1 Introduction

Fruits, being short-lived, are consumed either as fresh or processed within a limited time frame to savour their delightful flavours. However, the sheer volume of production during peak seasons often exceeds direct consumption capacity, necessitating transportation to geographically distant areas [1]. To address this challenge and make nutritionally rich juice products available across the map, fruits are processed through a method known as "fruit juice concentration." In this process, a significant portion of the naturally occurring water content is physically extracted [2]. This concentration is essential for practical packaging, storage, transportation and preservation from an economic standpoint. Traditional techniques for juice concentration commonly utilize heat, a method that, while successful in lowering moisture content, may unintentionally result in flavour deterioration, nutrient depletion, and changes in colour and aroma. On the contrary, vacuum concentration offers a more delicate approach, preserving the unique nuances that define karonda juice. By working under reduced pressure, this method minimizes exposure to high temperatures, reducing the likelihood of thermal damage and ensuring the final product retains its authentic taste and nutritional qualities.

Karonda (Carissa carandas L.), also known as the 'Christ Thorn Tree,' belongs to the Apocynaceae family. The fruits of karonda are characterized by their sour and astringent taste, ranging from acidic to sweet, accompanied by a distinctive aroma. Despite their unique qualities, Karonda fruits are not widely consumed fresh due to their high pectin content and latex exudation. In the processing industry, ripe karonda fruits find utility in the preparation of preserves. These fruits are noteworthy for their richness in iron (150 mg 100 g−1) and vitamin C (17.94 mg 100 g−1) Consequently, they possess antiscorbutic properties, making them beneficial in preventing anaemia, especially as the ascorbic acid content enhances the bioavailability of iron [3]. Karonda is also known for its reported efficacy in alleviating stomach-aches and acting as an anthelmintic [4]. In traditional medicine such as Ayurveda, unripe karonda fruits are employed for their astringent, appetizer, antipyretic, and antidiabetic properties. Additionally, root extracts are used to address conditions like lumbago, chest complaints, and venereal diseases.

Karonda fruits, when ripe, can be enjoyed as a dessert or utilized in the preparation of various processed products, including candy, jelly, squash, and chutney. Despite its versatility and nutritional benefits, the storage life of karonda is limited due to its soft flesh and high moisture content. Vacuum concentration emerges as a pragmatic solution to meet the rising consumer demand for convenient and less bulky and natural products. In response to the growing preference for beverages devoid of artificial additives and preservatives among health-conscious consumers, the vacuum concentration of karonda juice aligns seamlessly with this trend. This method offers a means to deliver a pure and unadulterated product.

The primary goal of the current study is to optimize process variables, temperature and time, in the vacuum concentration process of karonda juice. By focusing on these key factors, the research aims to enhance the efficiency and effectiveness of the concentration process, ensuring the production of a high-quality, natural beverage that resonates with the preferences of health-conscious consumers.

2 Material and methods

2.1 Preparation of samples

Karonda var. Konkan bold was procured from the fruit orchard of Regional Horticulture Research and Extension Centre (RHREC), University of Horticultural Sciences Campus, Bengaluru, Karnataka. Fully matured and ripe fruits were harvested that are of uniform size, shape and free of visible damages (DUS guidelines). The collection of karonda fruit is complied with the DUS guidelines. Fruits were washed with running tap water and the seeds were removed manually. After removing the seeds, the remaining fruit portion was hand crushed and used for further processing.

2.2 Karonda juice concentrating process

The concentration process of 250 mL Karonda juice commenced by placing it in a round-bottomed flask and subjecting it to a rotary vacuum evaporator (HS-2005 V, Hahnshin Scientific, Korea). This process entailed varying the heating temperature of the water bath across three settings (45, 50, and 55 °C) and adjusting the evaporation time to three different duration (90, 150 and 210 min). Subsequently, karonda juice within the flask was immersed in a water bath to facilitate the vacuum concentration process. The selection of maximum and minimum heating temperatures, as well as evaporation times, was based on preliminary experimentation to optimize the concentration process systematically.

