Importance and health hazards of nanoparticles used in the food industry

1 Introduction

Nanotechnology includes a set of disciplines, techniques, and devices to manipulate, restructure, and design matter at a nanoscale level (one billionth of a meter). The material containing particles, aggregates, or filaments of dimensions smaller than 100 nm is now called nanomaterial. Therefore, nanotechnology results in the formation of a diverse range of new structures and systems now called nanoparticles, nanodispersions, nanolaminates, nanotubes, nanowires, buckyballs, quantum dots, and other terms [1]. The modification and fabrication of materials at nanoscale creates small-sized particles with a very large ratio of surface area to volume. This has led to improved optical, electrical, mechanical, and functional characteristics of matter and is responsible for the successful current and future applications of this new interdisciplinary technology [2]. The size of nanoparticles, their distribution, number of interfaces or grain boundaries, the chemical composition of the constituent phases, and their interactions are the basic factors governing the unique properties of nanomaterials. These naturally occurring or engineered particles of the 21st century can be termed “magic bullets” because they can be targeted to deliver in a specific manner and thereby have high potential in various applications, including the manufacture of drug, textile, and food.

Agriculture and food production have been directly associated with human development and welfare as well as ecosystem maintenance. With the constant rise in world population, current environmental hazards, global climatic change, shortage of energy sources, and shrinkage of arable land, the use of modern technologies to increase and improve food production and food quality is imperative. The use of nanotechnology in food and feed processing is applicable at all stages of production, packaging, storage, transportation, and value addition. Nanotechnology has many beneficial effects in the food sector, including the costs involved, environmental hazards, disease detection to prevent losses, and management of farm practices. The enormous potential and ability of this powerful technology to combat various problems have already shown successes in the agroindustrial sector, and many more applications are under research and are yet to be commercialized [3]. The use of nanoparticles, as in crop disease detection and protection, smart delivery systems for bioactives, fertilizers, pesticides, and fungicides, less use of harsh chemicals in the food chain, encapsulation of enzymes for various processes, use of nanosensors in food packaging and transportation, and use of nanoparticles to detect food contamination and food adulteration, has revolutionized the food industry worldwide [4]. The most promising use of nanotechnology in food technology is the functionality that can be achieved by the relatively small amounts of these engineered nanoparticles, their increased interfacial reactions that enhance the effectiveness of their use, and their easy formulation, better handling, and lesser impact on the environment.

Food technology combines all the unit operations that take place from the farm to the time food reaches the fork. The use of lightweight sophisticated machinery and the design of nanoprocessor chips are the benefits of this powerful technology at the farm level of agriculture. Good agricultural produce depends on the adequate use of fertilizers, pesticides, and fungicides and their excessive amount that poses threat to human health and the ecosystem. The targeted release of nanofertilizers and encapsulated pesticides and herbicides offers the benefits of controlled release and prevent the excessive lumps of these chemicals to be dumped into the soil [5]. The use of nanolaminates and various functional ingredients, such as antioxidants, flavor enhancers, browning inhibitors, and enzymes in food packaging, plays an important role to prevent the deterioration of food from excessive ingress of moisture, dust, light, off-flavors, and off-odors. The combination of information technology and nanotechnology has led to the development of nanosensors. Nanosensors have effective applications in packaging, storage, and transit operations of food products as they can sense and signal the information regarding freshness, quality, and physical, chemical, and microbiological changes that are important parameters of food quality and safety. The incorporation of nanoparticles (silicates) in the packaging material can improve the mechanical properties and can be used to make biodegradable packaging material with improved barrier properties. The safety and quality of food products are of utmost importance to consumers nowadays and the sensitivity of nanosensors to detect minute concentrations of heavy metals, pathogens, and toxins in food items is another promising aspect of this nanoscale science in the food sector. The combination of nanobiosensors with intelligent packaging systems (active and smart packaging) is an extremely sensitive and rapid technique for food pathogen detection and food quality maintenance [6]. The advancement in nanofabricated tools has facilitated better detection of plant and animal diseases and the development of novel disease control approaches such as nanostructures to study the mechanism of bacterial colonization in plants and target drug delivery systems [7]. Nanosized functional ingredients manufactured as nutraceuticals and functional food have greater nutrient retention and absorption and are made as nanoemulsions for better bioavailability in the body. Nanotechnology provides an opportunity to modify the structure, properties, and interaction between various food components to design novel food with improved taste, texture, flavor, freshness, and stability [8].

