and fossil depletion indicators because of the consumption reduction. In addition, if the study focused only on the thermo-physical peeling stage, all of the environmental indicators improved between 17% and 20% in absolute values when the PEF technology was used. Further industrial tests should be carried out on other tomatoes varieties—especially those that are difficult to peel even in conventional treatments—in order to confirm these preliminary results.