Introduction

Food products have high nutritional value that lead to their easy contamination by microorganisms. Microorganisms contaminate food and generate toxins in self-defense to cause several diseases in humans. Food adulteration also induces ill effect to human health. The toxin and adulterant can induce vomiting, diarrhea to paralysis and even death. Being primary and fundamental need of humans, the food safety is a global concern. The health problem is any fold worse in case of warfare use of such toxins or chemicals. Hence, there is urgent need to develop quick and sensitive approaches to detect the harmful chemicals, bacteria and related toxins (Franz et al. 1997; Leggiadro 2000; Zhu et al. 2009). Due to their unique properties nanoparticles are increasingly employed to develop detection techniques for sensing contamination, adulteration and freshness of food materials.

Proper packaging of food product prevents them from moisture, contamination and spoilage. Traditional passive packaging systems act as passive barrier against air, dust and moisture. However, the passive packaging systems fail to address increasing concern of food safety and bioterrorism (Vermeiren et al. 1999), whereas intelligent packaging system can sense the quality of food products and protect the shelf life and brand name of packed food. Intelligent packaging uses various smart packaging devices like barcodes, time–temperature indicators, gas indicators and biosensors. As the existing barcodes-based protection tags are very easy to manipulate, nanoparticles-based invisible and sophisticated tags would be very useful to verify the originality of food products in future (Banu et al. 2006; Birtwell et al. 2008). Sensing is an important part of intelligent packaging system (Farahi et al. 2012). Present article describes the food sensing application of nanoparticles (Fig. 1). This article is an abridged version of the chapter published by Kumar et al. (2016) in the series Sustainable Agriculture Reviews (http://www.springer.com/series/8380).

Fig. 1
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Diagram showing various applications of nanoparticles in food industry. Use of nanoparticles in sensor ensures that food material is authentic and safe to consume

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Nanosensors of microorganisms, toxins and adulterants

The food items are good to consume only if they are fresh and free from adulterants and contaminants. Detection of food adulterant and contaminants at low level using routine detection system is a challenging task. So, nanoparticles were explored to detect the toxic chemicals and microorganisms with high sensitivity (Table 1).

Table 1 Nanoparticles-based detection of food pathogenic bacteria
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Detection of food pathogenic bacteria

Pathogenic bacterial detection in food materials is mainly achieved by identifying the bacterial genetic material or whole bacterial cell. Nanoparticles-assisted deoxyribonucleic acid (DNA) isolation and bacteria detection was less time-consuming and more sensitive than other conventional methods. Magnetic iron oxide nanoparticles have been used for isolating DNA of milk pathogenic bacterium Listeria monocytogenes. The DNA isolated from L. monocytogenes-contaminated milk sample was quantified using polymerase chain reaction (PCR) (Yang et al. 2007).

The 16s ribosomal ribonucleic acid is commonly used as a selective marker for PCR-based microbial detection. The PCR-based assay is costly and 16s ribosomal ribonucleic acid-based microarray method lacks sensitivity (Call et al. 2003). However, the nanoparticles-based detection of 16s ribosomal ribonucleic acid is easy and more sensitive (Joung et al. 2008). Various nanoparticles have been documented for detecting pathogenic bacteria in standard bacterial culture samples as well as complex food samples (Table 1).

Detection of food-contaminating toxins

Aflatoxins are a group of toxic and carcinogenic compounds found in food contaminated with Aspergillus flavus and Aspergillus parasiticus. Gold nanoparticles functionalized with anti-aflatoxin antibodies have been used for the detection of aflatoxin B1 (Table 2).

Table 2 Nanomaterial based detection of food-contaminating toxins. SW: single wall, MW: multi wall, CNT: carbon nanotubes
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Likewise, superparamagnetic beads containing anti-aflatoxin M1 antibodies and gold nanoprobes have also been used for the detection of aflatoxin M1 in milk sample (Fig. 2).

Fig. 2
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Dynamic light scattering-based detection of aflatoxin M1 using gold nanoparticle-based probes. In aflatoxin M1-contaminated samples, the aflatoxin M1 displaced nanoprobes by competitive binding to antibodies. The concentration of free nanoprobes was measured using dynamic light scattering reading that was proportional to quantity of aflatoxin M1 in test samples. Figure adapted with permission from (Zhang et al. 2013a). Copyright (2013) American Chemical Society

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Gold nanoparticles-based immunochromatographic strip method has been employed for the detection of aflatoxin M1 in milk. The aflatoxin M1-contaminated milk sample appears as colorless zone on the strip, while in the absence of aflatoxin M1 red color band appears (Wang et al. 2011). Contaminated seafood generally contains marine toxin, namely palytoxin. Carbon nanotubes-based electrochemiluminescent sensors have been designed for the ultrasensitive detection of palytoxin in mussel meat (Zamolo et al. 2012). Various nanoparticles have been reported for the detection of food-contaminating toxins (Table 2).

