Considerations When Brewing with Fruit Juices: A Review and Case Study Using Peaches

Abstract

Beer is traditionally brewed using barley, hops, water, and yeast. Historically, fruit has been utilized in brewing operations as a source of carbohydrates, color, and/or flavor. This practice has been increasing in recent years due to economic and market factors. While many scientific studies have shown that it is both possible and desirable to include fruits in brewing operations, there is little research published on how to properly evaluate their potential for use in brewing. This review aims to introduce and discuss the ways in which fermented products are impacted by the addition of fruit with respect to the following characteristics of the fruit and final product: sugars, volatiles, color, product identity, and microbial factors. To support this review, a case study is included in which peaches were evaluated for potential use as a fruit additive in a commercial brewing application. The peach juice, pre-addition beer, and final product were assessed based upon processing characteristics, sugar content and composition, color, volatile profile, and economic suitability for various applications in fermented beverages. This paper details the methods used to evaluate fruit as a guide for considering fruit as an addition to brewing formulations.

1. Introduction

Fruits are easy to use in brewing applications because of their inherent high simple sugar concentrations. The 2021 global fruit beer market was estimated to be worth USD 266.9 billion and is projected to grow by around USD 3.79 billion by 2027 [1]. There are several traditional styles (e.g., lambic) that typically contain fruit for alcoholic fermentation and volatile (flavoring) additions. Common fruits include cherries, raspberries, peaches, grapes, black currants, plums, and pineapples [2]. Increasing demand for fruit beer has influenced brewers to experiment by adding fruit juice to various styles of beer, often resulting in color and volatile changes. In past research, fruit additives have been used in fermentations and evaluated as carbohydrate sources, volatile enhancers, color enhancers [3], and as means of reducing raw-material costs, as well as with respect to their role in changing consumer perceptions of a product [4]. Most studies have been limited to how fruit performs in fermentation operations for a specific purpose. The goal of this study, however, is to provide a holistic assessment of parameters influenced by fruit addition so that brewers can successfully evaluate fruit for fermentations. This includes practical and sensory considerations (sugar types and concentrations, color, and volatile compounds), technical considerations (harvesting, preprocessing, juice extraction, adjustments, storage, and dosing), as well as product safety and identity (microbial factors and legal identity). Data concerning Florida peaches are included in a case study to evaluate the importance and relevance of each of these considerations.

1.1. Practical and Sensory Considerations When Using Fruit in Fermentation

Fruit is composed of many distinct components: sugars, volatiles, color compounds, bioactive molecules (antioxidants, etc.), organic acids, and non-fermentable solids. Knowing the anticipated juice yield (how much juice can be extracted from a given amount of fruit) and the concentrations of desirable properties are essential to brewing with fruit from both economic and processing standpoints. Both fruit and fruit juices are commonly assessed for their properties prior to addition in brewing formulations. Important sensory changes imparted to beverages by fruit include sweetness [5], acidity [6], color [7], and volatile characteristics [8]. The following sections provide additional detail on the variability in sugar-content profiles, volatile-content profiles, and colors of fruit juices when combined with a fermented product.

1.1.1. Sugars

Fruit juices contain high (8–20% wb) concentrations of sugars, typically sucrose, fructose, glucose, maltose, mannose, and maltotriose [5]. Sugar concentration can be measured via density measurement (commonly reported as specific gravity or °Brix), or by identifying each sugar individually via high-performance liquid chromatography (HPLC). The average Brix values for common fruit juices are listed by the United States Code of Federal Regulations (CFR) to provide a standard reference (21 CFR 101.30). Sugar is one of the major factors in overall flavor perception [9]; understanding the composition and concentration of sugars is part of identifying consumer preferences [10], as well as fermentability and economic advantages. The ability to perceive sweetness is strongly impacted by sugar composition, as shown in Table 1. Sucrose is used as the reference standard for sugar sweetness, against which other sugars are measured [11]. The quantity of each sugar also affects the perceived sweetness of the final product and should be taken into account when determining the impact of sugar on taste.

Throughout fermentation, yeast obtain energy by converting sugar into ethanol and carbon dioxide. However, some sugars are not fermentable by traditional beer yeast. The most-used yeast species in brewing fermentation are Saccharomyces spp., which can utilize sucrose, fructose, glucose, maltose, and maltotriose, in this preferential order, with some overlap [17]. Sugars and similar compounds unfermentable by Saccharomyces spp. include dextrins (hydrolyzed starch or maltodextrins), sugar alcohols [24], lactose [25], and beta-glucans [26]. Unfermentable components are often included in fermented products to either increase the perceived sweetness of the final product (e.g., lactose) or to increase the thickness and body and modify the mouthfeel (maltodextrin) [26,27]. As the fermentability of some sugars is dependent upon yeast strain, yeast selection becomes an important factor in maximizing sugar conversion [25]. By identifying the sugar profile of a fruit juice and the yeast strain of interest, a brewer can determine the types of sugars that will be fermented and the rates at which they will be metabolized.

1.1.2. Volatile Compounds

Flavor perception is a combination of both aroma (involving the orthonasal and retronasal olfactory systems) and taste [10]. Aroma is defined as a complex mixture of many volatile compounds whose composition and conglomeration are specific to a food [28]. A molecule can be considered volatile when it exhibits an appreciably low boiling point and can vaporize without additional energy (typically at standard temperature and pressure (STP)). These volatiles naturally form an equilibrium within their environment, which is an important characteristic when determining packaging [29], food matrix, and food identity [30]. To measure volatiles, gas chromatography is used.

