High-value products from fruit waste

Grape (Vitis spp.)

Grape is the world’s largest fruit crop with >65 million metric tons per annum with EU production accounting for up to 46% of this. In addition, global wine production in 2009 was over 27.2 million tonnes which equates to over 8 million hectares of vineyard. The wine production industry thus provides a significant waste stream in the form of both pomace (skins, seeds) and stems. In fact, the global amount of pomace produced equates to approximately 20% of the harvested grape. The majority of pomace is used to distil alcoholic beverages and to provide animal feed and fertiliser. However, there also exists a sizable opportunity for exploiting grape pomace for high-value products as novel actives in cosmetics products. Such a strategy is attractive from an environmental perspective and is both sustainable and cost-effective.
Vitis spp. contains a plethora of products of both primary and secondary metabolism. In fact, the chemical analysis of wine has to date identified more than 500 compounds. Diversity of secondary metabolism in grape is quite rich, with compounds ranging from simple phenols and phenylpropanoids to flavonoid pigments and stilbenoids present. However, the majority of simple phenols accumulate in the vacuole and would thus be extracted upon fruit processing; nevertheless, the majority of flavonoids, stilbenoids and proanthocyanidins are mainly localised in the skin and seeds of the fruit and thus provide a basis for closer scrutiny as potential actives in the pomace.

Anthocyanins

The group of flavonoids most associated with grape are the anthocyanin class of pigments which are water-soluble vacuolar compounds largely responsible for the colour of red wine; red grapes have been shown to contain on average between 0.09-0.26% w/w total anthocyanin.
However, although grapes are recognised as being the most abundant source of anthocyanins, through experience it has been shown that the majority of anthocyanins in grapes are extracted during the winemaking process leaving negligible levels in the pomace, therefore, the potential value of using the anthocyanin fraction in grape pomace is limited.

Flavonols

The other main group of flavonoid pigments present in grape skins are the flavonol class of compounds (1). These are often yellow compounds and all have intense absorption in the UV spectrum. The principal flavonols found in red grapes are glycosides of quercetin, kaempferol, myricetin, isorhamnetin, laricitrin and syringetin; in the white varieties myricetin, laricitrin and syringetin were shown to be absent (Table 1). The average total flavonol content of red and white grape varieties was shown to be 30 mg/kg and 10 mg/kg, respectively, although the concentration in the isolated pomace would be expected to be significantly higher.
The UV spectral characteristics of the flavonoids found in grapes make such actives potentially useful natural UV absorbers. It has been common practice for several years to incorporate synthetic sunscreens into cosmetics products to counteract the biological damage, such as ageing and wrinkling caused to skin by UV radiation. However, these UV absorbers generally have a low efficacy as broad-spectrum filters due to photo instability issues in the longer UV-A range. There is therefore a need to develop sunscreens exhibiting a wider spectrum of UV attenuation properties. Flavonoids present in grape waste may be ideal candidates for such functionality.
Flavonols have the multifunctional advantage that they are photostable and absorb well into the UV-A range. They can also function as potent antioxidants and scavenge reactive oxygen species generated by UV radiation which attack various biological targets such as DNA, proteins and cause lipid oxidation by reaction with molecular oxygen. These processes directly lead to ageing and wrinkling in the skin.
There is a significant regulatory barrier to the approval of new UV absorbers, hence the ability to include flavonols due to their multifunctionality allows the incorporation of activities that have strong UV absorbing capacity, but can be included as “antioxidant”, “skin protecting” and “skin conditioning”.