Detection of food-contaminating pesticides and chemicals

Nanoparticles have been used for the detection of pesticides, fertilizers and other toxic chemicals (Table 3). Among the various pesticides, organophosphates are the most common (Vamvakaki and Chaniotakis 2007). Gold nanoparticles have been used as colorimetric and fluorometric sensors for the detection of organophosphorus and carbamate pesticides (Liu et al. 2012). Cadmium selenide and cadmium selenide–zinc sulfide core–shell quantum dots have been explored for the pesticide paraoxon sensing (Ji et al. 2005). Selective binding of phosphate group containing pesticide parathion to zirconium dioxide/gold nanocomposite film electrode has been employed for developing voltammetric biosensors (Wang and Li 2008). MWCNT–silica nanocomposite-based potentiometer sensors have been documented for the detection of toxic cadmium ions (Bagheri et al. 2013).

Table 3 Nanoparticle-based detection of food-contaminating pesticides and chemicals. SW: single wall, MW: multi wall, CNT: carbon nanotubes
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Like pesticides, excessive use of fertilizers is also a big concern. Melamine is a fertilizer, and it is used as adulterant in protein-rich products such as egg, biscuits, candy and coffee drinks. Gold nanoparticles-based fluorescence sensors were able to detect even picomolar concentration of melamine in cow milk and infant formulas (Vasimalai and John 2013). Likewise, other nanoparticles have also been reported for melamine sensing (Table 3). Food dyes and preservatives are also toxic when used above permissible limit. Multi wall carbon nanotubes (MWCNT)–ionic liquid nanocomposites modified carbon–ceramic electrodes have been used for the detection of food dyes, sunset yellow and tartrazine (Majidi et al. 2013). Cobalt nitroprusside nanoparticles has been used for the detection of sulfite in sugar, dry fruits and wine (Devaramani and Malingappa 2012).

Chloramphenicol is a low-cost, but toxic broad-spectrum antibiotic. But it is still used to cure infections in bees, and as a result, honey gets contaminated with antibiotics. Poly(ethylene glycol dimethacrylate-N-methacryloyl-l-histidine methylester) nanoparticles have been used for the detection of chloramphenicol in honey samples (Kara et al. 2013). Sudan I is a carcinogenic red dye used as adulterant in chili powder. Multi-wall carbon nanotubes (MWCNTs) have been used for the detection of Sudan I adulteration in chili powder (Yang et al. 2010). Likewise, nanoparticles were used for the detection of Sudan I in chili powder, egg yolk, ketchup, tomato, chilli and strawberry sauce (Table 3). SWCNT and MWCNT–zinc oxide nanocomposite has been documented for the simultaneous detection of bisphenol A and Sudan I. Bisphenol A is a toxic contaminant released from plastic container (Sanchez-Acevedo et al. 2009; Najafi et al. 2014).

Nanosensors for food freshness detection

The food ingredients generally get spoiled on storage longer than shelf life and exposure of air and moisture. Laboratory-based food spoilage testing is not possible for individual packages. Alternatively, nanoparticles based on spot indicators are sensitive and can be easily labeled on individual packages (Realini and Marcos 2014; Jiang et al. 2015).

Food quality assessment due to improper storage

Besides adulteration and contamination by bacteria or toxin, some food products are perishable and prone to degradation. Some food items perish on change in storage time and temperature. Traditional time–temperature indicators are costly and lack flexibility in programming. So, nanoparticles-based time–temperature indicators have been developed to overcome these limitations. The change in color, size shape and spectral properties of nanoparticles has been explored for developing time–temperature indicator (Table 4).