Volatiles are categorized on the basis of perception and chemical structure. Those typically important for fermented beverages include (but are not limited to) acids, alcohols, aldehydes, esters, and ketones [31], some of which are Maillard products [32]. While the concentration of airborne volatiles is minuscule (parts per million (ppm) or parts per billion (ppb)), humans have developed the ability to detect these small concentrations due to their association with high-energy food sources [33,34]. Small changes in the concentrations of these compounds can greatly change the perceived flavor of a product [31]. There are other factors and components that can affect the impact volatiles have on food, such as pH, salinity, ethanol level, proteins, fats/oil, starches, and phenolic compounds [35].

Volatile compounds are either inherent to the raw material of a product, produced by microorganisms during fermentation as secondary metabolites, or are added in post-production. While many volatiles are desirable, some are considered defects or are only desirable at specific concentrations, such as diacetyl [36,37]. Volatiles produced during fermentation are strain-dependent, and the production of these compounds can be heavily influenced by multiple factors, including temperature, stress level, and the age of the yeast [16]. Additionally, the production of many volatile components requires precursor compounds (e.g., sugars, amino acids, and alcohols), which will control the compounds that are produced. Therefore, identifying the compounds and quantifying the concentrations of volatiles present is important in characterizing the effects a fruit juice or additive can have on a final fermented product. In past research, fruit additives have been added to fermentations and evaluated as volatile sources. Examples include cherry [38], peach [39], black mulberry [40], persimmon [41], and even vegetables, such as sweet potato [42]. There are many compounds included in a volatile analysis; however, some contribute more to the perceived aroma of a fruit. The most important peach volatile compounds have been described by El Hadi (2013), these being ethyl acetate, acetone, benzaldehyde, and acetaldehyde [43].

1.1.3. Color

Color in fruit juice is a function of various concentrations of natural compounds, some of which are inert, while others (such as anthocyanins, phenols, and carotenoids) have unique properties. Anthocyanins and carotenoids are pH- [44,45,46] and temperature-sensitive [47,48], while high amounts of phenols can affect the bitterness [49] and clarity of a product. Beyond the impact of individual compounds, food color alone is known to influence perceived flavor [50]. Colored fruit beverages appeared sweeter to panelists than non-colored beverages of the same base [51,52]. Stillman [53] also found that the color of a fruit-flavored beverage significantly influenced panelist identification of fruit flavor. The effects of color and color intensity can be interpreted as positive or negative traits for the consumer dependent upon the individual and product in question.

Color intensity is typically measured using a colorimeter or a UV–Vis spectrophotometer [54,55]. CIELAB or the L*a*b* color scale is the most commonly used color scale in the food industry [54]. All parameters of the L*a*b* color scale can have a negative or a positive value. The L* value correlates with the lightness or darkness of a sample, with a positive L* value being lighter and a negative L* value being darker. The a* parameter takes positive values as redder and negative values as greener, while the b* parameter takes positive values as yellow and negative values as blue. From these L* a* b* values, varying quantifications of color can be determined. The most common quantification, chroma (C*), is determined using Equation (1). This value represents the colorfulness of a sample and differentiates between this color and a grey color of the same lightness [54]. Additionally, the color change of a sample as a result of processing techniques and storage time can be determined as the total color difference ($\Delta E$), using Equation (2), as described by Pathare et al. [54].

(1)$C^{*}=\sqrt{a^2+b^2}$

(2)$\Delta E=\sqrt{\Delta L^2+\Delta a^2+\Delta b^2}$

where $\Delta L, \Delta a,$ and $\Delta b$ are the differences in color values.

This color system allows for universal translation and quantification of color, which can be used to determine ripeness and quality [56,57]. However, the color of the whole fruit and the resulting juice are often different [58,59]. Color values are known to vary based on individual fruits, the concentration of juice [60], effects of enzymatic [61] and non- enzymatic browning [62], and the degradation of pigments due to oxidative reactions over time [63]. Furthermore, pasteurization treatment can be used to mitigate effects on the overall color of fruit juice if browning reactions occur [64].

The major color-contributing compounds in most fruits are combinations of chlorophyll, anthocyanins, phenols, and carotenoids, with phenols and carotenoids being primarily responsible for the yellow/red color in peaches [65]. Research on wine [55,66] and beer fermentation [67] shows that the fermentation process can result in color loss via compound degradation and pH changes. Certain compounds, including polyphenols and carotenoid compounds, have been shown to decrease within the pH range 2.5–3.8 [46,68,69]. To determine the total amount of a color compound, samples are typically extracted in an appropriate solvent (e.g., methanol for polyphenols) and quantified in a spectrophotometer [70]. For the individual quantification of color compounds, HPLC can be used, and methods for extraction are dependent on the color compound of interest [71].

It is to be expected that adding colored juices, such as peach juice, at the start of fermentation will result in a lower color concentration compared to the original juice color, which may or may not be desirable. Therefore, the final color of a fermented fruit juice (whether fermented on its own or added as an adjunct to other fermented beverages, such as beer) will be dependent upon many factors and may not be representative of the raw material.

1.2. Technical Considerations

Now that the practical and sensory rationale for the addition of fruit to fermentations has been reviewed, it is pertinent to discuss technical steps that may be required. Some or all of the operations shown in Figure 1 are typically employed when fruits are used in commercial fermentations. For each step, a short background is given, followed by specific recommendations for brewers, and, finally, the steps taken in a sample peach ale preparation are detailed.

1.2.1. Harvesting/Reception

There are many factors that will cause variability in fruit sugar concentration, composition, and size. Important factors include cultivar, location, weather [72], techniques used for planting [73], irrigation [74], fertilization, date of harvest/storage conditions [75], and ripeness of fruit [76]. Fruits can often be categorized by their ripening patterns (climacteric, non-climacteric, or a mixture of the two), which correlate with ethylene production and respiration patterns [77]. Climacteric fruits (such as the peaches examined in this study) continue to ripen (yielding additional produced sugars) and soften post-harvesting [78], while non-climacteric fruits (such as strawberries) will degrade post-harvest without additional sugar or flavor production. The temperature used to store the fruits and juices will depend upon the fruit; however, many will be stored frozen, as textural considerations are negated once juiced.

