Types of Food Adulteration

Substitution

This refers to the complete or partial substitution of food ingredients or valuable authentic ingredients with lower-grade or cheaper alternatives. The substitution also includes false claims and unclear statements based on geography, animal species, plant species, various origins and production processes (Rahmati et al., 2016; Spink et al., 2019). Again, the goal of substitution is to derive economic benefits.

2.1.1 Products of Plant Origin

Generally, it is quite difficult to distinguish the origin of specific food ingredients from the appearance of the plant origin product after processing (Fritsche, 2018), particularly when the manufacturers of counterfeit food usually deliberately imitate the morphological and sensory characteristics of the original source. Thus, sometimes certain plant origin food products can be adulterated using ingredients from different types of plant sources and these adulterations might affect the further application of food products, for example, a common adulteration in the monofloral honey products. The monofloral honey, which is collected by bees from the nectar of a single plant and brewed, has unique characteristics in flavour, taste and even functionality. Flowever, due to the influence of regions and climate, the yield of monofloral honey can be very low (Gok et al., 2015). As a result, some honey manufacturers completely or partially substitute the valuable monofloral honey with a lower-grade or cheaper polyfloral one (Siddiqui et al., 2017). Moreover, the regulatory requirement for monofloral honey is not clear enough to stop the adulterations (Won et al., 2008).

2.1.2 Products of Animal Origin

Similarly, food products from animal origin can be adulterated via substitution too. This is because the substitution of a declared high-value animal ingredient using lower-grade adulterants leads to huge economic benefits. Again, it brings potential risks to food safety framework within a region, For example, the use of unapproved animal species in food manufacturing has resulted in inapplicability of inspection and quarantine (Manning and Soon, 2014). In addition, the absence of declared ingredients and the presence of undeclared ones may lead to compromised lifestyle habits, religious conflicts, and personal emotional discomfort (Sentandreu and Sentandreu, 2014).

According to the outcomes of six food authenticity investigations from 1995 to 2006, the composition of 15-39 per cent of meat products (such as hamburgers and sausages) in the USA, UK, Mexico, Switzerland, Turkey, and Brazil do not agree with their labels (Ballin, 2010). In these cases, cheaper and lower-grade meat tissues and/or parts are used to partially substitute the expensive and popular ones. The 2013 UK horse meat scandal can be referred to as a good example, where horse meat was used to partially substitute high-end beef for economic gain in Europe.

2.1.3 Geographical Origin

Geographic origin of the food products is becoming increasingly important, particularly due to economic and food safety reasons. Although the adulterated food products do not agree with their labels, they usually meet the food safety standards and nutrition requirements of the place of sale. Generally, it is difficult to identify these food adulterants from undeclared geographical origin because most of the time, foods from different origins are almost chemically identical. Moreover, the characteristics of agricultural products in the same region are even affected by multiple factors, such as season, climate, farming or feeding conditions (Lohumi et al., 2015). Food adulteration using ingredients/components from non- declared geographical origin can be detected using non-target methods, such as proton transfer reaction mass spectrometry (PTR-MS) analysis of oligosaccharides (Abbas et al., 2018) and/or gas chromatography (GC) spectra of fatty acids composition (Wojcicki et al., 2015).

Nowadays, it is to the interest of both the food industry and regulatory bodies to develop analytical/biotechnical methods to verify the geographical origin of food products. This is highly associated with the interests of domestic manufacturers and consumers. Moreover, the verification method can potentially promote the development of domestic agri-food industries (Markiewicz-Keszycka et al., 2017), for example, PTR-MS and non-targeted methods were developed and used to distinguish geographically distinctive authentic cheeses products from the adulterated ones in EU (Pustjens et al., 2016). Sousa et al. (Sousa et al., 2018) reported the determination of the geographical origin of fish based on extraction and characterization of DNA from the microorganisms present in the fish flesh.

2.1.4 Production System

There are different ways to manufacture food products to meet the specific requirements of various consumer groups. Organic or green food is one of the main categories, where no or only trace amounts of chemicals are used in the farming/production process to ensure the production process is close to the natural cycle. Generally, consumers consider organic food products as healthier, more environmental-friendly and more sustainable and are willing to bear the additional costs (Shafie and Rennie, 2012). So far, a great number of countries/regions have established relevant laws and regulations to define, standardize, and audit the manufacturing process of organic food products to ensure their authenticity. However, due to the high price of organic food products (usually it is one of the most expensive of its kind in the market), there have been a great number of food adulterations (Roman et al., 2017). One of the main ways to identify organic foods is to detect the chemical residues, particularly pesticides and/or other substances that should not have been used. So far, the authentication methods for organic dairy products, organic meat products, organic eggs, organic wheat, and organic fish (including wild) have been developed (Puertas and Vazquez, 2019; Rao et al., 2019; Su and Sun, 2017; Yang et al., 2019).