The last decade has seen tremendous growth in the study of the adverse effects of nanoparticles. Most of these reported studies are in vitro studies performed primarily by dosing a specific amount of nanomaterial to cells that are growing at the bottom of the plate. Further, the response of the cells is monitored to conclude what effect does that particular nanomaterial has on the cells. Different studies cannot be compared to each other for these are often carried out with arbitrary conditions and nanomaterials that are not well characterized. For now, we can make out that there are only a few studies that offer consistent results and are of value. Large-scale progress in the field of nanotoxicology has been seen, but there still is a void between validation and evidence-based studies. There is a need for high-throughput methods such as metabolomics along with evidence-based studies.

2 Nanotechnology usage in food – current scenario and public opinion

Food business stakeholders and research scientists have realized all the potential applications of nanotechnology in food, although the success of any emerging technology depends much on its cost-effectiveness as well as on the public perception of its risks and benefits [9]. Nanotechnology has numerous advantages to benefit agriculture and food industry, but its market usage is still marginal and uncertain in the food industry while it is used rapidly in other industrial sectors of medicine, biotechnology, information technology, and physical sciences. This limited market coverage of nanoagrotech products is due to the lack of adequate returns in comparison to the huge initial investment involved, scarcity of regulatory framework, and varied public perception. Large companies are busy patenting various applications to seek the security of their future operations, but industrialists do not identify any large economic gains due to the limited field usage. Various regulatory agencies are assessing the safety issues of the nanotechnology products and are putting up more consensus and harmonized approach toward the improvised food sector applications of nanotechnology [10].

Many studies in this regard have shown that public response toward the awareness of nanotechnology is neutral, but there is more deflection toward those applications where nanoscience is not directly applied to food. The use of natural food constituents is preferred more than nanostructured additives, so this dynamic technology has identified commercial applications in food packaging market with good consumer acceptance than the nanotechnology food themselves [9], [11]. The study conducted by Matin et al. [12] examined the perception of the general public toward the acceptance of nanotechnology in food science, which showed that 30% of the respondents supported the benefits of nanotechnology over risks, 44% were of the opinion that risks are coincident with benefits, and 26% perceived that risks are more than overall benefits. Consumer acceptance towards any novel food item is governed by individual preference or choice and by the knowledge of the science that is behind its production. The focus of nanotechnology toward sustainable environment and society is appreciable but not complementary with the risks perceived to health by the use of this technology [12]. It has been observed that people perceive nanotechnology risks similar to genetically modified food, thus reducing the consumption of such food. Also, government regulations and trust issues influence individual responses [11].

3 Nanotechnology applications in the food industry

4 Nanotoxicity

Choose any model organism and any nanoparticle and you will find contrasting or slightly different studies about the toxicological effect of the same nanomaterial. After years of research, we have only come to the conclusion that materials at nanoscale show drastically different properties and unexpected behavior. This unexpected behavior is what leads to our concerns about its toxicity. The interaction between engineered nanoparticles and various living organisms and the environment is still to be explored at large.

The last decade has seen a growth of about 600% in the number of papers published in the field of nanotoxicology, the study of adverse effects of nanomaterials on health and the environment [74]. Most of these reported studies are in vitro studies performed primarily by dosing a specific amount of nanomaterial to cells that are growing at the bottom of the plate. Further, the response of the cells is monitored to conclude what effect does that particular nanomaterial has on the cells. Different studies cannot be compared to each other for these are often carried out with arbitrary conditions and nanomaterials that are not well characterized. For now, we can make out that there are only a few studies that offer consistent results and are of value. Large-scale progress in the field of nanotoxicology has been seen, but there still is a void between validation and evidence-based studies. There are complaints about the misconceptions [75] and slow progress in the field [76].