1.2.2. Post-Harvest and Pre-Processing

Post-harvest operations are those conducted as soon as possible after harvest and are primarily designed for enzyme inactivation, microbial load reduction [79,80], breakdown of macromolecular components (such as starches or protein), as well as seed, skin, and debris removal [81,82]. As thermal processing can negatively affect antioxidant levels [79], phenolic contents [83], etc., producers often employ the processes with the lowest temperatures to accomplish their primary aims. While thermal processing is often used, chemical additions, such as sulfur dioxide [81], can also meet many of the same objectives.

1.2.3. Juice Extraction

The primary goal of juice extraction is to separate desirable compounds from fruit without capturing undesirable materials (typically seeds, skins, and sometimes fiber). Fruits are composed mostly of water (ranging from 75–95%) [84], and the desirable sugars and volatiles are largely water-soluble, while most fiber and seeds are not. Methods of juice extraction include hydraulic pressing, centrifugation, and enzymatic destruction of the solid matrix during the mashing stage of brewing. Most fruits require moderate filtration prior to further processing if a clarified product is desired. More information on considerations in juice processing can be found in the book Principles and Practices of Small- and Medium-Scale Fruit Juice Processing by Bates et al. [81].

1.2.4. Adjustments

After juice extraction is complete, physiochemical adjustments may be desired to control the properties of the final product. Examples of adjustments include enzyme addition (e.g., pectinase or amylase), pH adjustment, preservative addition, and oxidation enzyme inactivation. Pectinases and amylases are traditionally used on fruits to increase the total fermentable sugars [85,86]. Pectinases are used to break down pectic substances into simple sugars [87,88], which results in increased Brix and juice extraction, improved clarification, and reduced juice viscosity [7,89,90]. Furthermore, pectinase has also been shown to improve the aromatic profile of red dragon fruit juice followed by fermentation [91].

pH adjustment can be accomplished by acid addition. Common food acids used to adjust pH include ascorbic, citric, lactic, malic, and acetic acids. Both the type and amount of acid will affect the perception of sour taste [92,93,94]. For example, the acid profile of peaches includes malic, citric, oxalic, quinic, and shikimic acids [95,96]. The amount of acid in a product is quantified via titratable acidity (TA). Acids are also considered natural preservatives and have displayed antimicrobial activities [30]; therefore, adjustments in pH can assist in extending the shelf lives of products [79,81]. Furthermore, pH is closely related to the stability of bioactive compounds naturally found in fruits and fruit products [97]. It is important to note that the pH of a juice being added to a fermented product will affect the final product’s characteristics. These characteristics include but are not limited to sweetness perception [98], decrease in pH of the product [99,100,101], color [102], haziness [103], and volatility [104]. Therefore, pH and TA are important parameters to assess for consumer acceptability and the microbial safety of the final product.

Many fruits (including peaches) exhibit enzymatic browning or oxidation due to the presence of polyphenol oxidase (PPO) [105] and peroxidase (POD), amongst other potential degradation enzymes. To denature PPO, ascorbic acid or sulfur dioxide can be added [105], but the most effective method for controlling denaturation is heat treatment [81,82]. In order to reduce microbial load and assist the preservation of a product prior to storage and use, hurdle technologies can be applied [80].

For long-term storage, most fruit juices require stabilization via chemical preservatives, such as sulfiting agents. Sulfiting agents are known to assist with microbial stability, color retention, the prevention of non-oxidative browning, and mitigation of the effects of light degradation [106,107]. Other preservatives commonly used in fruit juices include sodium benzoate [108], potassium sorbate [109], and essential oils [110].

1.2.5. Storage

The logistics involved in the manufacturing of fruit juice often necessitate medium-term storage and transportation. Options for stabilizing fruit juice in preparation for and during storage include aseptic high-temperature–short-time processing, freezing, removal of water, or combinations thereof. For example, it is a widely practiced standard to store frozen concentrate at 68 to 72 °Brix and hold the concentrate at less than –20 °C [81]. Stored juices must conform to food regulations, as discussed in depth in Section 1.3.

1.2.6. Pasteurization

When used in fermentation, fruit juice may require a pasteurization step to reduce/eliminate microbial contamination, dependent upon many factors (including acidity, FDA regulations, etc.). Examples of pasteurization regimes include high-temperature–short-time (HTST), low-temperature–long-time (LTLT), high-temperature–long-time (HTLT), ultra-high-temperature (UHT), and high-pressure processing (HPP) regimes. Each method will affect volatile profile, color, and perceived flavor.

The exact method of pasteurization is dependent on the type of juice and on local regulations. After pasteurization, specific attention must be paid to the proper sterilization of equipment and storage vessels so as to not re-contaminate the product. Petruzzi outlines examples of pasteurization regimes for different types of fruit juices [80]. For example, commercial sterilization can be achieved for peaches using a HTLT treatment if the peaches are heated to 90 °C and held at that temperature for five minutes, then rapidly cooled to below 37 °C.

1.2.7. Dosing

The addition of a specific amount of fruit juice to a fermentation or final product blend is known as dosing. Both the amount of fruit or juice added as well as the timing of fruit addition will impact the final product. The point at which a fruit juice is added in the brewing process is primarily dependent on the purpose of the fruit addition—whether it be stylistic, aesthetically focused, or solely for flavor novelty. Points of addition include before primary fermentation, at the beginning of the fermentation, direct blending into the final product, or an intermediate point during the fermentation. For pre-fermentation additions, fruits can be added into the mash or the boil to provide a sugar source for yeast to metabolize and therefore fuel fermentation. Unfermentable sugars can be enzymatically broken down in the mash step, allowing the use of fibrous fruits (pumpkins [111]) and dense root vegetables (sweet potato [112] or beets [113]). Fruit juice that has already been heat-treated alone can also be added after the mash or boil stage and added directly into the fermentation broth prior to yeast addition (before primary fermentation). Fruit added at these stages will have little effect on the final beverage, other than providing sugar to fuel the fermentation. Any volatile compounds from these fruits are lost during the boil and fermentation stages. High-value fruits (such as berries) are rarely used at early stages, as most of the benefits of the addition (volatiles, sugars, and color) are lost.

