Wine producers commonly operate with a single bottling line for both aromatised wine-based beverages and regular wine; only for very large or specialised producers, a separate line for aromatised products is economically feasible. Despite the implementation of clean-in-place (CIP) systems, carryover of added aroma compounds from the aromatised product into subsequently bottled wine can occur due to absorption (scalping) and remigration from bottling line sealing polymers (Elss et al., 2007, Eckstein, 2011, Vetter, 2012, LUA, 2014, Freund and Seckler, 2016, BMEL, 2019, Beierle et al., 2020, Gottmann et al., 2021).

Even unintentional carryover of trace aroma compounds can result in prosecution by the EU and other legislation (European Parliament, 2009). Aroma concentrates added for aromatisation are usually of synthetic origin (Mosandl, 2004) and recent developments in enantioselective and isotope ratio analysis allows distinguishing aroma compounds from biological and synthetic sources (Schumacher et al., 1998, Langen et al., 2013, Lampe, 2015, Langen et al., 2016). So far, any detection of aroma compounds of chemical origin is illegal, regardless of their sensory relevance. The German Federal Ministry of Nutrition and Agriculture (Bundesministerium für Ernährung und Landwirtschaft, BMEL), in cooperation with the German food control authorities and wine industry, developed good manufacturing practices (GMP) to reduce the risk of aroma carryover (BMEL, 2019). If a bottling company complies with this guideline and aroma compounds used to aromatise are detected but have, more importantly, no sensory significance, these compounds will be classified as technically unavoidable, consequently not resulting in prosecution.

Literature exploring the uptake of aroma compounds into sealing polymers and their release during cleaning and contact within subsequently bottled wine is scarce. Cleaning procedures usually focus on the mechanical removal of residue from previous products as well as sterilisation of the bottling line to prevent microbial cross-contamination (Englezos et al., 2019), while the removal of aroma compounds from sealing materials has been largely neglected. To date, cleaning efficacy and its economic efficiency depend on each company's experience and are based on their experimental trials (Changani et al., 1997, Seiberling, 2008, Basso et al., 2017).

The achieved cleaning effect is primarily influenced by a combination of several factors: time, temperature, mechanical effects and the concentration and chemical nature of cleaning agents (Sinner, 1960, Tamime, 2009). Mechanical cleaning is limited to the polymer surface and can be neglected in terms of cleaning efficiency on absorbed aroma compounds. The aroma carryover risk was firstly described for aroma compounds including limonene and myrcene from orange lemonade bottled in reusable polyethylene terephthalate (PET) bottles, whereas the conducted basic bottle washing at 70 °C removed only 31 % and 45 %, respectively (Nielsen, 1994). Furthermore, Safa and Bourelle (1999) reported a sensorial perceptible carryover of 250–300 µg/L limonene from reusable PET bottles into water, despite prior cleaning. Within in the scope of our recent study, the GMP-oriented cleaning of ethylene propylene diene monomer rubber (EPDM) sealings polymers removed only 11–62 % of absorbed marker aroma compounds in a model system and further assessment of each cleaning parameter were required (Gottmann et al., 2021).

Besides frequent use of water, acidic and alkaline agents, oxidants such as peracetic acid, ozone and chlorine dioxide are widely applied to remove organic compounds and to sterilise bottling lines, hoses, barrels and other winery equipment (Sonntag and Gunten, 2012, Rincon-Delgadillo et al., 2013, Englezos et al., 2019, Pandiselvam et al., 2019, Kim and Huang, 2020, Gan et al., 2020, Stadler et al., 2020, Stadler and Fischer, 2020). Improved cleaning efficacy was shown for a 60 min treatment of enriched water with 2 mg/L ozone for the removal of d-limonene from EPDM and silicone sealings (Nishijima et al., 2014).

