CHIRAL CHEMICALS IN FOOD
Chemical analysis is important in the food industry as it helps in monitoring the quality, authenticity, and safety of the food. The chemical composition of food products is monitored regularly to ensure that the food product adheres to national and international regulations. Most countries have a standards association or a food and drug administration agency that monitors and/or regulates the quality and safety of foods.
Chemicals in food can be endogenous, additives, or contaminants. Endogenous chemicals are those chemicals that naturally occur in the food products, and these may include amino acids and flavors. Chirality is an important property of food flavors and aroma. For example, -asparagine has a sweet taste, while its antipode /-asparagine is tasteless (Engel. 2020) and (l/?,2S,4R)-(+)-borneol and (15,2/?,45)-(-)-borneol have a weak camphor-like odor and camphor- or turpentine-like odor, respectively (Bentley, 2006). The advances in enantioselective analysis supported the development of flavor and aroma analysis as powerful tools for food authentication.
The determination of the enantiomeric compositions of naturally occurring food constituents at trace levels allowed to provide the essential data for establishing the pathways and enzymes involved in the biosynthesis of chiral substances. Moreover, enantioselective analysis of chiral substances is also important to understand the effect of enantiomeric composition on the odor and taste of foods that has been extensively studied (Engel, 2020). Considering that the enzymes involved in the synthesis of chiral substances are highly enantiospecific, the enantiomeric composition of the chiral substances is normally consistent, hence the application of EF values as an indicator of food authenticity.
For example, raspberry flavor is primarily due to a-ionone that occurs naturally as the (R)-enantiomer (97-100%) as well as 6-octalactone, 5-decalactone, and terpenes-4-ol that occur naturally as the (S)-enantiomer (80-100%) (Hansen et al., 2016). Hence, a deviation of the EF values of these flavors from the natural enantiomeric composition suggests that the raspberry flavor is not natural. However, the application of EF values as indicators of food authenticity is challenging because flavor and aroma compounds may widely vary due to spatial, temporal, and local environmental conditions (Schafer et al., 2015). In addition, food storage and processing can result in shifts in the enantiomeric composition of the chiral compounds due to an enzymatic action or physicochemical treatment (Schafer et al., 2015). Hence, enantioselective analysis should be used in conjunction with other food authenticity techniques for improving the accuracy required.
Food Additives and Contaminants
Food additives are a diverse group of chemicals that are added to food to enhance or maintain their sensorial, physicochemical, biological, and rheological properties (Martins et al., 2019). They can maintain or improve the color, flavor, quality, texture, and stability of foods. Food additives can be classified according to their industrial use into 25 classes such as acidity regulator, anticaking agent, antioxidant, colorant, emulsifier, foaming agent, preservative, and sweetener. However, previous studies found that the gut microbiota were altered by different food additives using in vitro studies and mammal models (Cao et ah, 2020). Moreover, food additives have been shown to cause adverse reactions in some people (Wilson and Bahna, 2005). In fact, it was verified that azo dyes used as food colorants may cause proinflammatory responses in vitro, indicating a potential risk to human health (Leo et ah, 2018). The European Food Safety Authority (EFSA) recommends that the enantiomeric stability of chiral food additives during storage should be investigated (Bura et ah, 2019). Enantiomers can undergo chiral inversion whereby they change into their antipode. The EFSA further recommends that when the enantiomers of the food additives have different or unknown toxicological profiles, the enantiomers should be treated as different compounds (Bura et ah, 2019).
Chemical food contaminants pose a risk to human health. The main route of entry of chemical contaminants to food is through packaging material, processing, soil, and water. Examples of chemicals that can contaminant food through packaging material include flame retardants, plasticizers, antioxidants, and thermal and light stabilizers. Several studies detected brominated flame retardants (Shaw et ah, 2014), organic UV filters (Snedeker, 2014), and perfluorinated compounds (Rice et ah, 2014) in food packaging and contact materials. However, as shown in the previous sections, some brominated flame retardants, organic UV filters, and perfluorinated compounds are potentially toxic chiral compounds. The second class of food contaminants is environmental contaminants that enter the raw material of the food due to contact, interaction, or exposure to contaminated environment. Examples of such contaminants include pharmaceuticals and personal care products which can enter plant products that are irrigated with contaminated water or grown in soil amended with biosolids (Fu et ah, 2016). Additionally, pesticides are used in agriculture and can be taken up by plants. In fact, more than 35% of pesticides currently used possess at least one chiral center. Several studies have shown that the uptake of chiral pesticides in plants often results in the enrichment of one enantiomer over the other (Sanganyado et ah, 2020; Zhang et ah, 2018b, 2018a). In addition to land-based pathways, contaminant uptake in food can occur in aquatic ecosystems such as wild fisheries and aquaculture systems. Aquatic ecosystems are often the sink of most organic pollutants and hence, organic pollutants can bioaccumulate in fish (Meng et ah, 2009). Interestingly, bioaccumulation of some chiral pollutants such as triazole fungicides (Konwick et ah, 2006a; Wang et ah, 2015), fipronil (Baird et ah, 2013; Konwick et ah, 2006b), and lactofen (Wang et ah, 2018) has been shown to be enantioselective. These metabolism studies show that these chiral compounds should be treated as distinct compounds when assessing the safety of the fish for human consumption. The EFSA recommends that the distribution, behavior, and toxicity of the enantiomers shall be established individually to improve the robustness of the risk assessment (Bura et ah, 2019).