Nanoparticles have the unique property of increased surface area per unit volume. This renders them to behave completely different from their bulk counterparts. For instance, bulk gold is normally inert, but as soon as we transform this macroscopic gold to nanoparticles, it shows high reactivity and unique properties [77]. It is due to these unique properties that gold nanoparticles find vast applications from drug delivery to medical imaging. However, nanoparticles are more likely to react with various biological entities such as lipids and proteins or cells as a whole. Nanoparticles may cross the cell membrane entering various organs and activate inflammatory or other immune responses [78], [79].

To foresee the unknown consequences of nanoparticle usage, nanotoxicological studies are performed. A typical toxicity test involves cells or organisms subjected to a specific dose of chemicals (nanoparticles, in the case of nanotoxicological studies) and measuring the response of the cells over a period of time. The dose-response relationship from these experiments determines the optimum dose and acceptable limits for chemicals. However, unlike conventional chemicals and their toxicology studies, nanoparticles, as stated earlier have shapes, surface area, and surface electrical charge completely different from bulk counterparts. These might diffuse, aggregate, sediment, and change the physical and chemical properties of the media they are kept in. The major inference that we draw is that the conventional in vitro assays may misinterpret the results and the dose-response regimes. These conventional assays do not take into account the anomalous behavior of nanoparticles in the environment and their cellular uptake [80].

Most of the toxicity studies have been done at a much higher dose than the realistic dose [81], and as Paracelsus has rightly said, “the dose makes the poison”, these experiments exemplify the same [82]. Most of the substances considered toxic today are harmless in small quantities and are poisonous only when overly consumed. The exact quantification of the release of nanoparticles in the environment and occupational exposure is quite a challenge. The half-life and life cycle of nanomaterials based on modeling studies have been reported. These studies need improvements as data relevant to the industrial production of nanoparticles have not yet been much included in studies, such as the amount that is released at different life cycle stages of these materials and the forms in which these are released into the environment [82]. Nanomaterials may acquire different chemical and physical forms once released into the environment for they have remarkably different physical and chemical properties with respect to their bulk counterparts. These novel chemical and physical properties have a huge impact on various ways through which these may interact with biological components, their uptake, accumulation, and clearance through the body, and interaction with the environment at large. There is more than one factor that governs the toxicity of nanoparticles as shown in Figure 1.

Figure 1:

Different factors that may affect the overall toxicity of a nanoparticle.

It is for sure that due to the highly reactive surfaces nanoparticles in the environment cannot exist as bare particles. It has been observed that a corona of protein is acquired by a nanoparticle surface that decides the pathway through which cell uptake, accumulation, and clearance will proceed [83]. Further, the interaction between the nanoparticle and the biological membrane can be either physical or chemical. Physical interactions mainly result in the disruption of membranes and its activity, protein folding, aggregation, and various transport processes [84]. In contrast, chemical interactions mainly lead to reactive oxygen species (ROS) generation and oxidative damage [85]. The environmental interactions also add complexity to the determination of nanoparticle toxicity [86].

Human exposure to nanoparticles that are airborne cannot be avoided. Intentionally via nanotherapeutics or unintentionally via natural/anthropogenic particles, we are exposed to nanoparticles. The high reactivity of nanoparticles and the multiple entry routes aggravate the problem (Figure 2). These particles can travel large distances facilitated by Brownian motion and are likely to get deposited into our air sacs [87].

Figure 2:

Possible routes of entry of a nanoparticle into the body.

Nanoparticle toxicity has been dealt with approaches similar to conventional toxicity analysis. Most of the studies have suggested oxidative stress as a major parameter for nanotoxicity analysis.

As discussed earlier, nanoparticles may have varied shape, size, charge, solubility, and chemistry as a whole. CNTs, for example, have been extensively studied for its toxicological impact on living beings [88]. The toxicity potential of these nanotubes has come to light due to its striking similarity to different carcinogens such as asbestos.

