The food industry is very competitive and must satisfy the growing need for optimized processed and reduced food processing’s carbon footprint.
Food processing techniques include food preservation, transformation, and the extraction of nutrients from food, take an essential role in food manufacturing. Recently consumers have started to manifest their need for less processed and greener food products.
On the other hand, there is a growing need for derived ingredients such as food colors, antioxidants, and aroma extracted from foods.
The concept of green food processing refers to the idea of meeting both the needs of more optimized processed and the growing demand for greener food products by consumers worldwide.
How can food processing techniques become eco-friendlier? Here are a few principles that help understand which aspects it is possible to leverage to produce greener food products:
- Reduced consumption of energy.
- Reduced use of water.
- Reduced use of solvents.
- Reduced generation of hazardous substances.
- Reduced nutrient extraction time.
Conventional food processing techniques are more prone to losses of thermally-sensitive nutrients and phytochemicals and are often inefficient due to time- and energy-consuming processes. Instead, “green” food processing techniques involve less time, water, and energy.
Heating by microwaves is an example of how to destruct cells and liberate metabolites. Ultrasound and electric pulse fields represent other good examples since they allow the destruction of cells that accelerate mass transfer and shorten the processing time.
Innovative food processing techniques are a crucial factor in transforming the food industry into a champion of sustainability.
What follows is a detailed description of standard green food production techniques.
Instant controlled pressure drop technology
Instant Controlled Pressure-Drop (ICPD) is a food processing technique used for food drying and decontamination. In addition to working as a food drying method, this technology also works as a decontamination process for powders, pharmaceutical and cosmetic products, animal feed, and fresh-cut fruits and vegetables. ICPD works by exposing the raw material to saturated steam for a short time, followed by a rapid pressure drop near-vacuum. This mechanical stress can instantly vaporize water and produces an instant cooling of the sample, associated with a swelling phenomenon that causes the rupture of cells which kills bacteria and allows the extraction of metabolites from food.
The food product undergoes a textural change resulting in greater porosity and reduced diffusion resistance, and high quality of the extract.
The equipment used for this technique is composed by:
- an autoclave where the sample is placed at the beginning of the process and which is filled with steam,
- a controlled pressure valve,
- a vacuum system composed of a vacuum pump and a tank, and
- an extract collection trap to collect the extracts.
This technique is suitable for food transformation, preservation, and nutrient extraction. ICPD is also considered foodstuff decontamination and debacterization process and can eliminate even spores. Its applications are not limited to food but extend to cosmetic and pharmaceutical products as well. It is considered an alternative to the classic and more energy-consuming hot air drying and freeze-drying techniques and is particularly suited for heat-sensitive food products such as strawberries and other types of fruit. Interestingly, strawberries treated with this system tend to be richer in anthocyanins and phenolic compounds than the same fruit treated with other drying methods. Used with olives, ICPD can sensibly improve the yield of polyphenol extraction by up to three times by facilitating the diffusion of solvents. ICPD-treated legumes (particularly soy) tend to be less allergenic thanks to reduced IgE-binding ability.
Since ICPD can increase the porosity of cell walls, it is an efficient pretreatment method for the extraction and texturing of vegetable materials, for instance, to extract lipids and phytochemicals.
This technique is suitable for producing swell-dried products of both fruit and vegetables (e.g., apples, strawberries, bananas, tomatoes, onions, and others), which are ingredients in several healthy foods. In China, it is common to treat teabags with ICPD because it allows an improved diffusion of tea in both cold and hot water.
Pulsed electric field (PEF)
PEF treatment is also called “electropermeabilization” or “electroporation” and consists in the application of an electric field to cells to increase the permeability of their membranes. Molecular biologists often use electroporation to transfer exogenous DNA into a cell. It is an emerging green tool in food processes with many potential applications. The latter include food preservation (through enzyme or microbial inactivation), acceleration of freezing and thawing, osmotic dehydration, convective drying, fermentation, among others.
The level of effectiveness of this technique depends on several parameters, including treatment time, pulse width, and electric field strength, among others. Also, the physicochemical properties of the treated matrix (its conductivity and pH), the characteristics of the treated cells (size, shape, etc.), and their state (suspension, semi-solid, or solid) can influence PEF effectiveness.
PEF works for food preservation both at room temperature and in the presence of heat. At sufficiently high energy (> 1,000 kJ/kg), PEF can inactivate enzymes at room or mild temperature. However, in specific situations (for instance, when aiming to eliminate very resistant bacteria or spores, which are highly stress-resistant), the combination of PEF with heat, antimicrobial agents, or pH contributes to an increased lethality effect. A temperature greater than 60°C combined with 30 kV/cm allows an effective spore inactivation.
