molecules Review A Review on Stems Composition and Their Impact on Wine Quality Marie Blackford 1,2,* , Montaine Comby 1,2, Liming Zeng 2, Ágnes Dienes-Nagy 1, Gilles Bourdin 1, Fabrice Lorenzini 1 and Benoit Bach 2 1 Agroscope, Route de Duillier 50, 1260 Nyon, Switzerland; [email protected] (M.C.); [email protected] (Á.D.-N.); [email protected] (G.B.); [email protected] (F.L.) 2 Changins, Viticulture and Enology, HES-SO University of Applied Sciences and Arts Western Switzerland, Route de Duillier 50, 1260 Nyon, Switzerland; [email protected] (L.Z.); [email protected] (B.B.) * Correspondence: [email protected] Abstract: Often blamed for bringing green aromas and astringency to wines, the use of stems is also empirically known to improve the aromatic complexity and freshness of some wines. Although applied in different wine-growing regions, stems use remains mainly experimental at a cellar level. Few studies have specifically focused on the compounds extracted from stems during fermentation and maceration and their potential impact on the must and wine matrices. We identified current knowledge on stem chemical composition and inventoried the compounds likely to be released during maceration to consider their theoretical impact. In addition, we investigated existing studies that examined the impact of either single stems or whole clusters on the wine quality. Many parameters influence stems’ effect on the wine, especially grape variety, stem state, how stems are incorporated, Citation: Blackford, M.; Comby, M.; when they are added, and contact duration. Other rarely considered factors may also have an impact, Zeng, L.; Dienes-Nagy, Á.; Bourdin, G.; Lorenzini, F.; Bach, B. A Review including vintage and ripening conditions, which could affect the lignification of the stem. on Stems Composition and Their Impact on Wine Quality. Molecules Keywords: grape stems; whole clusters; wine; winemaking practice; phenolic compounds; antioxi- 2021, 26, 1240. https://doi.org/ dant activity; maceration technique; polyphenolic compounds 10.3390/molecules26051240 Academic Editors: Rocio Gil-Muñoz and Encarna Gómez-Plaza 1. Introduction For white winemaking, stems are generally kept during pressing because they allow Received: 12 January 2021 for better juice extraction yields. Given the short contact time, compounds are extracted Accepted: 22 February 2021 from the stems in relatively low levels. In red winemaking, the maceration phase—where Published: 25 February 2021 color is extracted from the grape skin and tannins from the grape seeds—occurs before pressing. Originally, stems were kept during this phase, but destemming practices appeared Publisher’s Note: MDPI stays neutral at the end of the 19th century, improving wine quality by reducing excessive astringency with regard to jurisdictional claims in and negative strong green tastes from the stems [1]. Initially used in cellars with high published maps and institutional affil- production capacity, the first destemming machines quickly trivialized this practice; to- iations. day, this technique is systematic for winegrowers in most wine-producing countries [2]. However, in some regions, using whole clusters of grapes is a matter of tradition. The stem is considered a natural additive that, if well mastered, brings complexity, freshness, and phenolic structure to the wine and facilitates chemical stability during aging [3] (e.g., Copyright: © 2021 by the authors. the Pinot Noir in Burgundy, the Cabernet Franc in the Loire Valley, or the Gamay in the Licensee MDPI, Basel, Switzerland. Beaujolais, Kakhethian wines from Georgia, etc.). In recent years, winemakers in Europe This article is an open access article and other countries, such as Australia and South Africa, have shown an interest in using distributed under the terms and stems, and several technical articles mention the advantages of this practice [4,5]. conditions of the Creative Commons Attribution (CC BY) license (https:// These winemaking techniques are not used for all grape varieties, nor for all vintages. creativecommons.org/licenses/by/ Since these techniques have very little research behind them, they are generally passed 4.0/). Molecules 2021, 26, 1240. https://doi.org/10.3390/molecules26051240 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x 2 of 41 Molecules 2021, 26, 1240 2 of 40 along by word of mouth. Therefore, it is difficult to know which stem conditions will lead alongto improved by word or of deteriorated mouth. Therefore, wine quality. it is difficult to know which stem conditions will lead to improvedStem composition or deteriorated has often wine been quality. analyzed to value winemaking by-products and has beenStem relatively composition well studied. has often Many been compounds analyzed to of value interest winemaking can be found by-products in stems’ and overall has beencomposition. relatively Their well richness studied. in Many polyphenolic compounds compounds of interest makes can be them found very in stems’interesting overall for composition.the food and medicine