2.3 Experimental design

Response surface methodology (RSM) in conjunction with central composite design (CCD), was employed to optimize the process parameters i.e., concentration temperature and evaporation time by generating a set of experimental runs. A calculation of experimental runs and optimum yield was performed using SAS version 9.3 statistical software. A CCD design approach was adopted incorporating two independent factors (Table 1) and seven responses (TSS, moisture (%), ascorbic acid content, TPC, total antioxidant activity (TAA), total anthocyanin content (TAC) and iron content). Response values were analysed by fitting the data in a second order polynomial model. The generalized second order polynomial model proposed for predicting response variables is given as,

$$ {{y}}\, = \,{{b}}_{0} \, + \,{{b}}_{{{1}}} {{X}}_{{{1}}} \, + \,{{b}}_{{{2}}} {{X}}_{{{2}}} \, + \,{{b}}_{{{1}}} {{b}}_{{{2}}} {{X}}_{{{1}}} {{X}}_{{{2}}} \, + \,{{b}}_{{{1}}} ^{{{2}}} {{X}}_{{{1}}} ^{{{2}}} \, + \,{{b}}_{{{2}}} ^{{{2}}} {{X}}_{{{2}}} ^{{{2}}}$$
Table 1 Independent variables and their coded and actual values used for optimisation
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In this equation, y represents the dependent variable (the estimated response), X1 represents concentration temperature and X2 represents evaporation time. Coefficients of the polynomial were represented by b0 (constant term), b1 and b2 (interactive coefficients). Model adequacy was tested by ANOVA. The fitness of the models was further affirmed based on statistical parameters such as coefficient of determination (R2), F test value and lack of fit. Interaction effects of independent variables were pictorially represented by 3-D response surface graphs for better depiction of results.

3 Physicochemical parameters

3.1 TSS (˚B)

TSS of samples were determined by digital refractometer at room temperature (28 ± 2˚C) (Erma Japan). It was expressed in °B.

3.2 Moisture content (MC) (%)

The moisture content of the sample was determined by using an electronic moisture analyser (Sartorius MA 35) and it was expressed in per cent.

3.3 Ascorbic acid content (mg 100 g−1)

Ascorbic acid content of concentrated samples was determined by modified method using 2, 6- dichlorophenol indophenol sodium salt described by [5]. The results are expressed as mg ascorbic acid equivalent per 100 g fresh weight using a standard curve of L-Ascorbic acid.

$$ {\text{Ascorbic acid }}\left( {{\text{mg 1}}00{\text{g}}^{{ - {{1}}}} } \right) = \frac{{{\text{Titre value }} \times {\text{dye factor}} \times {\text{ volume made up}} \times 100}}{{{\text{Weight of sample }} \times {\text{ aliquot of sample}}}} $$

3.4 Total phenol content (TPC)

TPC was determined by the spectrophotometric method, using Folin Ciocalteu Reagent (FCR) [6], and expressed in mg gallic acid equivalents in 100 g of fresh samples using following formula,

$$ {\text{Total phenol content }}\left( {{\text{mg GAE 1}}00{\text{g}}^{{ - {\text{1}}}} {\text{FW}}} \right) = \frac{{{\text{OD}}_{{{\text{7}}00{\text{nm}}}} \times {\text{ Std}}.{\text{ value }}\left( {\mu {\text{g OD}}^{{ - {\text{1}}}} } \right) \times {\text{ Volume made up }}\left( {{\text{5}}0{\text{ mL}}} \right) \times {{ 1}}00}}{{{\text{Assay volume }}\left( {{\text{mL}}} \right) \times {\text{ Weight of sample }}\left( {\text{g}} \right) \times {{ 1}}00}} $$

3.5 Total antioxidant activity (TAA) (FRAP index) (mg AAE 100 g−1)

The antioxidant activity of the concentrated juice was determined using ferric reducing antioxidant potential (FRAP) assay and expressed in ascorbic acid equivalents (AAE) in mg 100 g−1 [7].

$$ {\text{TAA }}\left( {{\text{mg AAE 1}}00{\text{g}}^{{ - {{1}}}} } \right) = \frac{{{\text{Standard value }} \times {\text{ Total volume }} \times {{ 1}}00}}{{{\text{Assay volume }}\left( {{\text{mL}}} \right)~ \times {\text{ Weight of sample }}\left( {\text{g}} \right) \times {{1}}000}} $$

3.6 Total anthocyanin content (TAC) (mg 100 g.−1)

Anthocyanin estimation was done as per the procedure of [8] and expressed as mg per 100 g.