Table 4 Nanomaterial-based time–temperature indicators for food freshness: nanomaterial-based food freshness indicators/detectors
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Freshness of packed food items is spoiled on exposure to oxygen exposure. Oxygen promotes the oxidation of antioxidants and, hence, induces the growth of bacteria. Colorimetric oxygen indicators have been produced for this purpose using methylene blue/titanium dioxide hybrid nanocomposite material (Gutierrez-Tauste et al. 2007). Oxygen sensor was also developed using luminescent metal–ligand complex functionalized poly-(styrene-block-vinylpyrrolidone) nanobeads (Borisov and Klimant 2009). Trimethylamine is generally produced as a result of metabolism of animal proteins with passage of time. So, presence of trimethylamine is also an indicator of loss of freshness. Tin dioxide–zinc oxide nanocomposite sensors were effectively employed for the detection of trimethylamine in fish samples (Zhang and Zhang 2008). Like zinc oxide microrods, polyvinylpyrrolidone-capped zinc oxide nanoparticles and branched iron oxide–titanium dioxide heteronanostructure have also been used for trimethylamine sensing (Tang et al. 2006; Lou et al. 2013). Likewise, xanthine and hypoxanthine found in meat undergo degradation with time to release hydrogen peroxide. Hydrogen peroxide was detected by gold nanoparticles (Cubukcu et al. 2007).

Sensing the quality of unstable key food ingredients during food processing and storage

Vitamins and other antioxidant components present in food products are easily degraded. Nanoparticles have been used for the detection of vitamins in food items (Table 5). Deficiency of water-soluble vitamin folic acid can cause anemia, carcinogenesis and heart attack. MWCNT and SWCNT–ionic liquid nanocomposites have been reported for the detection of folic acid in wheat flour, fruit juices and milk samples (Wei et al. 2006; Xiao et al. 2008). Nickel oxide nanoparticles have been used for the detection of vitamin, ascorbic acid (Karimi-Maleh et al. 2014). Likewise, N-(3,4-dihydroxyphenethyl)-3,5-dinitrobenzamide modified MWCNT has been used for sensing ascorbic acid and essential amino acid, tryptophan (Ensafi et al. 2012).

Table 5 Nanomaterial-based detection of unstable key food ingredients. SW: single wall, MW: multi wall, CNT: carbon nanotubes
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The antioxidant value of red wine is due to its phenolic content. Tyrosinase enzyme immobilized gold nanoparticles was used to detect the quality of phenol in red and white wines (Liu et al. 2003; Sanz et al. 2005). Hydrogen peroxide is used as an antioxidant in food industry, but higher concentration of hydrogen peroxide can induce toxic effects in humans. Various nanoparticles were reported for the hydrogen peroxide sensing (Table 5). The level of glucose, fructose, sucrose, d-sorbitol, l-malic acid, citric acid, succinic acid, l-glutamic acid, hydrogen peroxide and alcohol during food processing and stored product is used as indicators of food quality (Verstrepen et al. 2004; Terry et al. 2005; Vermeir et al. 2007). Nanoparticles have also been used to sense the quality of such food component as shown in Table 5.

Nanobarcodes for product authenticity

Barcodes are globally used as product authentication labels. Commonly used two-dimensional barcodes can be easily located and, hence, are more prone to damage, alteration and falsification. However, at the same time, nanoparticle-based invisible barcodes are hard to manipulate (Wang et al. 2015b). Unique nanoparticle-based encoding system and nanodisk codes have been reported recently. Nanodisk code is a sequence of surface-enhanced Raman scattering producing disk pairs that can be scanned with Raman microscope (Qin et al. 2007). Authors have documented linear arrays of gold nanodisk, silver nanodisk and silver–gold heterodimer nanodisk codes (Table 6). This approach can be further improved by using nanodisk codes with disk pairs of different metal compositions and their functionalization with different type of chromophores.

Table 6 Nanobarcodes for confirming product authenticity
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Fluorescent poly(p-phenylene vinylene)-based barcode nanorods have been developed for individual packet labeling (Li et al. 2010). Invisible nanobarcode tags containing 7400 and 68,000 unique barcodes have been reported (Banu et al. 2006; Birtwell et al. 2008). Fluorescent DNA dendrimer nanobarcodes have been reported for the detection of E. coli, anthrax, Ebola and severe acute respiratory syndrome pathogens in food and biological samples (Li et al. 2005; Lin et al. 2012). So, nanoparticle-based robust nanobarcodes are better than traditional barcodes.

Electronic nose and electronic tongue for artificial smell and taste sensing

Artificial detection of smell and taste of food products with human-like efficiency helps in producing food of desired quality and taste. Nanoparticles have been explored for designing electronic nose and electronic tongue (Table 7).