Blending fruit into the final product will ensure that the volatile compounds of the fruit are not expelled during the brewing process and are instead more representative of the fruit juice [3]. This introduction of fruit at the end of fermentation will lower the ethanol content of the final product through dilution. While the primary fermentation is complete at this stage, there will likely be residual microorganisms present within the product. If the fermentation is not correctly stopped or the fermentation microbes are not properly removed upon completion, refermentation may occur. Undesirable refermentation can be mitigated through filtration, temperature, or chemical additions. If uncontrolled, refermentation may lead to sensory changes, higher ethanol contents, and over-pressurized packaging.

The addition of fruits during an active fermentation will induce a secondary fermentation that can be stopped through temperature manipulation, filtration, or chemical addition. This refermentation occurs due to the sudden increase in fermentable sugars from the fruit additions [8]. During this refermentation, some of the volatiles from the fruit will be extracted, thereby creating a blended flavor profile which can be used to provide the desired balance between modified alcohol concentration and additional flavors.

The dosing ratio for fermented beverages (and the point in the brewing process at which juice is added) will be dependent on the end-product goal. Ratio and addition time may influence the amount of ethanol in the final product through the contribution of sugars during fermentation or by dilution of the final product. In many instances, the cost of juice will be lower than the cost of producing a fermented beverage, therefore creating an incentive for more beverages that include fruit additives. However, many fermented products have legal identities that prohibit or limit ingredients.

1.3. Product Safety and Identity

1.3.1. Identity of Fermented Products

In the United States, beer is defined as a fermented beverage that “must be brewed from malt or from substitutes for malt”, including “rice, grain of any kind, bran, glucose, sugar and molasses” ((CFR) Part 25.15 subpart A). Furthermore, the beer must “contain one-half of one percent or more of alcohol by volume, brewed or produced from malt, wholly or in part, or from any substitute for malt.” (CFR Part 25.11). However, if fruit juice is added as a beer flavoring, the product formula might be subject to a formula submission and require approval from the United States Alcohol and Tobacco Tax and Trade Bureau [114] if the fruit/fruit juice of interest is not on the preapproved additions list, which can be found in the TTB ruling 2015–1 [114]. If the fruit in question is not on the preapproved list, then it is possible to request a determination from the TTB for approval of said ingredient(s). The exemption policy must include a “detailed description of the proposed process”; evidence that the ingredient to be used in the production of a fermented beverage, such as “beer”, is “generally recognized as a traditional ingredient”; and “an explanation of the effect of the proposed process on the production” of the final product [114]. For any alcoholic beverage, there are national mandatory labeling requirements that must be displayed on the label, as per 27 CFR 7.22, as well as state requirements.

Peaches are on the preapproved list of fruit additions and are therefore exempt from a formula submission as they have been used as ingredients in many ales [101,115,116], Berliner weisse [117], and Peche lambics [118]. With fruit flavors impacting the perceptions of beer styles, it is important to test batches of beer with the desired additives to ensure that the fruit impacts the end product in the desired manner.

Multiple studies have shown that varying concentrations of antioxidants and other desirable bioactive compounds derived from fruits can be preserved in beer. Fruits that have been used in this manner include grapes [119], cherries [100], goji berries [120], and persimmons [41]. Marketing of beer and highlighting of the fruit used is an additional value-added step which has the potential to appeal to consumers [121]. However, the regulations regarding labeling requirements are country- and state-dependent and must be adhered to. In the United States, bioactive benefits, such as antioxidant properties, cannot be added to the label without meeting strict criteria necessitating clinical studies and governmental approval (21 CFR 101.9).

1.3.2. Microbial Concerns

In the United States, a food substance is generally regarded as safe (GRAS) if the food has been deemed safe “…through experience based on common use in food prior to January 1, 1958…” (21 CFR 170.30 (c)(1)). Beer is considered to be a safe product, as numerous hurdles have to be overcome to reduce the risk of pathogenic contamination, such as low pH, low available oxygen, hop compound, and ethanol requirements. However, raw fruit juice has many properties that are ideal for the growth and proliferation of undesirable microorganisms, thus necessitating regulation by the Food and Drug Administration (FDA) and appropriate safety precautions. Traditionally, the fruit processor ensures the safety of the final product by following the FDA protocols and regulations. As fruit juice is added to beer, the beer’s intrinsic antimicrobial potential becomes proportionately compromised and safety must be taken into consideration [122]. Pathogenic organisms relevant to fruit juice include Escherichia coli O157:H7, Salmonella, and parasitic Cryptosporidium [123], along with spoilage organisms (molds, yeasts, and aciduric bacteria).

Fruit juices are regulated by the FDA under regulation 21 (CFR 120), which dictates that all fruit juices be produced using a hazard analysis and critical control point (HACCP) plan [124]. Under this federal regulation, specifically 21 CFR 101.17(g), “any juice or juice ingredient that is not processed to…attain a 5-log reduction…must bear a warning label” for facilities deemed “small” or “very small”. Subsequently, the “Juice HACCP Rule” (21 CFR Part 120) was developed for all other juice processors. This rule states that these “other juice processors are now required by the juice HACCP regulation to apply HACCP principles to their processing operations and to have…measures to achieve a 5-log reduction in pathogens”. Consequently, many commercially produced fruit juices are pasteurized to eradicate vegetative spoilage organisms and human pathogens [125].