Compared to traditional thermal treatments, the use of PEF guarantees increased maintenance of foods’ nutritional properties. PEF-treated beverages have higher antioxidant and vitamin contents compared to pasteurized ones.
PEF can also effectively contribute to a reduced crystallization during freezing, which is a known cause of decreased food quality in frozen foods. Cell electroporation before freezing enables the introduction of cryoprotectants to prevent crystal formations inside the frozen cells. This technique works well with spinach leaves and potato strips impregnated with the cryoprotectant trehalose or with glycerol-treated apples.
The combination of PEF with osmotic dehydration increases water loss and the migration of solutes into the food matrix, with reduced losses.
PEF can also significantly reduce energy consumption and acceleration of cooling and drying time when applied before freeze-drying, with similar results observed for radiant and convective air drying. The latter show that PEF is an eco-friendly technique.
Another relevant application is the extraction of molecules of industrial interest from food, reducing the use of solvents and force fields (i.e., pressing, filtration, and centrifugation). Electroporation before applying these techniques significantly improves yields and saves energy.
In winemaking, PEF pretreatment before the macerating fermentation step allows more efficient polyphenol extraction and the production of a wine with better organoleptic features.
Traditionally, sugar is thermally extracted from beets. PEF pretreatment increases the efficiency of sugar extraction, with lower colloidal impurities (e.g. pectins) at lower temperatures.
The extraction of oils and juices is a delicate process since these products can quickly get spoiled. PEF pretreatment before mechanical extraction increases extraction yields and results in the production of oils and juices with higher polyphenol content.
Other potential combinations that increase yields and reduce energy consumption are filtration, distillation, (faster) fermentation, removal of undesired food parts such as skin or peel, and meat softening through turgor reduction.
The application of a moderate PEF treatment during cooking, frying, or meat curing improves the tenderness and organoleptic quality of the prepared food. Recently, a cooking device called Nutri-Pulse® e-Cooker® has become available on the market. The latter uses electroporation to cook food with increased nutritive properties, flavor, color, structure, and taste.
Supercritical fluids
Supercritical fluids (SCFs) are fluids heated above their critical temperature, i.e., the temperature above which a gas cannot be converted into a liquid by pressure alone and pressurized above their critical pressure (i.e., the pressure of a gas in its critical state). As an example, the critical temperature of water is slightly less than 374°C. SCFs have a solvating power similar to liquids, a viscosity similar to gases, and a diffusivity between gases and liquids. These characteristics make them suitable for being used as solvents, and the most common SCF is CO2 (SC-CO2). Sometimes a polar modifier like ethanol or methanol is used together with SC-CO2 to enhance its effectiveness as a solvent. A typical way of producing SCFs is to extract them from solids with autoclaves.
The advantages of using SCFs include the possibility to limit or avoid chemical solvents, which also implies that it is possible to obtain a solvent-free extract. Also, applying a depressurization after the solvating process makes the SCFs evaporate, which makes purification unnecessary. Notably, supercritical precipitates are sterile. Possible applications of SCF include:
- food preservation (through sterilization, microbial, virus, and spore inactivation),
- extraction of high molecular weight compounds such as oils,
- alcohol removal,
- hexane removal from vegetable oils,
- aromas and flavors extraction,
- purification of beta carotene from carrot oil,
- fractionation of fish oil triglycerides, and
- coffee and tea decaffeination.
Microwave extraction
The main difference between conventional and microwave heating is that in microwave heating, heat dissipation occurs inside the irradiated medium and no heat transfer occurs between the heating device and the heated object. Microwave heating is also faster, although the distribution of heat in the medium is not homogeneous and depends on the geometry of the heated object. Examples of applications of microwave to food processing include drying, tempering, thawing, blanching, sterilization, pasteurization, baking, and extraction.
Drying represents a traditional preservation method that prevents food spoiling by reducing food’s hydration. Microwaves are absorbed more in the product’s dry regions, resulting in high thermal efficiency, shorter drying time, and improved final quality compared to traditional drying. Microwave drying also maintains a higher product quality in terms of color, aroma, and texture. Vacuum Microwave Drying results in an even greater quality because it allows a higher retainment of volatile compounds sensitive to losses through thermal and oxidative degradation. The absence of air limits oxidation, making vacuum microwave drying optimal for berries, fruit gels, garlic, cabbages, and mushrooms.
Microwave blanching is a faster alternative to traditional blanching for destroying microorganisms on the product’s surface using less energy. Interestingly, microwave blanched food products tend to be firmer, have equal or better nutrient contents, and a similar color compared to products treated by conventional blanching.