Table 7 Electronic nose and electronic tongue for artificial smell and taste sensing. SW: single wall, CNT: carbon nanotubes
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Electronic nose

Electronic nose is an electronic device derived from aroma detection techniques. It can sense the smell like mammalian olfactory system. Electronic nose mainly consists of gas sensors that senses change in type, quality and quantity of odor/flavor. Nanoparticles help in better absorption of gas on sensor surface due to more surface area than macroscopic particles (Ranjan et al. 2014; Dasgupta et al. 2015). Electronic nose senses the characteristic volatile organic compounds present in food to ensure good quality, uniformity and consistency of raw material during mixing, cooking and of final product during packaging and storage processes (Wilson and Baietto 2009). Detection of ethylene gas level is useful for monitoring the harvesting, storage and processing of the fruits and vegetables. Excess exposure of ethylene gas deteriorates the quality of fruits and vegetables. Tungsten oxide–tin oxide nanocomposites have been employed for ethylene sensing (Pimtong-Ngam et al. 2007). Similarly, nanoparticles have been explored for the sensing of ethanol gas, aromas and other volatile organic compounds (Table 7).

Gold nanoparticles were used to modify an array of quartz crystal microbalance sensors to form electronic nose. So designed nose was used for the detection of extra virgin, virgin and non-edible lampante olive oil (Carlo et al. 2014). Likewise, zinc oxide nanoparticles have been used to scrutinize the quality of 17 commercially available Chinese vinegars (Zhang et al. 2006). Manganese dioxide-, titanium dioxide- and cobalt oxide-doped zinc oxide NPs have been used for the identification of five different types of Chinese alcoholic liquors, namely baiyunbian, Beijing erguotou, red star erguotou, zhijiangdaqu and jianliliangjiu (Zhang et al. 2005). Surface plasmon resonance-based immunosensor has been designed for the detection of characteristic fragrant compound, benzaldehyde in peach products (Gobi et al. 2008).

SWCNT field-effect transistor functionalized with human olfactory receptor 2AG1 protein has been employed for sensing fruit odorant amylbutyrate in apricot (Kim et al. 2009; Jin et al. 2012). Olfactory receptors-functionalized carbon nanotubes-based transistor has been documented for the selective detection of hexanal as olfactory indicator of spoiled milk and oxidized food (Park et al. 2012). SWCNT-based electronic nose has been used for sensing the femtomolar concentration of the seafood spoilage indicator, trimethylamine (Lim et al. 2013). SWCNT-based nanobioelectronic nose has also been used to sense other gaseous odorants selectively up to parts per trillion concentration (Lee et al. 2012a, b).

Electric tongue

Nanobioelectronic tongue sensor for bitter taste detection has been developed by functionalization of carboxylated polypyrrole nanotube field-effect transistor with human taste receptor protein, hTAS2R38. Interestingly, the nanotongue could selectively detect target bitterness compounds, phenylthiocarbamide and propylthiouracil with human-like efficiency (Song et al. 2013). Similarly, human bitter taste receptor protein was immobilized on SWCNT field-effect transistor to form tasters for bitter taste (Table 7). The bioelectronic tongue could discriminate between femtomolar concentration of bitter and non-bitter tastants (Kim et al. 2011). SWCNT field-effect transistor functionalized with nanovesicles containing heterodimeric G-protein-coupled human sweet taste receptors has also been used to develop bioelectronic tongue (Song et al. 2014). Floating electrode-based bioelectronic tongue has been designed for the detection of umami substances. In this study, carbon nanotube field-effect transistor with floating electrodes was hybridized with nanovesicles containing honeybee (Apis mellifera) umami taste receptor, gustatory receptor 10 (Fig. 3). As the umami taste substance, l-monosodium glutamate, binds receptor there is increase in flow of current to electrode. This system was successfully used for the detection of umami taste in chicken soup (Lee et al. 2015).

Fig. 3
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Image showing the basic design of floating electrode-based bioelectronic tongue for umami taste detection. Carbon nanotubes field-effect transistor with floating electrodes hybridized to nanovesicles containing honeybee umami taste receptor, AmGr10. Figure adapted with permission from (Lee et al. 2015). Copyright (2015) American Chemical Society

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The nanoparticle-based electronic nose and electronic tongue can act as substitute for cell-based assays in order to better understand the mechanism of human taste (Song et al. 2013).

Conclusion

Nanosensors ensure fast and effective detection of microorganisms, toxins and adulterants as compared to the existing traditional sensors. Nanoparticles are also very useful for the detection of degradable food ingredients like vitamins and antioxidant materials. Individual pack quality indicator and smart robust packaging materials are some other areas of nanoparticles use. Invisible nanobarcodes protect brands and prevent adulteration. Use of nanoparticles in electronic nose and electronic tongue has lead to artificial sensing of smell and taste with human-like efficiency. So nanoparticles have huge significance in food industry.