2. Materials and Methods

2.1. Study Overview

The following case study details the processing and analysis of peaches that were juiced and added to a commercial Gose-style beer. The juice extraction process, pasteurization, fractionated sugar analysis, fermentability assessment, volatile profile, and color assessment of the resulting juice are outlined as part of a case study for adding peach juice to a fermented product. During this study, peach juice was processed (as depicted in Figure 2) and added to the commercial beer product during the active fermentation. The final beer was analyzed against two controls (the original peach juice and the finished beer without peach juice) for the parameters of sugar content, ethanol content, volatile profile, and color. The methodology described in this case study aims to provide an example of how the success of fruit additions can be predicted prior to inclusion in a recipe.

In May 2019, ~1000 kg of ripe ‘UFSun’ peaches were hand-harvested from a research orchard located at the University of Florida (UF) Plant Science and Research Education Unit (PSREU) in Citra, Florida. The peaches were stored in plastic harvesting bins in a cooling room at 4 °C with relative humidity between 80% and 90% for ~7 days. Prior to juicing, the peaches were rinsed and steamed in a hooded blanching unit in 10 kg batches with 115 °C pressurized steam for eight minutes, then rinsed with ambient (~20 °C) cooling water for three minutes before pitting. The peaches were transferred into a hydraulic piston press and juiced at 197.3 atm. The resulting juice was collected in sanitized food-grade plastic drums. Pectic enzyme (BSG HandCraft, Shakopee, MN, USA) was added to the peaches at 1.52 g per gallon of juice (as per the manufacturer’s directions). The juice was subjected to a thermal commercial sterilization process to ensure that no microbial life affected the juice product (sugar consumption, ethanol production, etc.). The process for peach juice sterilization [80] involved the heating of the juice to 90 °C, which temperature was held for five minutes, followed by rapid cooling to below 37 °C. The juice was aseptically transferred into sanitized 11.4 L (3 gal) food-grade buckets, hermetically sealed, and then stored at −40 °C in a food-grade freezer. A quantity of 83 gallons of the pasteurized peach juice was then added to 527 gallons of a commercial beer (16% dosing rate by volume).

2.2. Sugar Content and Compositional Sugar Analysis

To determine the sugar composition of the peach juice, target sugars (fructose, glucose, and sucrose) were quantified via high-performance liquid chromatography (HPLC). Peach juice samples (25 mL) were filtered with Double Rings 201–125 cm ashless filter paper. The °Brix was measured using an Anton Paar DMA 35 digital densitometer (Graz, Austria). Samples were further filtered through 0.45 µm filters into glass sample vials. The refractive index detector signal (T = 35 °C) was monitored in positive polarity. A sample volume of 5 µL was injected using an autosampler. Sugars were analyzed using a Restek ultra-amino column (5 µm, 250 × 4.6 mm) at 30 °C oven temperature with a 75% v/v mobile phase of aqueous acetonitrile at 1 mL/min. The total sample run time was 20 min (15 min run, 5 min post time). An external standard curve was used to determine the concentrations of each sugar in all samples.

2.3. Volatile Profile

Peach juice samples (taken and frozen post-pasteurization) and beer samples (taken and frozen post-fermentation) were thawed on the day of experimentation and vacuum-filtered through 70 mm (2.8 in) glass microfiber Whatman filter paper (GE Healthcare Life Sciences, cat. no. 1822–070) into a clean Büchner flask, then poured into 40 mL (1.4 fl oz) amber sample vials (performed in duplicate). After filtration, 3 µL of antifoam reagent (Trans-400, Bristol, WI, USA) was added to each vial and vortexed using a Fisher digital vortex mixer at an instrument speed of 2225 for 20 s. Carvone (125 ppb v/v) was used as the internal standard. Two microliters were added to each sample vial prior to analysis. Run conditions for gas chromatography/mass spectroscopy analysis (GC/MS) were as follows: An autosampler drew 5 mL samples from each amber vial and purged them internally with a nitrogen flow rate of 200 mL/min for 30 min. Volatile compounds were adsorbed by a Tenax^® trap and desorbed at 180 °C. These compounds were transferred in-line to the unit injection port of an Agilent gas chromatograph (Santa Clara, CA, USA) with a quadrupole mass spectrometer detector (5975C MSD). Separation was completed using a ZB-WAX plus column (30 m × 250 µm × 0.25 µm) (Phenomenex, Torrance, CA, USA). GC oven conditions included a split-mode run with a 20:1 ratio, an oven temperature of 45 °C held for 7 min then increased, first, to 150 °C at a rate of 3 °C/min, second, to 210 °C at a rate of 10 °C/min, and third, to 240 °C at a rate of 30 °C/min and held for 4 min. The flow rate of helium gas was 1 mL/min. Volatile peaks were identified using the 2011 NIST mass spectral library, and mass spectral and retention index data were compared. All identified volatile compounds were compared to carvone at 125 ppb v/v, as previously described. Equation (3) was used to determine the concentrations of each identified volatile compound per individual sample:

(3)$\frac{\mathrm{Volatile} \mathrm{Compound} \mathrm{Concentration} }{\mathrm{Total} \mathrm{Area} \mathrm{of} \mathrm{Volatile} \mathrm{Compound}}=\frac{\mathrm{Internal} \mathrm{Standard} \mathrm{Concentration} }{\mathrm{Total} \mathrm{Area} \mathrm{of} \mathrm{Internal} \mathrm{Standard}}$

2.4. Color

Samples of unfermented peach juice, beer with no peach juice, and beer with peach juice were filtered with Double Rings 201–125 cm ashless filter paper, then the colors of the juice and beer samples were analyzed via a colorimeter and reported as L*, a*, and b* values in triplicate readings.

