Characterisation of the flavour of fresh bell peppers (Capsicum annuum) and its changes after hot-air drying : an instrumental and sensory evaluation

Fruits of <em>Capsicum annuum</em> and <em>C. frutescens</em> are commonly used in the diet because of their typical colour, pungency, taste. and distinct aroma. The fruits are eaten fresh or processed, as unripe (green) or ripe (e.g., red, yellow, orange, white) peppers. In the last decade. attention is shifting towards flavour as an important quality parameter for fruits and vegetables. The flavour of fruits and vegetables, as perceived during consumption, is the overall sensation provided by the interaction of taste, odour, mouth feel, sight, and sound. The composition of non-volatile compounds influences mainly the sensory perceived taste, while the aroma is affected by volatile compounds.In this thesis, the flavour, including taste, aroma, non-volatile and volatile compounds, of fresh and dried bell peppers were evaluated. The major Dutch bell pepper cultivar for export, i.e., <em>C. annuum</em> cv. <em></em> Mazurka, was used as basic cultivar throughout the thesis, others were included for comparison of results.<strong>Instrumental and sensory flavour evaluation of fresh bell peppers</strong><strong><em>Development of non-volatile bell pepper compounds</em> .</strong> In chapter 2, some growth characteristics and changes in compositions of non-volatile compounds (sugars, organic acids) during ripening of cv. Mazurka and cv. Evident were investigated. The development of the fruits showed a growth (between 4-7 weeks after fruit set) and ripening phase (7-10 weeks). During growth, fresh weight increased rapidly, while during ripening the colour turned from green to red, which was reflected in a decreased level of chlorophyll <em>a</em> (i.e., green pigments) and an increased level of carotenoids (i.e., orange/red pigments). Two high performance liquid chromatography (HPLC) methods were developed for the analysis of sugars and organic acids in bell peppers. The sugars detected were, sucrose, glucose, and fructose, and the major organic acids detected were ascorbic, citric and malic acid, while oxalic, fumaric, shikimic, and pyroglutamic: acid were present at lower concentrations. The levels of sugars and organic acids showed distinct ripening related patterns, i.e., glucose, fructose, citric and ascorbic acid increased while sucrose and malic acid decreased.<em><strong>Relationships between non-volatile compounds and taste</strong> .</em> Based on these time series, three distinct ripening stages (i.e., green, turning, and red) of cv. Mazurka and cv. Evident were selected for instrumental and sensory evaluation, in Chapter 3. The composition of sugars and organic acids were analysed and the fruits were evaluated by a sensory descriptive panel on flavour attributes, perceived while eating. The flavour of green bell peppers was characterised by bitter taste and grassy, green bell pepper, and cucumber aroma, while red bell pepper aroma, sweet and sour taste were typical for the turning and red fruits. The relationships between sugars and sweetness, and organic acids and sourness, and dissociated organic acids and sourness were examined using principal component analysis (PCA). It appeared that sweetness, which was typical for the ripe stages, closely related to glucose, fructose, total sugar, and dry matter content, while sourness, also characteristic for the ripe stages, closely related to total citric and total ascorbic acid. PCA on calculated undissociated and dissociated acids revealed that, mainly undissociated ascorbic acid and citric acid in the "-1" and "-2" dissociation forms related closely with sourness. Based on these results and the available literature, it was suggested that undissociated ascorbic acid might be directly important for the sour perception, while the dissociated forms of citric acid influenced the sourness by increasing the proton (H <sup>+</SUP>) concentration.<em><strong>Development of volatile bell pepper compounds</strong> .