However, when considering the toxicity, it is not only the nanoparticle but also the various other factors one should consider. CNTs, for instance, can be single walled or multiwalled, functionalized or nonfunctionalized, may or may not be conjugated with the metal catalyst, or may be hydrophobic or hydrophilic depending on the functional group attached and many more such factors need to be considered [89]. As it would become cumbersome to test each and every parameter for toxicity, toxicologists have identified key parameters or tests that allow scientists to screen safer nanomaterials.

A major step in the direction was taken by Xia et al. by carrying out a systematic study on how nanomaterials can induce oxidative damage to cells [90]. The basic idea of the study was to know the mechanism and thus compare the toxicity potential of various nanomaterials. This can be done simply by looking at the generation of reactive species within the cell. Oxidative stress results from the imbalance between oxidants (ROS, peroxide, etc.) and antioxidants (vitamin C, glutathione, etc.).

An increase in the number of oxidants in the cell can have damaging effects on the cell. There is abundant literature on pollution particles such as carbon soot and other nanosized pollutants that lead to the generation of ROS and can lead to oxidative stress (Figure 3) [91]. With the study conducted by Nel et al. we can now compare the oxidative stress profiles of conventional particles to those of particles that are being newly synthesized every day. The data can be further extrapolated to newer materials [89]. If carefully thought out, this and other similar studies can serve as an important building block toward a more efficient screening system for nanotoxicology. Although the study provides insights into the oxidative damage that can be caused by different nanomaterials, a few avenues were left unexplored. First, the experiments should be conducted on more than one cell line; also, primary cell culture (such as human macrophages) should also be used to make sure that the cell culture model is foolproof. Second, different dose regimes should be considered and doses used should be comparable to the dose of the nanomaterial being released into the environment. Third, different media formulations need to be tested as most of the studies have been carried out with fetal calf serum (FCS). FCS contains high levels of different antioxidants. These antioxidants may mask the oxidative damage caused by nanomaterials. Last but not the least, some supplementary assays need to be done to complete the picture. For example, both intracellular and extracellular oxidative stress need to be monitored.

Figure 3:

Various routes of particle entry and stress generation.

The field is in its early days and there is so much to explore. The fact that their particles can distort lipid organization and overall membrane structure [92] is an evidence in itself that the nanoparticles may affect biology as a whole. There is an urgent need for information to better understand the nanoparticle-biological interactions and processes. These interactions primarily involve biomolecules such as proteins, but there are studies that show us different routes to monitor the same. Granick et al. have emphasized on physical interactions of nanoparticles and found that nanoparticles can modulate the lipid membrane phase structuring (Figure 4) [93], [94].

Figure 4:

Different modes in which a cell can uptake substances. A similar mechanism is what nanoparticles are expected to follow.

Therefore, it would be too early to jump to conclusions or to prefer a single field of study. For now, we can say that understanding the manner in which nanoparticles interact (physical or chemical) with biological molecules or living matter can open up loads of opportunities to the field of toxicology. The knowledge and mechanism of oxidative stress seem to be the aptest parameter and appeal maximally to discriminate between toxic and nontoxic materials. The near-future goal of nanotoxicologists seems to develop more studies based on the works of Nel et al. and other similar groups. This will help us learn about the mechanism responsible for nanomaterial-induced toxicity and lead us to safe and profitable nanotechnology.

The National Academy of Sciences report entitled “Toxicity testing in the 21st century: A vision and strategy” emphasizes on new technologies from biotechnology and bioinformatics to revolutionize the toxicity testing [95]. It also emphasizes on learning lessons from alternative methods and their validation. New problems should be dealt with new solutions, and nanotoxicity is one big example. Most of the toxicology tools that are being used for the assessment of toxicity potential of products, mainly nanoparticles, rely on high concentration or dose regime animal studies. These methods have remained unchanged for years. Knowledge in biological sciences doubles really quickly. We now have tons of knowledge and data as we had 60–70 years back. We cannot fuel our cars with a more advanced fuel if we do not change its engine. Similarly, what we need today are better predictive models and tools to minimize time and costs.