To minimise the risk of aroma carryover, less absorptive sealing materials are of interest. The chemical nature of a sealing polymer defines its absorptive behaviour. Commonly used polymers are EPDM and highly fluorinated rubber (FKM), which vary in their flexibility, inertness, stability, application field and temperature tolerance (Vetter, 2012, Freund and Seckler, 2016). However, the cost is often the deciding factor for industry usage, with EPDM being the more economical option over FKM (Abts, 2010). EPDM polymers available on the market differ in their formulation, particularly with the incorporation of filling materials such as minerals, talc, natural organics and synthetic inorganics into the polymer mesh to reduce costs and control physical properties (Müller et al., 2022). The absorption behaviour of the polymers usually becomes secondary or even redundant. However, importantly, the selected filler influences the crystallite formation of the polymer. With increasing crystallinity, the migration of molecules diminishes, as movement is more restricted in crystalline phases than in amorphous phases and aroma absorption can be influenced (Charara et al., 1992, Richter and Schacker, 2016).

This work addresses how novel fillers in EPDM polymers influence the absorption of aroma compounds and how different cleaning methods affect their release. We used seven aroma compounds previously characterised in their sensory properties, which are commonly used in high concentrations in aromatised cocktails with the aroma of peach (γ-decalactone, δ-decalactone and γ-undecalactone), apple (ethyl 2-methylbutanoate) and raspberry (α-ionone), as well as mulled wines with clove (eugenol) and cinnamon (trans-cinnamaldehyde) (Gottmann et al., 2022). Using a recently developed model system (Gottmann et al., 2021), aroma absorption was studied by monitoring experimental EPDM polymers enriched with nine different fillers in comparison with two commercial sealings. Moreover, the impact of cleaning temperature, time and chemical additive on cleaning efficacy was investigated in this model system.

We previously reported the principles of the uptake of aroma compounds from model wine into EPDM sealing material and their incomplete desorption during cleaning, which bear the risk of subsequent contamination (Gottmann et al., 2021). It is the objective of this study to mitigate the absorption of aroma compounds by improved polymer materials and to enhance their removal through improved cleaning procedures. Studying the behaviour in sealings, we report concentrations of aroma compounds per volume (µg/cm³). Due to different fillers, the density of the polymers varied by a factor of two (Supplemental Table 1).

Diffusion phenomena in solid material are strongly dependent on the concentration gradient and are governed according to the first Fick's law (1) by the diffusion flux (J), the diffusion coefficient (D), the concentration (c) being active along the length and thickness of the matrix (x) (Sajilata et al., 2007, Licciardello et al., 2009, Karbowiak et al., 2010, Číhal et al., 2015).

(1)

J = - D ∂ c ∂ x

To minimise the diffusion flux J of aroma compounds into and within sealing polymers, the diffusion coefficient D is the only factor which can be freely modified. The concentration of aroma compounds is predetermined by the composition of the bottled aromatised beverage, while x is defined by the dimensions of the sealings which are installed within a bottling machine. To alter the diffusion coefficient D, we incorporated different filler substances into the polymer, which should mitigate best aroma absorption, but have as little as possible impact on material properties such as durability and flexibility. Increased crystallinity of the polymer due to interaction with appropriate filler substance diminishes absorption because, in crystalline phases, diffusion of aroma compounds is more hindered, as it is the case in amorphous phases in polymers (Charara et al., 1992, Richter and Schacker, 2016). Due to their nonpolar properties the incorporation of PE wax and cork into EPDM enlarged the absorption of volatiles, which is contradictory to the mitigation objective (Barker et al., 2001, Brajkovich et al., 2005, Blake et al., 2009, Karbowiak et al., 2010, Adams et al., 2019). The observed difference between Carbon Black N220 and N990 is rationalised by the fact that Carbon Black N220 consists of small particles with a larger surface per unit of weight, which enhances absorption versus the larger-sized N990 particles. Uptake of aroma compounds decreased largely by incorporation of micro-cellulose or kaolin into the EPDM polymer and absorption declined more effectively with a larger portion of both materials. This beneficial effect can be explained by the crystalline nature of cellulose and the layered silicate mineral kaolin (Al₂Si₂O₅(OH)₄). However, as displayed in Figure 1, this improvement could not achieve the same inertness towards the absorption of aroma compounds as it was measured for fluorine-based rubber (FKM). FKM sealings feature excellent thermal and chemical stability due to the high binding energy of the carbon-fluorine bond and the high density of the polymer. The linear molecular structure also favours semi-crystalline structures with high melting points, which hinders diffusion within the sealing (Abts, 2010). Although aroma carryover can be efficiently decreased by implementing FKM sealings into bottling equipment, FKM is not commonly used due to its approximately ten times higher price as well as its low fatigue resistance, which severely restricts its use in sealing moving parts of the bottling machine. The merits of FKM to mitigate the migration of several aroma compounds as reported in this paper has been so far only published in German technical literature, studying aroma carryover during simulated bottling of flavoured water just by sensory evaluation and no chemical analysis of aroma compounds in the sealing itself (Eckstein, 2011; Vetter, 2012; Freudenberg Sealing Technologies, 2016; Freund and Seckler, 2016; Beierle et al., 2020).