</em> In chapter 4, the same ripening stages of cv. Mazurka were selected for evaluation of the volatile compounds, for which samples were cut into pieces or completely homogenised. Sixty four compounds were analysed and identified in the headspaces of the different samples by gas chromatography (GC) and GC-mass spectrometry. A trained sniffing port panel detected and characterised 30 odour compounds, of which 21 could be identified. The GC-sniffing profiles of all ripening stages and sample preparations had several odour compounds in common, i.e., 2,3-butanedione (caramel odour), 1-penten-3-one (chemical/pungent, spicy), hexanal (grassy), 3-carene (red bell pepper, rubbery), (Z)-β-ocimene (rancid, sweaty), octanal (fruity), and 2-isobutyl-3-methoxypyrazine (green bell pepper). Furthermore, major ripening and preparation related differences were observed. During ripening from green to red, many volatile compounds having green-related odour notes e.g., (Z)-3-hexenol (grassy, lettuce, cucumber), nonanal (mushroom, herbal), 2-sec-butyl-3-methoxypyrazine (carrot, lettuce, grassy), linalool (floral, green bell pepper), (2)-3-hexenal (grassy, green bell pepper, fruity), and 1-hexanol (fruity, green bell pepper, herbal), decreased or even disappeared. Only the levels of (E)-2-hexenal and (E)-2-hexenol, which have almond, fruity, sweet odours, were higher in the turning and red fruits. Disruption of the cell structure by homogenization of the fruits favoured oxidation of unsaturated fatty acids by lipoxygenase and other enzymes, and stimulated the formation of related alcohols, aldehydes, and ketones; several of these compounds had distinct odour notes.<em><strong>Biochemical aspects of the formation of specific volatile bell pepper compounds</strong> .</em> The development and origin of these enzymatic formed volatiles were studied in more detail in Chapter 5. Extracts of green bell peppers of cv. Mazurka were analysed for the presence of lipoxygenase (LOX); a pH-optimum of 5.5-6.0 was obtained and enzyme activity was 100% and 51% inhibited by two specific LOX inhibitors (10 mM of <em>n</em> -propyl gallate and 340 μM of 5,8,11,14-eicosatetraynoic acid). During bell pepper ripening, LOX activity decreased 70% between 6 and 8 weeks after fruit set. The composition of enzymatic formed volatiles showed four typical ripening related patterns. The origin of these compounds was investigated by adding specific LOX substrates to homogenates of green and red bell peppers. Addition of linoleic acid to green homogenates increased significantly the levels of hexanal, hexanol, 2-pentylfuran, (E)-2-heptenal, (E)-2-octenal, (E)-2-nonenal, but also (2)-3-hexenal, whereas addition of linolenic acid to the same homogenates increased the levels of (2)-3-hexenal, (E)-2-hexenal, (Z)-2-hexenal, (Z)-2- pentenal, (E)-2-pentenal, (Z)-2-pentenol, (E)-2-pentenol, and 1-penten-3-one; similar qualitative effects were observed in the red pepper homogenates. A biochemical pathway was proposed for the formation of volatile bell pepper compounds upon tissue disruption based on experimental data and comprehensive literature.<strong>Influence of hot-air drying on the instrumental and sensory flavour evaluation of fresh bell peppers</strong><strong><em>Effects of hot-air drying on the composition of non-volatile and volatile flavour compounds</em></strong><em>.</em> In chapter 6, the composition of non-volatile and volatile odour compounds were analysed before and after drying of four bell peppers with distinct flavours, i.e., green and red cv. Mazurka, white cv. Blondy, and yellow cv. Kelvin. Hot-air drying affected both the composition of non-volatile and volatile compounds. The levels of glucose, fructose, ascorbic, citric, and oxalic acid decreased, while the levels of sucrose, malic, fumaric, and cis-aconitic acid increased. Glucose, fructose, and ascorbic acid possibly participated in the Maillard reaction during heating. The effects of hot-air drying on the composition of volatile bell pepper compounds could be distinguished in three groups i.e., 1) compounds which decreased or disappeared, 2) compounds which increased or were formed, and 3) compounds which showed no clear changes.The first group represented the majority of volatiles, which apparently evaporated during drying. Most of these compounds were partly retained in the dried bell peppers, but some disappeared completely such as, (Z)-2-pentenal, (E)-2-pentenal, (2)-3-hexenal, (Z)- 