Studying the properties of individual nanoparticles, their exposure route, exposure time, the right dose and the right model system can be time-consuming, tedious and expensive. It is here, where high throughput screening methods and computational approaches slide in to save our day. These can rapidly screen and prioritize nanoparticles for toxicology assays and thus accelerating the process of establishing a relationship between material and its biological behavior [96]. Quantitative nanostructure-activity relationship [97] can help us predict the cytotoxicity of a number of metal nanoparticles [98]. Another important aspect to be considered for the field to progress is a detailed characterization of the nanomaterial in question, this would help researchers to use that data to design toxicology assays, properly interpret the results obtained and ensure that data can be reproduced and compared by others. Although researchers have initiated the knowhow of material properties, interactions, and toxicity mechanism, the coming years still have a big challenge to understand the physical and chemical properties, interactions and responses.

Most of the modern toxicology studies are animal based in vitro or in silico; however, there is a need for evidence-based toxicology. With such vast knowledge and advancements, the current toxicological assays need to be revamped and new tools, such as proteomics, functional genomics, high-throughput screening, and metabolomics, to name a few, should be incorporated more and more to these studies. Incorporation of these advanced tools will lead to the minimization of a number of false positives and accelerate and validate the evaluation of toxicity of nanoparticles. The onus of nanotechnology and its safe applications is on the shoulders of scientists and the public must be well informed on the benefits and risks associated with the field.

5 Harmful effects of nanoparticles on humans

The use of nanotechnology in food irrespective of its wide benefits confers the possible adverse environmental, social, and health risks as these particles are believed to enter the ecosystem through the delivery of pesticides in agriculture or through application in processed food such as the packaging sector, thus raising the toxicity concerns about their usage [99]. The enhanced risk of nanoengineered particles is due to the higher reactivity of these nanoparticles and increased bioavailability of smaller particles to our bodies leading to long-term pathological effects. Nanomaterials can enter the food chain through:

1. Direct incorporation of nanoparticles in novel food as nanoemulsions, nanocapsules, and nanoantimicrobial films.

2. By use of nanomaterials in food manufacturing, processing, preservation, and trackings such as the use of nanolaminates, nanosensors, and CNTs.

The level of human exposure to nanoparticles greatly depends on the specific area where it is used in the food industry and the concentration of usage with exposure risk being higher in the fields where nanomaterials are added directly to food products as carriers of novel food ingredients. Some of the toxic effects of nanoparticles used in food are presented in Table 1. The migration of nanoparticles from food packaging materials and the behavior of nanoparticles upon entering the body are still being evaluated at an extensive level [115].

As discussed earlier, nanoparticles can cause oxidative stress to human body cells and can traverse from lungs to blood, cell nuclei, and central nervous system leading to the inflammation of the gastrointestinal tract, Parkinson’s syndrome, Alzheimer’s disease, as well as the impairment of the DNA. Adverse effects on kidney, liver, and other vital organs have been reported due to long-term exposure to nanoparticles [116].

6 Migration issues of nanoparticles

Recent studies have documented the migration of silver nanoparticles into food products with an amount below the permissible limits and there is a lack of safety assessment data in this regard. The result of a study conducted by the National Packaging Products Quality Supervision and Inspection Centre showed that silver migration from fresh nanosilver bags increased in all the food solutions with an increase in time and temperature, suggesting the release of nanosilver particles by dual sorption diffusion and embedding [117]. In addition to the migration of inorganic silver metal, migration trends of modified cellulose nanocrystals have been studied and the migration level of cellulose nanocrystal in isooctane was higher in 10% ethanol but was still within the permissible European Union legislation limits [118].

