Fermentation-induced changes in the nutritional value of native or germinated rye

Abstract

Processing is important both for the sensory and nutritional quality of grains. In the case of whole grains, it especially may influence the levels and bioavailability of the bioactive compounds. Yeast, lactic acid and mixed fermentation were shown to vary in their effects on native and germinated wholemeal rye flour. The presence of Saccharomyces cerevisiae was important for enhanced levels of the analysed compounds. The levels of folates, free phenolic acids, total phenolic compounds, lignans and alkylresorcinols were increased especially by fermentation of germinated rye, which also resulted in lower pH when compared with native rye flour, the level of folate was increased up to seven-fold and that of free phenolic acids by up to ten-fold after germination and fermentation. Tailored fermentation thus offers a tool to further increase the bioactive potential of wholemeal rye.

Introduction

Cereal foods are important components of the daily diet, providing carbohydrates, protein, dietary fibre (DF) and many vitamins. Recent epidemiological studies have indicated a protective role of whole grain foods against several diseases associated with westernised societies such as type 2 diabetes (Murtaugh et al., 2003; Pereira et al., 2002; Simin et al., 2000), cardiovascular disease (Jacobs and Gallagher, 2004) and certain cancers (Larsson et al., 2005). DF has long been considered to be the major health protective component of grains. In addition to containing macronutrients and DF, whole grains are also rich in vitamins, especially B vitamins, and good sources of minerals, particularly trace minerals. Whole grains are also sources of many phytochemicals, including phytoestrogens, phenolic compounds, antioxidants, phytic acid and sterols (Fujino and Ohnishi, 1983; Manore et al., 1990; Penalvo et al., 2005; Seitz, 1992). The bioactive components of whole grains, as well as those of vegetables, fruits and berries, are being intensively studied for their effects on health. These compounds vary widely in their chemical structure and function and therefore, many possible mechanisms have been postulated (Slavin et al., 1999, Slavin et al., 2001).

In this study, the effects of fermentation and germination on various phenolic compounds (phenolic acids, lignans, alkenylresorcinols), folates, sterols and benzoxaxinoids were studied. Phenolic compounds such as lignans, alk(en)ylresorcinols and phenolic acids are potential bioactive compounds due to their antimicrobial, anti-oxidative and anti-carcinogenic effects (Pratt, 1992). Folates are cofactors in many enzymatic reactions and deficiency increases the risk of mutations and DNA breakdown (which may lead to cancers). Furthermore, sufficient intake of folate lowers elevated level of serum homocysteine, which is a risk factor for coronary heart disease (Brouwer et al., 1999). Plant sterols also have cholesterol-lowering effects if consumed in sufficient quantities (Cater, 2000). The health effects of benzoxazinoids (Bxs) are not well understood but these compounds have been postulated to be anti-inflammatory (Otsuka et al., 1988) and have been shown to prevent the proliferation of certain cancer cells in vitro (Zhang et al., 1995).

Bioactive compounds are concentrated in the germ and the outer layers with the outer parts also being the richest in DF (Glitsø and Bach Knudsen, 1999; Lampi et al., 2004; Liukkonen et al., 2003; Nilsson et al., 1997a; Piironen et al., 2002). If most of the bran and some of the germ are removed in the milling process, the levels of both DF and of these compounds are much lower.

Processing is a prerequisite for the consumption of whole grains. However, knowledge of the effects that processes such as milling, baking, malting and extruding have on the levels and availabilities of bioactive compounds in grains is currently still scarce. This type of knowledge becomes especially important when the development of whole grain foods is considered. Rye is an important source of whole grain foods in Eastern and Northern European diets, with traditional uses in various soft and crisp breads being based on wholemeal flour.

Grains contain living cells, and have variable levels of different biocatalytic activities. During fermentation and malting, when grains are hydrated in ambient conditions, both endogenous and added enzymes and micro-organisms start to modify the grain constituents. Many biochemical changes occur during sourdough baking and germinating or malting, which affect product properties such as structure, bioactivity, flavour, stability and digestibility (Heiniö et al., 2003; Katina et al., 2004, Katina et al., 2007; Salmenkallio-Marttila et al., 2001). In rye baking, the amounts of folates and phenolic compounds increase during the fermentation phase with the starter type being an important factor affecting the process (Kariluoto et al., 2004). The levels of phytate (Frölich et al., 1986; Larsson and Sandberg, 1991), alkylresorcinols (ARs) (Verdeal and Lorenz, 1977) and tocopherols (Piironen et al., 1987) are reduced during sourdough baking process, whereas the levels of lignans did not change greatly (Nilsson et al., 1997b).

During germination (i.e. hydrothermal treatment in ambient conditions) the biosynthetic potential of grains is exploited and a number of hydrolytic enzymes are synthesised. The reactions in germinating grain lead to structural modification and the synthesis of new compounds, some of which have high bioactivity and can increase the nutritional value and stability of the grains (Kaukovirta-Norja et al., 2004). The germination of rye grains for 6 days at 15–25 °C has been shown to lead to 2–3.5-fold increases in the contents of folates and methanol-soluble phenolic compounds (Liukkonen et al., 2003).

Previous studies have established the potential of both sourdough and germination to increase the nutritional quality of wholemeal rye. However, the combination of these two processes has not yet been studied. Some recent work, however, shows that fermentation is increased and product quality altered (Di Cagno et al., 2003; Katina et al., 2006) if exogenous enzymes are combined with sourdough fermentation. The aim of the current work was to combine fermentation with germination of rye, in order to determine if these have synergistic effects on bioactive compounds. Different types of fermentation (yeast fermentation, heterofermentative and homofermentative lactic acid fermentation, combined and spontaneous fermentation) were studied in the processing of both native and germinated (high enzyme activity) rye.