A comprehensive evaluation of the efficacy of removing aroma compounds from sealings by commonly applied cleaning agents, temperatures and duration of treatments for bottling equipment did not reveal any significant improvement toward pure water at the same temperature (Figure 2C). All cleaning agents by themselves, as well as their dissolvent water, are highly polar compounds. Deviating absorption of aroma compounds of varying polarity in the same EPDM sealing revealed that only 1 % of the most polar trans-cinnamaldehyde could be absorbed from a solution, while the least polar α-ionone was absorbed by 25 % (Gottmann et al., 2021). As cleaning agents such as citric acid, caustic soda and water exceed by large the polarity of the studied aroma compounds, it is obvious that neither the cleaning agents nor the water enter the sealing and, thus, cannot interact directly with absorbed aroma compounds in deeper layers of the polymer.

Oxidative cleaning additives commonly used in sterilisation and sanitation, such as ozone, peracetic acid (PAA) or chlorine dioxide, are discussed in the industry to enhance the removal of aroma compounds from sealings. Indeed, oxidative additives oxidise volatiles according to the Criegee mechanism resulting in an oxygen-substitution or a ring cleavage leading to a rise in polarity of the educts (Staehelin et al., 1984, Mvula and Sonntag, 2003, Sonntag and Gunten, 2012, Wenk et al., 2013). This effect was proven to be effective in the water by the direct exposure of aroma compounds to oxidative agents (Table 1). The unsaturated aroma molecules α-ionone, eugenol and trans-cinnamaldehyde were partially or completely degraded by oxidants, while the saturated lactones and ester remained unaffected in their concentration even throughout three days of contact. The observation that none of the treatments with added oxidants in this study could significantly enhance the depletion of aroma compounds from the sealings in comparison to water (Table 2) is another proof that these highly polar agents do not migrate into the EPDM polymers at all. In an earlier study, ozonated water (2 mg/L, 60 min, 20 °C) enhanced the removal of limonene from EPDM and silicone sealings, while for 1-hexanol, furfural and 4-vinylguaiacol only minor effects were observed (Nishijima et al., 2014). Our findings are corroborated by Stadler et al. (2020), who did not observe any change in oak volatiles in wine barrels using a short 2 min treatment with 10 mg/L ozonated water, although ozone penetrates porous oak better than solid EPDM polymers.

The fact that elevated temperature is by far the most effective cleaning parameter to remove aroma compounds is rationalised by the comprehensive definition of the diffusion coefficient D in Fick’s law (2). D is determined by the maximum diffusion (D₀ ), the activation energy (E_A ) required to initiate diffusion, the universal gas constant (R) and temperature (T). Besides the fixed R, D₀ depends on the matrix and E_A on the compound studied. Thus, the only parameter that can be freely varied is temperature.