A modeling behavior was used to quantify the migration of nanoclay from PET beer bottles showing that migration will occur only when 1 nm radius small-sized particle will combine with a polymer of low dynamic viscosity. Two different sized TiO₂ in low-density polyethylene (LDPE) packaging films were tested for migration behavior. The result showed that 100 nm TiO₂ in LDPE film migrated more into the food matrix with the migration trend depending on the compatibility of the TiO₂ with the packaging film and food stimulant liquid. In another experimental model, nanosized titanium nitride was blended with LDPE at 0, 100, 500, and 1000 ppm concentration with food stimulants used as ethanol, 95% iso-octane, and 3% acetic acid. In the results, titanium migration from 1000 ppm was highest with ethanol and iso-octane samples showing no migration [119]. Thus, from the series of exhaustive experiments, it was concluded that the migration of nanomaterials in contact with plastic packaging is negligible. The critical point in migration estimation is the drawback and limitations in detection and quantification techniques. Inductively coupled plasma (ICP)-atomic emission spectroscopy and ICP-mass spectrometry are both used for the efficient identification of nanoparticles, but there are certain limitations in these methods, which make it necessary to include a pre-separation step before digestion. Therefore, for the accurate quantification of nanoparticles, it is important to develop an efficient separation characterization and quantification step [14].

The primary concern is to analyze the extent to which these particles migrate into the food and what happens when the nanoparticles are ingested into the mouth and then absorbed by various organs and then metabolized until they are ejected from the body. Some studies have shown that silver, TiO₂, tin, and CNT enter the gastrointestinal tract from the blood. The physical properties of nanoparticles such as size, surface charge, mass, crystal structure, surface porosity, chemical composition, and state of agglomeration determine the adverse effects of these particles on target organs such as the spleen, liver, kidney, and brain. When nanosized hydrophilic and positively charged particles enter the bloodstream, blood circulation increases drastically. These nanoparticles may pose a threat to all the organs; however, long-term exposure studies are limited to unknown consequences that still need investigation [16]. Dermal exposure, ingestion, and inhalation are the possible ways for nanoparticles to enter the body. Pulmonary inflammatory problems and vascular disease from long-term exposure to carbon nanoparticles have already provoked health concerns among people. TiO₂ nanoparticles could penetrate the skin dermis layers interacting with the immune system through lymph nodes and cause oxidative damage to skin by generating hydroxyl radicals. The acute and chronic toxicity of nanoparticles when inhaled is because smaller particulates of 4 μm travel deeper to the alveolar region with particles of greater size. The particles once into the body are able to cross the blood-brain barrier and can cause pulmonary granuloma, oxidative damage, and pneumonia. However, the general statement regarding the toxicity of all nanosized particles cannot be made until it is evaluated through extensive experimentation trials. From the standpoint of nanoparticles being used in food, it is important to consider the particle size and mass to determine the toxicity. In some experimental trial, it has been shown that 20%–30% of nanoparticles made of polystyrene size 50 and 100 nm were absorbed by intestinal mucosa with penetration being faster for small-sized particles [120].

It is pertinent that nanoengineered particles interact with other food constituents when ingested, but how these particulates are metabolized is still an area that needs research. The use of nanosized emulsion or stabilizer in ice creams is absorbed more in the gut due to small size. In the case where nanoparticles are used in packaging matrix, a model formulated by Simon et al. suggests that the migration of nanomaterial from packaging film increases with decreasing polymer dynamic viscosity and nanoparticle size [121].

Nanoparticles interact with biomolecules and organelles and result in the formation of bio corna, which ultimately have a negative effect on cells such as immunotoxicity, cell death, and genotoxicity [122]. Nanoparticles can cause an alteration in epigenetics by DNA methylation, histone modification, and post-transcriptional changes of gene expression and later can be transferred to generations. Fifty-five types of nanoparticles and 35 patents are identified worldwide in food applications [123], [124]. Nanosized TiO₂, SiO₂, and MgO₂ are used in food processing for various functions without the particle size being mentioned on the label and their properties can differ from conventional additives, thus creating consumer concerns. The detection of nanoparticles in food material is still a complex process and needs validation as various physicochemical processes may alter the properties of nanoparticles and pose a challenge for their acute quantification. Ingested nanoparticles are taken up by secondary organs and absorption varies from organ to organ. When nanoparticles are in contact with biological fluid, they can change the properties due to change in pH and ionic strength and can interact with proteins, lipids, blood cells, and genetic material. Lung inflammation, granuloma, and focal emphysema were demonstrated by in vivo studies of SiO₂ nanoparticles. Gold nanoparticles are widely used in various food packaging applications and have been found to cause chromatin changes in the nucleus of human lung fibroblasts as well as significant changes in gene expression and epigenetic changes in mouse fetus leading to the risk of lung cancers [125]. Toxicological studies of silver nanoparticles have shown that these particles enter the intestinal mucus barrier and are associated with increased ROS generation damaging cell membranes, DNA, and chromosomes as well. Even at low concentrations, the toxicological effects of silver nanoparticles have been evident, whereas abnormal cell damage, shrinkage, apoptosis, and skin cancers have been observed at higher concentrations [9].