2.5. Commercial Brewing Conditions

The commercial style selected for experimentation with peach juice was a Gose-style beer. The brewers performed a two-part fermentation, the first stage of fermentation involving lactic acid and performed with Lactobacillus cultures for souring. Once the lactic acid fermentation was complete, wort was transferred to a cylindroconical vessel and fermented with ale yeast at a fermentation temperature of 20 °C. After the lacto-fermentation, the starting gravity for the wort was 9.1 °Plato (°P). Prior to the addition of peach juice into the wort, the gravity was 1.8 °P. Approximately 321.8 L (85 gal) of peach juice was aseptically pumped into the fermentation vessel and mechanically circulated through the tank to mix with roughly 2044.12 L (540 gal) of beer. The terminal gravity one day after the peach juice was added was 1.6 °P, with a terminal pH of 3.26. The beer was then cold crashed with 2 °C water.

Samples were taken of the peach juice prior to addition, the beer before the peach addition (referred to as “non-peach beer” throughout this document), and the final product (referred to as “peach beer”), and each was used for analysis.

2.6. Statistics

The sugar contents in °Brix for all juice samples were analyzed using a single analysis of variance (ANOVA) test in Microsoft Excel (α = 0.05). Compositional analysis of sugars via HPLC was analyzed using average and standard deviation functions in Microsoft Excel to combine duplicates for comparison. Volatile compounds were analyzed with average and standard deviation functions in Microsoft Excel to combine duplicates for comparison. Volatile concentrations were calculated using Equation (3) in relation to concentration as compared to an internal standard, carvone. Concentrations were compared between juice types using the Microsoft Excel single-factor ANOVA function (α = 0.05). Color data were analyzed using single-factor ANOVA in Microsoft Excel to distinguish significant differences at α = 0.05 between the color parameters L*, a*, and b*.

3. Results and Discussion

3.1. Extraction of Peach Juice

The results of the juicing process are detailed in Table 2. The juice yield was 51% (mass of fruit/mass of juice). This measurement was incorporated into the assessment of the economic viability for this fruit inclusion. The variability between each batch of peaches can be influenced by techniques used for planting [73], fertilization, date of harvest/storage conditions [75], and ripeness of fruit [76]. Accepting that cultivar, location, and weather conditions can all affect juice yield parameters [72], the average yield was compared to values found in the literature and these were found to be similar (56% [126]). The peach juice had a pH of 3.5. Although pathogen growth is known to be inhibited at this level, the peach juice could not be considered stable due to the likely presence of spoilage microorganisms (naturally found on all fruit). Therefore, prior to storage, the peach juice was pasteurized and stored aseptically to prevent recontamination.

The peach juice had an overall Brix of 9.05°, which includes all soluble solids. The average Brix for unconcentrated peach juice, according to the CFR, is 10.5 (21 CFR 101.30). Similar to variability in yield, Brix can be affected by many parameters, including cultivar, location, the maturity of the fruit, and post-harvesting conditions [22]. Brix values for peach juice have been reported as low as 8 and as high as 18 [22,127]. When the juice was assessed for fermentable sugars via HPLC, it was found that sucrose, glucose, and fructose made up 7.45 °Brix (or 82%) of the soluble solids (Table 2). The highest of these sugar concentrations was sucrose (76.6%), followed by fructose (12.1%) and glucose (11.4%). These sugar compositions fall within typical rages for peaches, with sucrose being the most abundant sugar [128]. The remaining solids are composed of unfermentable sugars, dextrins, fiber, sugar alcohols, and non-sugar solids [24]. Sugar alcohols, such as sorbitol, raffinose, and galactinol, are known to fluctuate in post-harvest fruit [20]. These typically account for less than 10% of the total sugar composition [129]. If desired, enzymes (such as pectinase) can be used to convert some of the non-fermentable sugars into fermentable sugars. However, non-fermentable sugars are commonly added to increase the total perceived sweetness [115,130], increase body and mouthfeel [131,132], and/or affect the haziness/turbidity [133] of a beer. The product style and target market should be the determining factors regarding which sugars are desirable and which should be added post-fermentation. With the sugar profile of the fruit juice known, the brewer can determine the fermentability and amount of residual sugars that will be added to the product. If an HPLC is unavailable, a simple test for fermentable sugars present in media is ASBC WORT-5 [134], which uses a simple controlled fermentation to determine the amount of sugars usable by a particular yeast strain.

The peach juice used in this study was considered a poor candidate for addition prior to and at early points during active fermentation due to its low initial Brix value (10.3). Fruits with a Brix value of 15% or higher are traditionally added pre-fermentation, as less fruit is required to achieve acceptable ethanol levels [135]. Additionally, volatiles native to the peaches would be lost during the mashing/fermentation process, minimizing the effect of the juice additive on the perceived flavor of the beverage. Furthermore, peach juice used as an adjunct is not economically viable, as the average cost of fresh peaches is considerably higher than the cost of malted barley (USD 0.43/pound vs. USD 0.08/pound) [136,137]. This calculation does not account for the loss conversion efficiency of the juicing or milling process, making the actual costs higher.

The peach juice used in this case study was added near the end of the active fermentation, with 83 gal of juice added to 527 gal of beer (16%). The anticipated benefits of adding peach juice near the end of fermentation were the minimization of the degradation and loss of the volatile compounds native to the fruit. This is important to establish the fruit characteristics present and can be helpful in determining a target market. The peach beer saw an increase in Brix and pH and a decrease in total alcohol content proportional to the addition of juice. A dosing rate of 16% was determined experimentally based upon the internal sensory preferences of the commercial brewer. In industry practice, the ratio of fruit juice added to beer ranges enormously.