(2)

                    D = D 0 × e x p - E A R T

The rising temperature warms up the sealing polymer and stimulates the diffusion of aroma compounds in the polymer matrix according to the Arrhenius equation with the rate constant (k) and the pre-exponential factor (A).

(3)

                    k   = A × e x p - E A R T

As a result, depletion of aroma compounds close to the surface of the sealing will increase compared to a cleaning temperature at 55 °C or even 22 °C with lower diffusion coefficients for the aroma compounds in the polymer.

A more holistic approach can be considered using the second Fick's law. Derived from the first law with a static approach, the second Fick's law (4) predicts the change in concentration in a given area as a function of time (Sajilata et al., 2007, Licciardello et al., 2009, Karbowiak et al., 2010, Číhal et al., 2015).

(4)

                    ∂ c ∂ t = D ∂ 2 c ∂ x 2

Studying the time-dependent depletion of aroma compounds at 85 °C in Figure 3, an equilibrium was revealed between absorption and depletion established after 20 minutes of cleaning. Bearing in mind that cleaning agents can only interact directly with aroma compounds remaining or emerging from deeper polymer layers on the surface of the sealing, diffusion of aroma compounds from deeper layers to the surface of the polymer seems to be the limiting step for the depletion of aroma compounds during the cleaning process. This diffusion is governed by the concentration gradient in the polymer matrix, the polarity of the aroma compounds and the density and crystallinity of the sealing polymer, which is a function of the polymer composition and added fillers. Any cleaning or oxidative agents are by far too polar to penetrate the sealing; only ethanol seems to be nonpolar enough to exert a minor depletion enhancement, but only after 24 h of exposure (Canals et al., 2005, Coelho et al., 2019). Highly nonpolar compounds such as α-ionone and γ-undecalactone seem to benefit more from this small effect which is confirmed by a two-fold increase in depletion. According to the principles of Le Chatelier, the exchange of the cleaning agents during the cleaning process should improve the cleaning efficacy to a large extent due to newly established equilibria. Surprisingly, exchanging the hot cleaning water after 30 min did not lead to further significant removal of the aroma compounds from the polymer.

Table

The comprehensive study of aroma compounds varying in chemical structure and polarity revealed their absorption and desorption behaviour within the sealing polymer. A first strategy to mitigate aroma carryover is to increase the crystallinity and density of the sealing polymer to restrict the uptake and diffusion within the polymer. The most promising kaolin and cellulose fillers did not modify the physical properties of the sealing polymers. Statistical analysis of all cleaning parameters clearly proved temperature as the most important and effective factor, which was rationalised by different physical laws. None of the cleaning agents were able to improve the depletion of aroma compounds for the simple reason that they only act on the surface because they are too polar to penetrate the nonpolar sealing polymers. Only exposure to ethanol solution over a long time of 24 hours showed a minor benefit in aroma depletion. Based on the developed model, aroma compounds will never be completely removed from a sealing polymer because diffusion has no spatial orientation and only follows a concentration gradient. Thus, depletion of aroma compounds ceased after 20 minutes at high temperature, as diffusion to the sealing surface was too slow to deliver further aroma compounds from deeper layers and also led to diffusion on the contrary direction. Energy-consuming high cleaning temperatures can only be partially compensated by very long exposure periods of 24 h or even longer.

Considering the fact that complete depletion of aroma compounds from sealings is not possible, aroma carryover into the subsequently bottled wine cannot be prevented to 100 %. Thus, it requires further studies to determine the sensory relevance of aroma carryover through sensory tests and the application of odour-thresholds of aroma compounds determined in a wine matrix.

This IGF Project 20220 N of the FEI is/was supported via AiF within the program for promoting the Industrial Collective Research (IGF) of the German Ministry of Economics and Climate Action (BMWK), based on a resolution of the German Parliament.