Religion and/or lifestyle also greatly affect how food is produced, such as halal or kosher food certification, which is produced free from any prohibited components proscribed by Islamic or Jewish food laws, for example, to guarantee the production of halal food, several methods have been developed and used to verify the origin of animals and animal products, as well as the presence of alcohol (Soon et al., 2017). Generally, the adulteration of halal animals can be divided into two categories:

• (i) incorporation of other meats into the meats which are allowed according to Islam food laws, such as cattle, goats, sheep, chickens, ducks, rabbits, etc. (Farag, 2020) and (ii) incorrect slaughtering process of the animals specified by Islamic law (Nakyinsige et al., 2012). The former type of adulteration can be detected using analytical and/or biotechnical methods, such as Fourier- transform infrared (FTIR) and NMR spectroscopies, biosensors, differential scanning calorimetry (DSC), and chromatography methods including liquid and gas chromatography hyphenated with mass spectrometry (LC- MS and GC-MS), etc. (Rohman and Fadzillah, 2018) while the assurance of food authenticity in the latter type mainly relies on the management program. Although the government and/or regulatory bodies are currently responsible for ensuring the food authenticity by detection of end products and inspection of the manufacturing process, the food manufacturers are expected to prevent the occurrence of food adulterations (Chen et al., 2014).
• 2.1.5 Processing

Some food materials are treated in specific ways to further improve their flavors and/or increase the value of the food product. However, concealment or improper claims of the production process might harm the interests of consumers, for example, frozen and thawed meat and fish sold as fresh ones (Uddin and Okazaki, 2004) and ordinary espresso sold as a coffee product which has undergone additional processing steps (de Carvalho Polari Souto et al., 2015). In Nascimento et al.'s work (2017), a new type of amperometric sensor was developed for the rapid detection of cheese made using raw milk instead of the heat-treated one, based on lysine content analysis.

Addition

The intentional addition of a non-authentic substance to the adulterated food products to mask their inferior quality but claim them as high-grade foods is another type of food adulteration (Johnson, 2014). As shown in Fig. 3, the numbers of the records for food adulteration via the intentional addition have been increasing significantly in the last 20 years. Among all the food categories, seafood, dairy and meat are the most prone food products in the past decade.

The 2008 Chinese milk scandal can be referred to as an example of this intentional addition. Before this scandal, the 'diluted' infant formula in China was reported occasionally, which resulted in marasmus in some infants. As a result, the government body specified the minimum protein content in various dairy products (Chaisson and Martinson, 2008). However, some milk manufacturers intentionally added melamine into the raw milk to 'increase' its apparent protein content, which is usually determined by the Kjeldahl method (nitrogen accounts for 66 per cent of the mass of melamine). Approximate 300,000 Chinese children were reported to have become victims in this milk scandal, among which around 54,000 were hospitalized and six died from kidney stones (Ellis et al., 2012; Johnson, 2014; Poonia et al., 2017; Spink et al., 2017). Later on, melamine was also found in feed, eggs, candy and other dairy commodities. This scandal seriously damaged the reputation of the Chinese food and feed industry, leading to a dramatic decrease in the export of these products and domestic consumer confidence (Poonia et al., 2017).

Honey products can be adulterated by addition of non-authentic substances too. In the early days, the adulterants used in honey were mainly sucrose, starch, honey flavor, sodium carboxymethyl cellulose and pigments to increase consistency, color and flavor of honey. Due to the advance of contemporary analytical and/or biotechnical technology, these adulterants in honey can be quickly detected (Da Silva et al., 2016). Therefore, nowadays, the adulteration of honey involves the addition of corn, high fructose and rice syrup (Wang et al., 2015). The content of fructose and glucose in these syrups and the ratio of these two carbohydrates are found to be very close to those of real honey. Moreover, the physicochemical properties of the adulterated honey products almost meet the standards of honey products, leading to the challenge of honey authentication process.

Removal and/or Intentional Decrease in the Content of Authentic and Valuable Ingredients

Food products can be adulterated by removal and/or intentional decreased content of valuable components too. The sale of 'defatted' paprika, where the valuable capsaicin has been extracted, as the original one can be a good example (Horn et al., 2018). Similarly, some honey manufacturers intentionally remove the pollen or characteristic residuals from the beehive in adulterated honey by filtration, making it difficult to trace the source (Zabrodska and Vorlova, 2014).