3.2. Volatile Chemical Profile

The volatile profiles of the peach juice, non-peach beer, and final product are shown in Table 3. The GC/MS volatile profiles identified 23 volatile compounds in the non-peach beer base. It should be noted that many foods have over 20 volatile compounds in their volatile profiles; however, very few volatiles in a food are used to create a flavor profile recognizable to humans [138]. Characteristic compounds are classified based on their highest factor over the human odor threshold [31]. The volatile profile of peach juice (a total of twelve compounds) indicated a high concentration of fruity/green odor compounds and nutty odors. These compounds are well-documented in the literature as being associated with peaches. Of the twelve compounds present, four of them (ethyl acetate, acetone, benzaldehyde, and acetaldehyde) are responsible for the characteristic aroma of peaches [43]. Volatile analysis indicated that, of these four compounds, three were naturally present in the non-peach beer, and, of these three, only one compound (benzaldehyde) was found in the peach juice and not in peach beer. Since benzaldehyde was not present in the non-peach beer, it is logical to assume that it was diluted to non-detectable levels in the combined final product. The combination of the peach juice and the non-peach beer increased the concentration of acetaldehyde and ethyl acetate but decreased the concentration of acetone. Overall, the concentrations did not substantially increase or decrease with the inclusion of peach juice. This impact is most likely due to the ratio of peach juice to non-peach beer being relatively low (16%).

As the peach juice was added near the end of fermentation (one day before crashing and filtration), some of the compounds associated with "green" or “unfinished” beer (2-octenol and hexanal) present in the peach juice were metabolized. These compounds were not detected within the final product (Table 3). Conversely, alcohol volatiles (i.e., isobutyl alcohol, isoamyl alcohol, 1- hexanol, 1-octanol, and phenylethyl alcohol) present in the non-peach beer decreased in concentration after the addition of peach juice due to dilution. Based on the volatile profiles of the peach juice and non-peach beer, it was predicted that the peach juice would have little impact (negative or positive) on this style of beer. This was due to many of the volatiles present in the non-peach beer overlapping with the characteristic volatiles of the peach juice. The observed effect in the final peach beer demonstrated that the peach addition created a final product indistinguishable from the original beer (with respect to volatiles).

3.3. Color and pH

Colorimeter reading results are displayed in Table 4. Measurements for lightness (L*) were not substantially different between the samples. The largest visual difference was the pink/red color of peach juice with an a* value of 11.79, compared to both beer samples, where a* was −0.47 for non-peach beer and 0.38 for peach beer. Non-peach beer had the highest b* value, 18.73, with peach beer having a b* value of 16.88 and peach juice having a value of 9.03. This indicates that the peach juice was significantly more red and somewhat less yellow than the two beers. All samples were significantly different (p < 0.05). The change in color from non-peach beer to peach beer due to the addition of peach juice ($\Delta E$) was 2.11. This represents a small change in color due to the addition of the juice and is visualized in the final column of Table 4.

The peaches used in this study were blanched prior to pressing, which would inactivate the enzyme polyphenol oxidase (PPO is typically present in peaches), helping to preserve a red-like color in the juice. This red color of the peach juice remained through the sterilization process; however, the color of the fruit juice addition was impacted by pH changes, particle dispersion, and the degradation of pigments due to oxidative reactions [63]. All of these factors can be expected to reduce the overall effect of fruit juice color on the color of the final product. The pH value of the peach juice was found to be 3.98, which is within the peach juice range reported in the literature [128]. For Gose beer, the pH range has been reported as 3.2–3.62 [141,142]. The final pH of the peach Gose beer used in this study was 3.26, which is within the expected range based on the interpolation of initial pH values and a 16% dosing ratio. As the peach juice was blended in a low-pH environment of 3.26, the pH-sensitive color-contributing compounds from the peach juice (phenols and carotenoids) had little effect on the peach beer, as shown in Table 4. Due to this environmental change in pH as well as dilution, the pink/red color was not retained in the final peach beer. Regardless of how much peach juice was added, the pH of the final product would not have exceeded the pH of the juice, thereby ensuring that the peach Gose beer was an uninhabitable environment for pathogenic microorganisms (pH < 4.6).

4. Conclusions

This paper reviewed the practical, technical, safety, and identity considerations of fruit additions in the brewing industry. A case study assessing the inclusion of peach juice in a commercial beer was used to highlight how each aspect may affect a final fermented product. The peach juice used in the study was evaluated for sugars (concentration and composition), economic viability, volatile profile and concentrations, color, and pH. The three predominant sugars in the analyzed peaches were sucrose, fructose, and glucose. While these sugars are fermentable, the other sugars which make up peaches, including sorbitol and inositol, are non-fermentable. Various methods of using both fermentable and non-fermentable sugars based on the characteristics for a desired final product were discussed. The inherent volatile profile of peaches was very similar to that of the chosen commercial beer, overlapping specifically in fruity and green aromas, resulting in little volatile differentiation. The importance of understanding the volatile profile similarities and differences (whether analyzed or researched) of a novel fruit additive versus a specific beer style was demonstrated. Knowing the volatile characteristics for a novel fruit addition and a specific beer style allows for theoretical compatibility analysis to ensure that the fruit additive has the desired effect on the final product. The economics of fruit addition will ultimately depend upon a cost–benefit analysis, the benefits being highly dependent upon how the fruit is processed and utilized. This will vary for all fruit and fresh-produce commodities. Even so, the analytical methodology presented in this paper can serve as an outline for how intrinsic attributes of fruits and their juices can be assessed and best utilized for success in brewing operations at all scales.

