Organic Chemistry

Module 2. Alcohols, aldehydes and ketones

Lesson 6
IMPORTANCE OF CARBONYL COMPOUNDS IN FOOD FLAVORS

6.1 Introduction

Carbonyl compounds are molecules containing the carbonyl group, C=O. Flavor compounds interact with olfactory and lingual receptors. Volatile compounds are generally responsible for odor perception and nonvolatile compounds for taste. The possibility of the interaction of flavor compounds at subthreshold concentrations giving rise to a detectable flavor has many implications in flavor research. Carbonyls have categorized into various fractions such as painty, oily, mushroom, metallic, tallowy, and cucumber.

In some foods, natural flavors may result from a number of compounds, none of which exist at their flavor threshold. The same phenomenon could account for off-flavor development. However, in the case of off-flavors it is conceivable that when any objectionable compound reaches its flavor threshold, the defect would become evident. The carbonyls may arise due to carbohydrate or citrate metabolism, lipid oxidation or amino acid degradation in various dairy and food products.

The Maillard reaction is one pathway falling under nonenzymatic browning (caramelization, Maillard reaction, and ascorbic acid browning). Of these pathways, the Maillard reaction plays the major key role in flavor development. This reaction is responsible for some of the most pleasant flavors enjoyed by man. There is no question that freshly baked bread, a steak, a freshly brewed cup of coffee, or a piece of chocolate is appreciated by the consumer. Yet none of the characterizing flavors existed in the product until the food processor (or cook) heated the product to develop the flavor.

Generally speaking, the Maillard reaction is a reaction between carbonyls and amines. The carbonyls in foods most often are reducing sugars, while the amines come from either amino acids or proteins. In the flavor industry, the carbonyl may be a pure compound (e.g., diacetyl) and the amine, simply ammonia, or an amine. The major end products of the Maillard reaction are melanoidins and other nonvolatile compounds. The major pathway leading to the formation of carbonyls is the Strecker degradation. This reaction occurs between dicarbonyls and free amino acids. The dicarbonyls involved have vicinal carbonyls (carbonyl groups separated by one double bond) or conjugated double bonds. While these carbonyls typically are intermediates in the Maillard reaction, they may also be normal constituents of the food (e.g., ascorbic acid), be end products of enzymatic browning (e.g., quinones), or be products of lipid oxidation.

Carbonyl compounds make a particularly significant contribution to the flavor of fermented dairy products. Diacetyl, characterized as having a buttery, nut-like aroma, is one of the most important carbonyls to the flavor of these products. Diacetyl is produced via the fermentation of citrate. The most important citric acid fermenters are Leuconostoc citrovorum, L. creamoris, L. dextranicum, Streptococcus lactis subspecies diacetylactis, S. Thermophilus, and certain strains of Proprionibacterium shermani. The metabolic pathway leading to the synthesis of diacetyl involves the degradation of citrate to acetate and oxaloacetate, and the oxaloacetate is then decarboxylated to form pyruvate. Pyruvate plus acetaldehyde forms α-acetolactate, and ultimately, diacetyl. Diacetyl is relatively nontoxic to the bacteria cell so excess pyruvate is channeled into diacetyl. Unfortunately diacetyl is not stable in most cultured food products. The microorganisms that synthesize diacetyl also contain diacetyl reductase that reduce diacetyl to acetoin and 2, 3-butanediol.

A second dairy product where carbonyls are considered as flavor impact compounds is yogurt. Carbonyl compounds comprise the main aromatic substances in yoghurt, among which acetaldehyde is the compound that contributes most to the typical flavor of yogurt. Acetaldehyde is a metabolic end product of L. bulgaricus and/or S. thermophilus during the fermentation of milk to yogurt. Pure acetaldehyde possesses a pungent irritating odor but at dilute concentrations it gives a pleasant fruity aroma. Acetaldehyde imparts yogurt its characteristic green apple or nutty flavor. Acetaldehyde is an indispensable aroma compound in yogurt; good flavored yogurt results when proper levels (23–40 mg/kg and at least 8–10 mg/kg) of acetaldehyde are produced . Diacetyl is an important aroma compound that gives the buttery flavor and it may improve yogurt flavor quality at elevated concentrations. Diacetyl reportedly contributes to the delicate, full flavor and aroma of yogurt and is especially important for products that contain low acetaldehyde concentrations. Diacetyl is a diketone, derived by the fermentation of citrate present in milk and dairy mixes. Small quantities of diacetyl, ranging from traces to 0.90 mg/kg or more contribute to the pleasant and delicate flavor and aroma of yogurt. The typical concentrations of diacetyl in yogurt ranged from 0.2 mg/kg to 3 mg/kg. Acetoin is a common flavor substance in many cultured dairy products. Acetoin has a mild creamy, slightly sweet, butter-like flavor that is similar to that of diacetyl. Meanwhile, the flavor of acetoin is considerably weaker than that of diacetyl and it tends to reduce the harshness of diacetyl. Typical acetoin concentrations in yogurt ranged from 1.2 to 28.2 mg/kg. Diacetyl combined with acetoin imparts the mild, pleasant, buttery taste, and they are critical to the rich perception of yogurt. Acetone and 2-butanone are both contributed by milk and are described as two volatile compounds of minor importance to flavor contribution in milk products. Acetone has a sweet, fruity aroma and is known to influence the aroma and flavor qualities of yoghurt. 2-butanone is significant for eliciting yogurt odor and contributes to the “fruity” flavor. Although each of these carbonyl compounds constitutes a recognizable aroma alone, yogurt flavor is determined by a balanced mixture of the important volatile compounds. For example, a 1:1 acetaldehyde and diacetyl ratio would give a preferred typical yogurt flavor, while too much acetaldehyde compared to diacetyl would lead to a “green” off-flavor. Also, the ratio of acetaldehyde to acetone plays a significant role in the development of yogurt flavor, and a ratio of 2.8:1 results in the desired “fullness” flavor.

Carbonyls are quite important to the flavor of cheeses. The 29 carbonyls have been identified in Cheddar cheese. Carbonyls (methyl ketones) may arise in fermented products initially via lipase activity of the starter culture. Dairy products contain a significant quantity of α-keto acids which are readily hydrolyzed from the triglyceride by microbial lipases and then decarboxylated to form odd carbon number methyl ketones. Methyl ketones and aldehydes may also be formed via microbially induced lipid oxidations. The oxidation may be initiated by microbial lipases, hydrogen peroxide produced by microorganisms and/or lipoxidase-like activity. Carbonyl compounds typically are significant to the flavor of most fermented food products.

Some of examples of carbonyl compounds found in dairy/food products are mentioned at below.

Table 6.1 Examples of carbonyl compounds