$$ {\text{Total OD 1}}00{\text{g}}^{{ - {{1}}}} = \frac{{{\text{OD }} \times {\text{ Volume made up }}\left( {{\text{mL}}} \right)}}{{{\text{Weight of sample }}\left( {\text{g}} \right)}} \times 100 $$
$$ {\text{TAC }}\left( {{\text{mg 1}}00{\text{g}}^{{ - {\text{1}}}} } \right) = \frac{{{\text{Total OD 1}}00{\text{g}}^{{ - {{1}}}} }}{{{{87}}.{{3}}}} $$

3.7 Iron content (mg 100 g−1)

Iron content of karonda vacuum concentrated juice was estimated as per the procedure given in AOAC [9] using atomic emission or adsorption spectrophotometer. Iron content was expressed as mg per 100 g.

3.8 GC–MS profiling

The hexane extract of vacuum concentrated karonda juice was analysed using GC–MS employing a Shimadzu QP2020 series gas chromatograph with an SH-Rxi-5Sil column. Helium served as the carrier gas at a constant flow rate of 1.20 mm min−1 under 68.3 kPa pressure, with an injection volume of 1.0 μL and a split ratio of 1:50. Detection was conducted using a flame ionization detector at 320 ˚C. The temperature program for the oven included initial holding at 50 ℃ for 2 min, followed by ramping up to 220 ℃ at 10 °C per minute, and maintenance at 310 ℃ for 5 min at a ramp rate of 15 °C per minute. All samples were analyzed and results documented by comparing the mass spectrum with a library through computer analysis.

4 Statistical analysis

Each experimental run was triplicated. Combined ANOVA processing by SAS version 9.3 statistical software. The optimal concentrate parameter of karonda juice predicted with significance level below 5%.

5 Results and discussion

The observed values of response variables are given in Table 2. The generalized second-order polynomial model proposed for predicting response variables were generated. The models were compared based on their coefficient of determination (R2), adjusted coefficient of determination (R2 -adj), predicted coefficient of determination (R2 -pred), and the probability (p) of lack of fit.

Table 2 Central composite design with experimental and predicted values for TSS, MC, Ascorbic acid, TPC, antioxidant activity, TAC and Iron
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5.1 Effect of vacuum concentration process parameters on total soluble solids and moisture content

The ANOVA results from Table 3 indicate that the RSM model's effect is not significant for both TSS and MC, suggesting that the response surface model adequately explain the variability in the experiment, yet indicating its fit is inadequate. The RSM equation describing the effects of process variables on total soluble solids and moisture content of vacuum concentrated karonda juice in terms of actual levels of variables is given as:

$$ {{Y}}_{{({{TSS}})}} = {{ 81}}.{{43 }} + {{ 67}}.{{7}}0{{X}}_{{{1}}} {-}{{ 2}}.{{34X}}_{{{2}}} {-}{{ 6}}.0{{1X}}_{{{1}}} {{X}}_{{{2}}} + {{ 34}}.{{39X}}_{{{1}}} ^{{{2}}} {-}{{ 18}}.{{97X}}_{{{2}}} ^{{{2}}} $$
$$ {{Y}}_{{({{MC}})}} = {{ }} - {{17}}.{{96 }} - {{174}}.{{13X}}_{{{1}}} + {{ 51}}.{{94X}}_{{{2}}} + {{84}}.{{93X}}_{{{1}}} {{X}}_{{{2}}} - {{1}}0{{2}}.{{75X}}_{{{1}}} ^{{{2}}} + {{ 8}}.{{93X}}_{{{2}}} ^{{{2}}} $$
Table 3 ANOVA of the second order polynomial model for the various responses for vacuum concentration of karonda juice
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Figure 1a and b illustrates the 3D relationships among concentration, temperature, and evaporation time on TSS and moisture content, respectively. For TSS, higher temperature levels positively impacted TSS, while prolonged evaporation time led to a decrease, exacerbated by the interaction between temperature and time. Conversely, moisture content exhibited a negative correlation with temperature and time. Notably, TSS in vacuum-concentrated karonda juice at 50℃ for 150 min was 28°B whereas when concentrated at 55℃ for 150 min resulted in highest TSS i.e., 89°B, aligning with findings in litchi juice by [10], indicating increased TSS with temperature. The decrease in moisture content alongside increased TSS suggests moisture evaporation during concentration. The profound effect of concentration time on moisture content, rather than temperature, is highlighted, possibly due to water's viscosity dependency on temperature. [11] similarly observed these dynamics in pineapple juice concentration.