Author Contributions

Conceptualization, A.J.M., S.J.C. and A.S.; methodology, S.J.C., E.D. and A.B.R.; validation, S.J.C.; formal analysis, A.J.M., S.J.C., C.A.S., K.A.T.-W. and P.J.S.; investigation, A.J.M., S.J.C. and P.J.S.; resources, A.J.M., P.J.S., A.S., E.D. and A.B.R.; data curation, S.J.C., S.R.M. and A.J.M.; writing—original draft preparation, S.R.M., S.J.C. and A.J.M.; writing—review and editing, S.J.C., S.R.M., A.J.M. and K.A.T.-W.; visualization, K.A.T.-W., S.R.M. and S.J.C.; supervision, A.J.M., C.A.S. and A.S.; project administration, A.J.M. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

This study did not require ethical approval.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

This study was supported by the University of Florida’s Food Science and Human Nutrition Department, Horticultural Sciences Department, and the First Magnitude Brewing Company, who made donations of the resources necessary for the completion of the project.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Process flow diagram of processing treatments for incorporation of fruit into fermentation. Bulleted lists include examples of processing options but are not comprehensive.

Figure 2. Flow diagram for the processing of fruit used in this case study.

Table 1

Relative sweetness of various sugars and concentrations of corresponding sugars in fresh peaches and ale-style beer.

Sugar	Relative Sweetness	Typical Sugar Levels in Ale (g/kg)	Typical Sugar Levels in Peaches (g/kg)
Sucrose	1.00 1	0 2	61–98 3
Fructose	0.8–1.70 1	0 2	1.3–19.5 3
Glucose	0.6–0.75 1	0 2	6.3–28.7 3
Maltose	0.30–50 1	0 2	Ranges *
Galactose	0.30–0.35 1	0 2	0
Lactose	0.2–0.4 1	0–10 2	0
Mannose	0.60 1	0 2	0
Trehalose	0.45 1	Dependent on Plato 2	0 3
Sorbitol (sugar alcohol)	0.5–0.6 1	nd	1–7 3
Inositol (sugar alcohol)	0.5 1	nd	<1.1 3
Dextrins	Variable	0–50	Variable

Note: The relative sweetness values provided in the table are based on liquid samples. The properties of the food matrixes containing sugars have also been shown to impact the relative sweetness of products [12]. ¹ Sugar relative sweetness [5,13,14,15]. ² Beer [16,17,18]. ³ Peach [19,20,21,22,23]. * Ranges depend upon harvest and storage parameters.

Table 2

Average juice yield, pH, Brix, and compositional analysis of fermentable sugars from peaches pressed hydraulically at 200 bar.

Juice Yield	pH	Brix(°Brix)	Sucrose(g/L)	Glucose(g/L)	Fructose(g/L)
51% ± 8%	3.98 ± 0.07	9.05 ±1.06	56.01 ± 0.04	9.09 ± 0.03	9.58 ± 0.68

Standard deviations reported for 10 kg batches. ± reported as 1 standard deviation.

Table 3

Volatile compounds identified in plain Gose beer, Gose beer with peach juice, and in peach juice alone.

Compound	Odor Descriptors	Concentration (mg/L)
		Non-Peach Beer	Peach Juice	Peach Beer
Alcohols
Butanol	Mildly alcoholic 1	0.43	-	0.51
Propanol	Rubbing alcohol 2	3.97	-	4.01
4-Penten-1-ol		0.41		0.33
2-Methyl 2-propanol	Mothball 2	-	0.11	-
Isobutyl alcohol	Winey, fusel 1	23.5	-	19.8
Isoamyl alcohol	Whiskey, fusel oil 2	146.9	-	128.7
Pentanol	Fusel-like 2	3.97	-	4.01
1-Hexanol	Green, fruity, sweet alcohol 1	0.26	-	0.19
1-Octanol	Fresh, orange, rose 2	2.51	-	2.12
2-Octenol	Fresh, fatty, green, herbal 2	0.188	0.105	-
Phenylethyl alcohol	Floral, rose, dried rose, bready, sweet 2	2.66	-	2.12
Aldehydes
Acetaldehyde	Fruity 2	0.297	0.189	0.549
Benzaldehyde	Bitter almond 2	-	0.38	-
Pentanal	Fermented, winey, bready, fruity, nutty 1	-	0.13	-
Hexanal	Green, fatty, leafy, fruity, woody 2	0.99	1.25	-
Esters
Ethyl acetate	Fruity, pineapple, brandy-like 2	44.96	5.1	47.80
Isoamyl acetate	Sweet, winy 2	5.71	-	6.86
Ethyl lactate	Buttery, creamy, coconut, fruity 2	2.84	-	1.85
Ethyl butyrate	Fruity, pineapple 2	0.19	-	0.28
Ethyl hexanoate	Fruity, pineapple–banana, winy 2	1.42	-	2.48
2-Ethylhexanol		-	0.14	-
Ethyl octanoate	Fruity, floral, winy, apricot 2	3.36	-	3.95
2-Phenylethyl acetate	Rose, honey-like 2	0.67	-	0.75
Ethyl-9-decenoate	Waxy, sweet, fruity 1	-	-	0.25
Ethyl decanoate	Oily, cognac-/brandy-like 2	0.41	-	0.43
Methyl acetate	Fruity 2	-	0.09	-
Ketone
Acetone	Fruity, apple, pear, sweet 1	0.19	0.39	0.17
2,3-Butandedione	Buttery 2	-	0.1	-
Terpene
Camphor	Mothball 1	0.24	-	0.20
Linalool	Floral, fresh 2	4.73	-	3.95

n = 2 for each sample; a hyphen indicates that the compound was not detected for that sample. Odor attribute information is from [139] (¹) and [140] (²).

Table 4

Colorimeter data for non-peach beer, peach beer, and peach juice.

Sample	Chroma(C*)	L*	a*	b*	L*a*b* Color
Non-peach beer	18.72	88.47 A	−0.47 A	18.73 A	[Image omitted. Please see PDF.]
Peach beer	16.88	89.03 B	0.38 B	16.88 B	[Image omitted. Please see PDF.]
Peach juice	14.85	85.47 C	11.79 C	9.03 C	[Image omitted. Please see PDF.]

n = 3 for each sample; different letters in each column indicate significant differences (p < 0.05).

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