Use of polyphenol-rich grape by-products in monogastric nutrition. A review

Highlights

• The addition of grape by-products to the diet or to meat inhibits meat lipid peroxidation.

• Polyphenol-rich grape by-products are effective in modifying the intestinal microbiota.

• Grape by-products have great potential as a source of functional feed or additive.

Abstract

Plants and their biologically active chemical constituents present numerous opportunities for improving animal production by inclusion in the diet. In recent years, interest has grown in the antioxidant and antimicrobial properties of a number of polyphenols found in different plants. The by-products of the wine industry (grape pomace, skin and seeds) and wine polyphenol extracts contain a wide range of bioactive compounds. However, studies on grape by-products are very limited, despite their richness polyphenolic substances. In this context, the purpose of this review is summarize recent advances of research in grape by-products including the phenolic composition, mechanism of intestinal and hepatic conjugation, plasma transport and elimination in bile and urine, and biological activities such as antioxidant and antimicrobial effect. Given their antioxidant activity, the inclusion of these by-products in feed rations would not only enhance the oxidative stability of the meat and reduce the amount of additives like vitamin E but also improve meat quality through direct addition of these natural antioxidants, thereby helping to meet consumer demand for healthier meat products. With respect to antimicrobial activity, they enhance the growth of specific beneficial bacteria strains in the intestinal tract while competitively excluding certain pathogenic bacteria.

Introduction

By-products from the winemaking industry are widely available in Mediterranean countries. The grape is the world's largest fruit crop, with an annual production of more than 67 million tonnes (FAO STAT, 2010). It is estimated that around 20% of the total weight of grapes used for wine is made up of grape pomace (GP), the solid residue left over after the juice is extracted from the grapes. This by-product, consisting of the seed, skin, and stem, is used in the production of ethanol by fermentation and distillation and in the extraction of tartaric acid. With respect to animal feed, the nutritional value of these by-products is relatively low. It is estimated that some 3% of production is used mainly in the manufacture of maintenance feed for ruminants and to provide some of the dietary fibre needs of rabbits (Nicodemus et al., 2007). GP has also been used to extract the oil contained in the grape seed.

Grape pomace is particularly rich in a wide range of polyphenols. Formerly known as tannins, these polyphenols were considered to be anti-nutritional factors as their presence in certain ingredients, such as legumes, sunflowers and sorghum had negative effects on animal nutrition. The major limitations of using GP in monogastric feed are the high level of lignified cell wall fraction and the high tannin content. However, studies carried out in vivo and in vitro over the last few years have shown the beneficial effects of administering these bioactive compounds because of their antioxidant and antimicrobial activity (Alonso et al., 2002, Torres et al., 2002, Viveros et al., 2011).

The use of polyphenols is recommended to limit lipoperoxidation and preserve animal health and product quality (Georgiev et al., 2014). Lipid peroxidation is a matter of great interest, both for the food industry and for consumers, as it leads to the development of unpleasant smells or tastes in food products, which may become potentially toxic. Meat from domestic animals is particularly exposed to lipoperoxidation processes, not only because it is at higher risk of oxidative stress but also for reasons of diet. The inclusion of polyunsaturated fatty acids (PUFA) in the diet is a generally accepted practice for raising the energy content of the feed or producing animal products with a fatty acid composition that better meets nutritional guidelines (Bourré, 2005). However, a high level of polyunsaturation accelerates oxidative processes, leading to a deterioration in the flavour, texture and nutritional value of the meat (Mielnik et al., 2006). Manipulation of the gut functions and microbial habitat of domestic animals with feed additives has also been recognized as an important tool for improving growth performance and feed efficiency. The increasing antimicrobial resistance of pathogens isolated from humans and animals, combined with the ban on the use of antibiotics as feed additives, has hastened research into alternative options for more antimicrobial efficient in animal production (Hughes et al., 2005).

In short, increased efforts are now being directed towards a broader revaluation of polyphenol-rich residues from wine processing for obtaining high-value co-products (GP, seeds, skin and grape polyphenols). The purpose of this review is to understand the mechanism of their biological activities with a view to exploring the potential application of these bioactive compounds, and to outline recent scientific advances supporting their beneficial qualities, bioavailability and use as ingredients or additives in monogastric nutrition.