Their richness industries, in polyphenolic in relationcompounds to their antioxidant makes them potential. very interestingIn some stud- for theies, foodunits and used medicine to express industries, stem extract in relation compos toition their is antioxidant very specific potential. and makes Insome it impossible studies, unitsto compare used to results. express Therefore, stem extract such composition data are not is verypresented specific in andthis makesarticle it[6–10]. impossible This re- to compareview gathered results. information Therefore, from such the data literature are not presented on stem chemical in this article composition [6–10]. This to examine review gatheredhow these information compounds from contribute the literature to variatio on stemns in chemical aroma and composition taste when to examinestems are how in- thesecluded compounds during winemaking. contribute Although to variations stems in are aroma only and used taste for whenred wines, stems we are also included exam- duringined data winemaking. on the chemical Although composition stems are of only white used grape for redvariety wines, stems. wealso We examinedthen compiled data onthe themain chemical results compositionobserved when of whitewhole grape clusters variety of grapes stems. or We single then stems compiled werethe incorpo- main resultsrated into observed the winemaking when whole process, clusters from of grapes a technological, or single stemschemical, were and incorporated sensorial perspec- into the winemakingtive. process, from a technological, chemical, and sensorial perspective. 2. Grape Stems 2.1. Morphology 2.1. Morphology The stem is the skeleton of the grape cluster or bunch. The longest part, the rachis The stem is the skeleton of the grape cluster or bunch. The longest part, the rachis (main axis), is branched with peduncles, and a pedicel attaches each grape berry to the (main axis), is branched with peduncles, and a pedicel attaches each grape berry to the stem (Figure1). stem (Figure 1). Figure 1. Bunch of grapes and stem morphology. The stem’sstem’s finalfinal sizesize is is reached reached around around veraison veraison [1 [1].]. For For each each grape grape variety, variety, the the number, num- length, and distance between two ramifications varies. Along with other morphological ber, length, and distance between two ramifications varies. Along with other morpholog- criteria, these components determine the compactness of the bunch [11]. The stem accounts ical criteria, these components determine the compactness of the bunch [11]. The stem for 3 to 7% of the total bunch weight, depending on the grape variety, number of grapes on accounts for 3 to 7% of the total bunch weight, depending on the grape variety, number the bunch, and its sanitary state [1,2,12]. of grapes on the bunch, and its sanitary state [1,2,12]. 2.2. General Composition 2.2. General Composition This part of the review aims to summarize the main compounds found in grape stems. An estimationThis part ofof their the quantificationreview aims to based summa on therize available the main data compounds is shown infound Figure in2 .grape This compositionstems. An estimation is close to of the their one quantification described by Foulonneaubased on the et available al. which data is similar is shown to that in Figure of the vine’s2. This leaves composition and tendrils is close [2]. to For the each one typedescribed of compound, by Foulonneau available et data al., which were summarized. is similar to Itthat should of the be vine’s noticed leaves that and the comparisontendrils [2]. ofFo publishedr each type data of compound, is difficult, asavailable their proportions data were cansummarized. be impacted It should by different be noticed factors, that such the as co grapemparison variety, of published vintage, maturation data is difficult, state, asas welltheir asproportions differences can in extractionbe impacted techniques by different and fa units.ctors, such as grape variety, vintage, mat- uration state, as well as differences in extraction techniques and units. Molecules 2021, 26, 1240 3 of 40 Molecules 2021, 26, x 3 of 41 FigureFigure 2.2. GeneralGeneral compositioncomposition ofof grapegrape stems.stems. 2.2.1.2.2.1. WaterWater AsAs thethe stem’s stem’s main main component, component, water water accounts accounts for 55 tofor 80% 55 ofto stem 80% weight of stem [2,3 ,weight13,14]. In[2,3,13,14]. 1976, Rice In et 1976, al. measured Rice et al. themeasured moisture the of moisture fresh grape of fresh stems grape from stems ten grape from varieties,ten grape fivevarieties, reds (Concord,five reds (Concord, Ives, Baco Ives, noir, Baco Red hybrids,noir, Red and hybrids, Cascade) and andCascade) fivewhites and five (Aurore, whites Concord(Aurore, Concord CP, Delaware, CP, Delaware, Niagara, Niagara, and Catawba) and Catawba) planted planted in New inYork New state,York state, USA USA [14]. The[14].values The values ranged ranged between between 68.4 68.4 and and 79.1% 79.1% of stemof stem fresh fresh weight weight (FW).