2-pentenol, (E)-2-pentenol, (E,E)-2,4-hexadienal, and (E,Z)-2,4-hexadienal. The majority of these compounds were ascribed to the enzymatic breakdown of unsaturated fatty acids upon tissue disruption (Chapter 5). Therefore, it was suggested that the disappearance of these compounds was due to heat inactivation of enzymes more than evaporation during drying. Sniffing port evaluation showed that the compounds, which decreased or disappeared after drying, had mainly "fresh" odour notes like lettuce/grassy/green bell pepper ((Z)-3-hexenal), fruity/almond ((E)-2-hexenal), fruity (octanal), lettuce/green bell pepper ((Z)-3-hexenol), and grassy/green bell pepper (Z)-2-hexenal,The second group included compounds which increased or were formed during drying. The increased levels of 4-octen-3-one, (E)-2-heptenal, (E)-2-octenal, (E,Z) and (E,E)-2,4-heptadienal, decanal, and (E)-2-nonenal were possibly due to autoxidation of unsaturated fatty acids. Whereas, the increased levels of 2- methylpropionic and 2- and 3-methylbutyric acid, 2-methylpropanal, and 2- and 3- methylbutanal seemed to be due to Strecker degradation. Sniffing port evaluation showed that the latter three compounds had distinct cacao, sweaty, and spicy odour characteristics.<em><strong>Changes of the aroma profile of fresh bell peppers due to hot-air drying.</strong></em> In chapter 7, the effects of hot-air drying on the fresh aromas of the four different bell peppers were investigated in more detail. The percentage composition of odour active compounds in the fresh and dried samples were analysed, and the fruits were evaluated by a sensory descriptive panel on aroma attributes, as perceived while smelling. On one hand, hot-air drying decreased the intensity scores of several characteristic fresh aroma attributes. For example, in the green and white fruits, mainly "green" attributes like herbal, grassy, green bell pepper, and cucumber decreased after drying. Moreover, the intensity of the aroma attributes fruity/fresh and floral decreased in the green, white, and red but not in the yellow samples. GC-analysis of the odour active compounds showed that the percentage peak areas of (2)-3-hexenal, 2-heptanone, (Z)-2-hexenal, (E)-2-hexenal, hexanol, (Z)-3-hexanol, (E)-2-hexenol, and linalool, which possessed green, vegetable-like, fruity, and floral characteristics, decreased during drying, which may be responsible for the decreased intensity scores of corresponding aroma attributes in the dried samples. On the other hand, hot-air changed the aroma profile. Five extra attributes, i.e., cacao, caramel, nutty, savoury, and dried tomato, were necessary to describe the aroma of the bell pepper samples after drying. The major odour active compounds detected in the dried samples were 2-methylpropanal, 2 and 3-methylbutanal hexanal, and nonanal. Also, the levels of 2,3-butanedione, 2,3-pentanedione, 4-octene-3-one, (E)-2-octenal, and (E)-2-nonenal were significantly higher in the dried bell peppers. Several of these compounds having cacao, spicy, sweaty/rancid, and sweet odour characteristics may contribute to aroma attributes like savoury, sweettsickly, rancid/sweaty, cacao, and caramel, which were characteristic for all dried samples.<em><strong>Modelling the lipid oxidation, Strecker degradation, and Maillard reaction products during hot-air drying of bell peppers</strong> .</em> In Chapter 8, the complex behaviour of volatile bell pepper compounds during drying at different time and temperature conditions was described by four distinct kinetic mechanisms.This thesis provided basic knowledge about the flavour of fresh and hot-air dried bell peppers. This knowledge can be used to study effects of, for example, harvest time, growth conditions, sample variation, cultivars, and seasons on the flavour of fresh and/or dried bell peppers. Furthermore, in this thesis a biochemical pathway was proposed for the enzymatic development of bell pepper volatiles, which can be used as basis for detailed studies on the enzymatic formation of bell pepper volatiles. Finally, the complex behaviour of volatile compounds during hot-air drying of bell peppers was described by four different kinetic mechanisms. This kinetic approach may be used to investigate the behaviour of volatiles in other dynamic processes.