SECTION II. Analysis, Fate, and Toxicity of Chiral Pollutantsin Food

Current Trends in Enantioselective Food Analysis

INTRODUCTION

Chirality is a common phenomenon in many biologically active compounds such as polysaccharides and proteins. It arises when a molecule has either an asymmetric carbon or perpendicular lines that are not symmetrical and cannot freely rotate against each other (Sanganyado et al., 2020). Many synthetic compounds available on the market have at least one asymmetric carbon atom, also known as a chiral center, and exist as enantiomers. These enantiomers are not superimposable, and they rotate the plane of polarized light either to the right (dextrorotatory) or to the left (levorota- tory). Enantiomers interact differently with different biological receptors, transport systems, and enzymes; hence, their enantiospecific biological activities (Alvarez-Rivera et al., 2020).

Food products owe their taste, texture, and aroma to a diverse group of molecules some of which are chiral. The odor and taste of a foodstuff may depend on the enantiomeric composition of the chiral food molecules (Figure 8.1) (Alvarez-Rivera et al., 2020). For example, spearmint and caraway fruits have different aroma despite arising from the same chemical carvone. The observed differences in aroma are due to the stereochemistry of carvone. (R)-carvone has a distinct spearmint

Examples of stereospecificity in taste and flavor of natural products

FIGURE 8.1 Examples of stereospecificity in taste and flavor of natural products.

aroma while (S)-carvone has a spicy scent (Rocco et ah, 2013). Citrus peel contains a chiral cyclic monoterpene aliphatic hydrocarbon called limonene. (R)-limonene has a distinct orange smell while (S)-limonene a distinct lemon smell. Enantiomeric differences have been observed in the taste and flavor of food chemicals such as asparagine and alapyridaine. The D-asparagine is sweet while the L-enantiomer is tasteless. Alapyridaine (N-(l-carboxyethyl)-6-hydroxymethyl-pyridinium-3-ol inner salt) is produced through the Maillard reaction during food processing when glucose and L-alanine are heated. Despite being tasteless, (S)-alapyridaine can enhance various tastes such as the salty, sweet, and umami tastes of NaCl, glucose, and monosodium L-glutamate, respectively (Soldo et ah, 2003). Unlike its antipode, (R)-(-)-alapyridaine did not enhance the sweet, salty, and umami taste of beef bouillon (Hofmann et ah, 2005). Human sensory receptors are three-dimensional biomolecules that can stereorecognize enantiomers.

Enantiomeric composition can also influence the nutritional value of foodstuff. Humans oftentimes metabolize one enantiomer while the antipode is eliminated unchanged without contributing any nutritional value. Previous studies showed that the D-form of Vitamin C had lower nutritional value compared to the L-form (Dabbagh and Azami, 2014). The human body can metabolize L-vitamin C but not the D-form.

The enantiomeric composition of food molecules can change during food processing and storage. Previous studies demonstrated that fermentation process shifts the enantiomeric composition of food molecules in addition to adding other chiral molecules (Armstrong et ah, 1990). Wine owes its taste from the molecules that come from the grapes, yeast fermentation, and oak wood. Oak wood barrels releases lyoniresinol, a compound with two chiral centers, during wine aging. Previous studies showed that (+)-lyoniresinol had a strong bitter taste, (-)-lyoniresinol was tasteless while its dia- stereomer е/л'-lyoniresinol exhibited slight sweetness (Cretin et ah, 2015). These results showed that enantiomeric composition of lyoniresinol in wine can affect wine taste and consumer perception. Natural flavors and fragrances are chiral compounds. Biosynthetic processes in plants are highly stereospecific and result in the formation of almost enantiopure flavors and fragrances (Sanganyado et ah, 2020). In contrast, industrial synthesis of flavors and fragrances yield racemic mixtures. For example, natural apricot flavor is due to (R)-(+)-y-decalatone (94-100%), however commercial products were found to contain racemic mixtures of y-decalatone (Ravid et al., 2010). Amino acids in fruit juices are mostly found in the L-form (D’Orazio et al„ 2017). Hence, enantioselective analysis can play a key role in food authentication.

8.1.1 Chirality in Food Safety

Organic pollutants discharged from agricultural, industrial, and domestic activities enter the environment and subsequently the food supply. For example, pesticide use in agriculture results in contamination of edible crops. The use of biosolids for soil amendment and recycled wastewater can result in uptake of pharmaceutical and personal care products by the crops (Fu et al., 2016). Antibiotics are extensively used in animal husbandry; hence, several studies have detected antibiotics in fish, beef, pork, and milk (Hernandez et al., 2007; S. Li et al., 2018; McEachran et al., 2015). Besides environmental transfer, organic pollutants can be introduced into food during preparation. packaging, storage, and distribution. For example, polycyclic aromatic hydrocarbons can be released when the food is prepared at high temperatures while plasticizers and flame retardants can be released by the food packaging into the food (Shaw et al.. 2014). Exposure to environmental organic pollutants through food consumption may cause adverse health effects in humans such as cardiovascular diseases, endocrine disruption, cancer, diabetes, congenital disorders, and dysfunctional immune systems (Guo et al., 2019). However, some organic pollutants such as pesticides, pharmaceuticals, personal care products, and flame retardants are chiral compounds with enantio- specific toxicities to humans (Liu et al., 2019; Sanganyado, 2019; Sanganyado et al., 2017; Stanley et al., 2007, 2006; Stanley and Brooks, 2009). Hence, understanding the enantiomeric composition of organic food contaminants is critical for accurate human risk assessment.

8.1.2 Scope of Chapter

There is a need for precise and accurate analytical methods that can determine the enantiomeric ratios of the compounds present in the food as a measure of quality assurance, food safety, and security (Zor et al., 2019). This chapter discusses the developments in various aspects of the analytical process in the determination of chiral food components and contaminants. The mechanism of enan- tioseparation will be discussed first followed by the application of different separation techniques in food safety and quality assessment. Table 8.1 shows the current applications of enantioselective analysis in food.

TABLE 8.1

An Overview of Current Applications of Enantioselective Analysis in Food

Application of Enantioselective Analysis in Food and Beverage Studies

Determination of aroma components and development of flavors closely approximating natural flavors

Identification of markers to assess authenticity and adulteration of foods and beverage

Evaluation of processing and storage time effects

Age dating

Investigation of health-promoting compounds

Control and monitoring of fermentation processes, and microbiological activity in general

Analysis of chiral metabolites

Biotransformation of persistent pollutants

Source: Reprinted from TrAC Trends in Analytical Chemistry, Vol. 52, Anna Rocco, Zeineb Aturki. Salvatore Fanali. Chiral Separations in Food Analysis, pp. 206-225, 2013. With permission from Elsevier.

Chiral molecular interaction showing that the left enantiomer has a stronger affinity for the active site on the chiral selector while the right enantiomer binds to the active site weakly

FIGURE 8.2 Chiral molecular interaction showing that the left enantiomer has a stronger affinity for the active site on the chiral selector while the right enantiomer binds to the active site weakly. (Reprinted from Water Research, Vol. 124. Edmond Sanganyado. Zhijiang Lu, Qiuguo Fu, Daniel Schlenk, Jay Gan. Chiral pharmaceuticals: A review on their environmental occurrence and fate processes, pp. 527-542. 2017. With permission from Elsevier.)

 
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