Bioactive compounds in leek (Allium ampeloprasum var. porrum): analysis as a function of the genetic diversity, harvest time and processing techniques

摘要

Leek (Allium ampeloprasum var. porrum) is one of the most important vegetables cultivated outdoors in Belgium. Besides their economic importance, they are a source of several bioactive or health-promoting compounds including 4 important chemical groups that have perceived benefits to human health, i.e. the S-alk(en)yl-L-cysteine sulfoxides (ACSOs), polyphenols, vitamins and fructans. Moreover, epidemiologic studies elucidated the reduction of the risk of prostate, colorectal, stomach and breast cancer when leek is consumed. On the one hand, the present study identified and quantified bioactive compounds in fresh leek for a range of leek cultivars and as a function of harvest time. On the other hand, the behaviour of these compounds was investigated during different processing/stabilisation techniques.Although leek is a popular vegetable in many European countries, the limited scientific knowledge concerning its health-promoting compounds is in sharp contrast with the well-documented health aspects of its related Allium species, i.e. onion, shallot, garlic. The objective of this PhD thesis was to develop knowledge on the presence of health-promoting compounds in leek in order to complement the many reports focusing on these compounds in related Allium species. On the basis of novel scientific knowledge on health-promoting compounds in leek obtained in the present study, the dissertation aims to stimulate innovation in leek breeding, production, marketing and the consumption pattern of leek.Chapter 1 included a general introduction and the objectives of the present study. Chapter 2 gave a brief summary concerning the current position of leek production in Belgium and abroad. Leek was described from two different angles: leek as a crop and leek as a food product. Moreover, the presence and properties of bioactive compounds was discussed in Chapter 2. Chapter 3 described the experimental design, including the selected plant material, the sampling procedure and further sample preparation for each experiment. The analytical and statistical methods were discussed in this chapter as well.In Chapter 4, statistically significant differences among 30 commercial and less common leek cultivars were discussed in terms of antioxidant capacity, polyphenols, ascorbate, ACSOs and fructan content. The antioxidant properties of the leek extracts were influenced by leek tissue (white shaft/green leaves) and type of cultivar (summer/autumn/winter type) to a large extent, whilst the manner of breeding (F1 hybrids/open pollinated) had no significant influence on the antioxidant properties. The green leaves of most cultivars contained a higher antioxidant capacity, ascorbate and polyphenol content, while the white shaft was rich in ACSOs and fructans. The dissimilarities between the three leek types could be explained by their genetic background, but because of their different harvest times, stress factors such as temperature, solar radiation, pathogens etc. to which the plants were subjected may also partly explain the different accumulation patterns of health-promoting compounds between the summer, autumn and winter leek. Therefore, Chapter 5 elucidated the effect of harvest time on the antioxidant properties in 9 F1 leek cultivars, harvested at 4 times during the leek growing season. Harvest time had a clear effect on antioxidant levels, in addition to the effect of cultivar. A clear distinction between harvest in September and harvest in November was observed, together with the difference between September/November and the other 2 months (January/March). Harvest in January or March resulted, for most of the cultivars and both parts, in a higher antioxidant capacity and polyphenol levels. Harvest in September had a positive influence on the ACSO content. Chapter 6 elucidated the difference between leek and some of its related Allium species with regard to the antioxidant properties. Based on the results, the antioxidant properties of the white shaft of leek were closely related to the antioxidant potential of the bulb of A. kurrat (Egyptian leek) and A. cepa (onion), while the green leaves of leek resembled the antioxidant profile of A. schoenoprasum (chives) and A. fistulosum (bunching onion). A. odorum (Chinese leek) and A. cepa (cv. Red Creole, red onion) were the species with different antioxidant properties compared to leek. These species were higher in methiin and flavonoid content, respectively. The evaluation of the impact of post-harvest storage and domestic food processing on the health benefits of vegetables is also of great practical importance and was demonstrated in Chapter 7 and Chapter 8, respectively. The antioxidant capacity was highly influenced by cooking but not by storage. In general, the antioxidant capacity and the total phenolic content in the white shaft and green leaves of the entire and packaged leek was stable during 13 days of storage at 4 °C. A slight increase in isoalliin level could be observed after a cool storage period. The difference between the antioxidant properties of the white shaft of the entire leek and the packaged leek was minimal, except for the lower ACSO values in packaged leek. An obvious increase could be observed in the antioxidant capacity of the steamed green leaves. Boiling had a negative effect on total phenolic content in the white shaft and green leaves. In contrast with boiling, steaming did not have an influence on the polyphenolic content. It is remarkable that blanching resulted in a slight increase in the ACSO content. Applying a longer-duration thermal treatment to leek samples negatively influenced the content of methiin and isoalliin. In general, steaming seemed to be responsible for a better retention of the bioactive compounds present in leek.The white shaft is used in many culinary applications, whereas the green leaves are considered inferior and are, therefore, usually only used in soups or even left behind on the fields or during processing. The application of 2 alternative value-adding valorisation methods, including lactic acid fermentation and drying, were therefore investigated in Chapter 9 with regard to the retention of the antioxidant properties. The results of Chapter 9 demonstrate the higher antioxidant capacity and polyphenol content upon fermentation especially in the green leaves. These results indicated the nutritional relevance of fermentation, which can be a promising stabilisation technique for leek. Fermentation does not require extensive materials and is an inexpensive way to stabilise and preserve plant material such as leek. Leek samples subjected to 3 drying methods retained their antioxidant capacity, with the exception of the higher ORAC value of air-dried green leaves and the higher DPPH value of the air-dried white shaft. Similarly, air-drying resulted in the highest total phenolic content compared with freeze-drying, while freeze-dried and refractance window dried samples exhibited equal amounts of polyphenols. The analysis of individual polyphenols revealed again that, air-dried samples contained higher quantities of polyphenols than freeze-dried leek, while freeze-dried leek exhibited higher levels of polyphenols compared to refractance window dried samples. Although air-drying was the best drying technique in retaining the antioxidant capacity and polyphenols, air-drying resulted in high losses of the ACSOs compared to freeze-drying. In fact, refractance window drying was the best drying technique to retain methiin. The present study revealed novel scientific knowledge on the content of health-promoting compounds in leek. This information can help to stimulate innovation in leek breeding, production, marketing and the consumption pattern of leek. In addition, the present study can give the onset to valorise the green leaves of leek, plant material which is rich in antioxidants.