Stilbenoids

Stilbenoids constitute the final class of principal phenolics found in grape skins and are plant phytoalexins, produced in response to abiotic and biotic factors such as fungal attack; resveratrol (2) is the principal stilbene found in grape skins. It has been shown that the total stilbenoid content measured in a series of Brazilian wines could reach levels >87 mg/L. Therefore the feasibility of incorporating stilbenoid fractions isolated from grape pomace into skincare products targeting dermatophytic fungi should be considered. Certainly, resveratrol has been shown to inhibit the growth of several skin bacterial and fungal pathogens.
Apart from antimicrobial activity, stilbenoids are being recognised for a wide range of different activities. For example they have been shown to possess anti-inflammatory, and antioxidant activity. The putative antimicrobial and antioxidant activity of resveratrol also provides opportunities for exploiting the functional preservative properties of stilbenoid fraction from grape pomace in skin care formulations as a substitute for parabens. Additionally, as the stilbenoids display characteristic UV absorption maxima at 300-320 nm, they may also potentially function as UV-B filters in skin care formulations. Furthermore, the compound resveratrol has become a hot topic in the health promoting community in recent years due to its wide-ranging potential health promoting properties such as anti-aging in animal models attributed to quenching damaging free radicals, repairing damaged DNA and preventing cell damage from nuclear radiation. Based on their antifungal, antioxidant and potential UV-B absorbing properties stilbenoids from grape may constitute high value products from grape pomace for incorporation into skin care products.

Proanthocyanidins

Proanthocyanidins (PACs) or condensed tannins consist of oligomers and polymers of catechin and flavan-3,4-diol units (3). In grapes, they consist of the flavan-3-ol subunits (+)-catechin, (–)-epicatechin, (–)-epicatechin-3-O-gallate and (–)-epigallocatechin. They are reported to be present in both grape skins and seeds at levels between 2-3% w/w and 6-10% w/w, respectively.

The biological, pharmacological and medicinal properties of grape PACs have been extensively reviewed. They have been shown to have a broad spectrum of biological, pharmacological and therapeutic activities against free radicals and oxidative stress in both in vitro and in vivo models; grape PACs have also shown significant anti-inflammatory activity in human tissue models. The anti-ageing properties of grape PACs have also received attention with research showing that they inhibited the damage of DNA due to age-related processes. Mouse model studies showed that grape seed PACs have the ability to induce dermal wound healing by enhancing vascular endothelial growth factor. These properties may be exploited for performance in cosmetics products targeting areas such as skin cell repair under the umbrella of helping to maintain healthy skin.

Apple (Malus spp.)

World apple production for 2011/2012 is estimated to be about 65 million tonnes. Of this figure, the EU accounts for about 16% and concentrated juice processing consumes about 20% of the total global apple production. Such consumption generates a significant amount of pomace consisting of skin, pulp, seeds and extraneous matter such as the remnants of stems. Uses for this pomace include the extraction of pectin, distillation of ciders, pet food and as culinary additives. However there is ample opportunity for exploiting apple waste for high value actives that may have specific applications in the cosmetics industry. Apples have been long associated with health benefits, for example, we have all heard of the phrase “an apple a day keeps the doctor away”; through an understanding of the phytochemistry of the fruit and the array of biological properties it possesses, science has provided credibility for this.

Phenolics

Apples are known to be rich in phenolic compounds which can represent 0.3% of the fresh apple. The principal class of phenolics present in apple peel are the flavonoids which consist mainly of galactosides, glucosides and arabinosides of quercetin (see Table 1). The total levels of flavonoids present in apples with skins were reported to be between 72-80 mg/kg for fresh fruit. Catechins are also present along with phenylpropanoid derivatives such as caffeic acid (4) and chlorogenic acid (5).
The biological activity most associated with apple phenolics is their putative antioxidant activity. Several apple flavonoids including quercetin based derivatives have shown strong radical scavenging activity against peroxyl radicals. As discussed for grapes, apple phenolics may be potentially useful in skincare products as both UV absorbers and antioxidants to prevent ageing and wrinkling in the skin. They may also have a function as natural preservatives as some, particularly the hydroxycinnamates, are known to possess antimicrobial properties.

Triterpenoids

About 13 different triterpenoids have been identified in apple peel which are mainly derivatives of ursolic acid (6a) and oleanolic acid (6b). These compounds have been reported for their anti-cancer activity. Several triterpenoids isolated from apple peel have shown antiproliferative activity against human liver, breast and colonic cancer cell lines. However they may also possess other useful activities such as wound healing, anti-inflammatory and antimicrobial properties.