7 Environmental impacts of nanoparticles

The growing use of nanoparticles in different products results in the release of a substantial quantity of the nanoparticles into the environment, which probably ends up in terrestrial and aquatic ecosystems. The release of the nanoparticles into the environment may be by any of the three ways viz., naturally, unintentionally, or intentionally. Natural nanoparticles are produced from a forest fire, volcanic eruption, ocean spray, clouds, dust storm, and soil erosion. Unintentional nanoparticles are generated through welding, metal smelting, smoking, mining, fossil fuel-burning industrial waste, and vehicle exhaust. The effect of natural and unintentional nanoparticles on the environment is well documented. The ecotoxicity of these nanoparticles depends on the physicochemical properties (chemical composition, surface chemistry, surface charge, particle size, size distribution, shape, crystal structure, agglomeration state, and porosity) of the nanoparticles [126]. Some common problems such as atmospheric visibility and building soiling are due to the optical properties of the nanoparticles. Because of the black carbon content in the diesel particles, it strongly absorbs light, which can contribute to the global warming.

Nanoparticles that are produced to be used in different processes and materials are engineered nanoparticles. These nanoparticles may be released into the environment through various ways depending on their use. The ecotoxicity of the engineered nanoparticles has been extensively researched, especially the effect on aquatic ecosystem (Table 2).

8 Prospects in nanotoxicology research

The toxicity of different nanoparticles that have a potential to be incorporated in food has already been studied by different researchers. There are many studies that depict the nature of nanoparticles as toxic or nontoxic. During the preparation of this manuscript, it was observed that most of the studies on the toxicity of nanoparticles have been conducted with the homogenous nanomaterial only. As one of the main characteristics of a nanoparticle is the enhancement of its reactivity, it is quite possible that, when a nontoxic nanoparticle is incorporated in food, it may get converted to a harmful form or vice versa.

Food has different roles in the body and the composition of the food is important with respect to that role. A food may contain a functional ingredient that is specific to that food; for instance, beef contains vitamin B12. During the processing of such food, the main aim is to reduce the loss of such functional ingredients. Food, by its nature, is a pool that presents enormous possibilities for biochemical interactions and the incorporation of a highly reactive species of nanoparticles into food may trigger different reactions. The interaction of nanoparticles with such functional ingredients and other constituents is another area of research that needs to be explored.

9 Conclusion

Every development has its own shortcomings, but if it deals with food it turns out to be a serious issue. Despite the miracle properties of the nanoparticle, it may have some harmful effects on the human health. Due to the increasing health consciousness, knowledge, and vast Internet availability, there is a huge pressure on the scientific fraternity to come clear about the safety of nanomaterials used for food purposes. Different studies have been carried out on the safety of nanoparticles, where some nanoparticles have been confirmed to have toxicity and harmful effects on the humans. This type of research is continuously ongoing to support the full evidence of the toxicity or nontoxicity. In this review, we tried to compile as much data that are currently reported on the nontoxicity. However, we could not make a significant contribution in confirming the toxicity of nanoparticles associated with the food by one way or another. It seems that a lot of work is needed to confirm the toxicity of the nanoparticles used in food apart from their individual toxicity. The interactions of nanoparticles with the food systems need to be estimated, which might also have an effect on the digestibility of the food constituents.