Large amounts of produce are frozen each year so that they can be available all year round. Microwave thawing is a technique to defrost frozen food faster and using less energy. The ice is insensitive to microwave energy, but the latter can heat the unfrozen water inside the product. Food products contain water in liquid form. However, its uneven distribution leads to partial thawing and thermal runaway, i.e., a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. The latter methods make it challenging to maintain a uniform temperature during the thawing process. Microwave thawing is typical for fish fillets and meat blocks.
Microwave tempering is a cost-effective technique to temper (i.e., improve the hardness and elasticity of) raw materials faster, with no drip loss, without tempering rooms.
Microwave baking reduces baking time by more than 90% compared to conventional baking. Baking cakes by microwaves results in better textural properties such as higher moisture content and firmness.
Microwaves are also suitable for pasteurization and sterilization, which work well with mashed potatoes, biphasic food products (e.g., salsa con queso), green beans, and mashed carrots. Microwaves also work well for liquid pasteurization with reduced energy costs.
Faster food extraction techniques that need lower amounts of solvents reduce pollution and costs and are obviously on demand. Solvent-free microwave hydrodistillation and hydrodiffusion are examples that find their application in the extraction of essential oils from aromatic plants and fruits. These techniques combine microwave heating and distillation at atmospheric pressure. The internal heating caused by microwaves makes the plant cells distent and induces the rupture of the glands and oleiferous receptacles. After extraction of the essential oil, a cooling system outside the microwave oven condenses it. Compared to conventional methods, microwave hydrodistillation causes lower CO2 emissions and a lower environmental impact.
The use of microwaves in food processing allows a reduction of the latter to seconds or minutes, which consequently reduces processing costs, makes manipulation simpler and guarantees a final product with higher purity. Also, post-treatment of wastewater is not needed.
Ultrasound-assisted food processing
Ultrasound has a frequency between 18 and 100 kHz, and the human ear cannot hear. High-power ultrasound has an intensity higher than 1 W/cm2. Ultrasound in food processing work through acoustic cavitation: micro-bubbles created in a liquid phase that subsequently collapse due to pressure changes. The latter implosion can disrupt the surface of solid matrixes, which enhances mass transfer and accelerates diffusion. Ultrasound’s effectiveness is related to the acoustic frequency, the temperature, and the pressure applied. Lower frequencies can generate larger bubbles which then collapse more violently. On the other hand, higher frequencies cause more collapses per unit of time.
Since the ultrasound increases solubility by reducing the size of solid particles, some cleaning baths use ultrasound for solid dispersion in a solvent. As its name suggests, cleaning baths are suitable for cleaning small objects by immersion. They can also work for degassing liquids without increasing the temperature. In the latter case, the acoustic waves make the bubble inside the liquid grow to a size that will make them rise on the liquid surface releasing the gas they contain outside the liquid. The beer industry uses this technique to degas beer before bottling.
Another application of ultrasound is mould cleaning, which removes surface coatings without leaving any residual material in the mould. Notably, ultrasound cutting, suitable for fragile, heterogeneous (e.g., bakery products) or fatty foodstuffs (cheese), uses a blade attach through a shaft to an ultrasonic source.
Meat is traditionally tenderized by mechanical pounding, which also works to make low-quality meat more palatable. Ultrasound can enhance meat tenderization by helping break the integrity of muscular cells and stimulate enzymatic reactions that make the meat tender. Tenderness is considered the most relevant factor that consumers consider to judge meat palatability.
Ultrasound also represents a new preservation technique able to eliminate microbial activity by disrupting bacteria cells. In combination with heat treatment, it makes food sterilization faster, reducing the thermal treatment duration and the resulting nutrient loss. Saccharomyces cerevisiae is more sensitive to ultrasound compared to other vegetative forms. Also, ultrasound work for inactivating Staphylococcus aureus, Pseudomonas fluorescens, Listeria monocytogenes, and Escherichia coli.
High-power ultrasound works for enzyme inactivation, which is helpful for the stabilization of some food materials without modifying the foodstuff’s flavor, color, or nutritional value. In general, though, ultrasound for enzyme inactivation works best in combination with other treatments, such as short-time heat treatments.
Ultrasound-assisted extraction is used thanks to its ability to break cell walls. It is an emerging technology that can accelerate aroma extraction, pigments, antioxidants, and other organic compounds from foods.
Conclusions
Consumers are demanding safer, more nutritious and eco-friendly food products. This trend represents both a challenge and an opportunity for the food industry to renovate itself and contribute more significantly to sustainable development goals, particularly goal 12: “Ensure sustainable consumption and production patterns”.
Environmental studies (such as life cycle assessment, LCA) are inherently complex, mainly because they need to include the whole food production chain from agricultural production to processing, storage, packaging, distribution, consumption, and waste management.
A future trend will be the selection of the technology that best suits each specific food or ingredient.