Fig. 1
figure 1

Response surface for a TSS (°B), b MC (%), c Ascorbic acid (100−1 mg), d TPC (GAE 100−1 mg), e TAA (AAE 100−1 mg), f TAC (100−1 mg) and g Iron (100−1 mg) of concentrated karonda juice as a function of (X1) temperature (45-55 °C) and (X2) time (90–210 min.)

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5.2 Effect of vacuum concentration process parameters on ascorbic acid

The ANOVA analysis (Table 3) affirms a well-fitted model for ascorbic acid, with a significant model p-value of 0.033, indicating a strong fit. Additionally, the model displays promising predictive capabilities, boasting an impressive R2 value of 0.91 and an adjusted R2 of 0.79. The RSM equation describing the effects of process variables on ascorbic acid content (Y) of vacuum concentrated karonda juice in terms of actual levels of variables is given as:

$$ {{Y}}_{{({\text{ascorbic acid content}})}} = {{ 12}}.{{5}}0{{ }} + {{ 1}}.{{25X}}_{{{1}}} + {{ 1}}.{{2}}0{{X}}_{{{2}}} {-}{{ }}0.{{42X}}_{{{1}}} {{X}}_{{{2}}} + {{ }}0.{{87X}}_{{{1}}} ^{{{2}}} {-}{{ 5}}.{{97X}}_{{{2}}} ^{{{2}}} $$

Figure 1c illustrates the three-dimensional relationships among temperature, and time on ascorbic acid, as per the generated model. Notably, the linear model showcases a positive effect, resulting in an increase in ascorbic acid levels with higher temperatures and time. However, the quadratic model of concentration and time, along with the interactive effects of concentration, temperature, and evaporation time, demonstrate a notable negative effect, leading to a decrease in ascorbic acid. Interestingly, the quadratic model for temperature also shows a significant effect on the concentration process. Across various experimental combinations, a wide range of ascorbic acid concentrations, ranging from 4.52 to 14.66 mg 100 g−1, was observed. The minimum value of 4.52 mg 100 g−1 was obtained with concentration temperature of 45˚C and concentration duration of 90 min. Notably, the maximum value (14.66 mg 100 g−1) was attained with a concentration temperature of 55 °C and a concentration duration of 150 min, likely attributable to water removal during the concentration process.

5.3 Effect of vacuum concentration process parameters on TPC

Based on the proposed model, the linear and quadratic effects of concentration, temperature, evaporation time, and their interactive effect had a significant impact on TPC. The analysis (Table 3) revealed a model p-value of 0.032, indicating a well-fitted model. Moreover, the model demonstrated strong predictive abilities with an R2 of 0.91 and an adjusted R2 of 0.79. The RSM equation describing the effects of process variables on total phenolic content (Y) of vacuum concentrated karonda juice in terms of actual levels of variables is given as:

$$ {{Y}}_{{({{TPC}})}} \, = \,{{548}}.0{{7}}\, + \,{{5}}0{{9}}.{{59X}}_{{{1}}} {-}{{93}}.{{82X}}_{{{2}}} {-}{{49}}.{{13X}}_{{{1}}} {{X}}_{{{2}}} \, + \,{{282}}.{{67X}}_{{{1}}} ^{{{2}}} {-}{{246}}.0{{8X}}_{{{2}}} ^{{{2}}} $$

Figure 1d illustrates the three-dimensional relationships among concentration, temperature, and time on TPC, as per the generated model. TPC of concentrated samples fell within the range of 119.58 to 386.97 mg GAE 100 g−1. It was observed that at lower temperatures (45℃) and times (90 min), the concentrated TPC was lower (119.58 mg GAE 100 g−1). Notably, when karonda juice was concentrated at 55 °C for 150 min, the highest amount of TPC was recorded, reaching 386.97 mg GAE 100 g−1. This increase can be attributed to the evaporation of water during processing, resulting in a smaller diminution of bioactive compounds, decreased activity of polyphenoloxidase, concentration of polyphenols, and stabilization of anthocyanins. However, with a duration increase to 210 min, there was a decrease in TPC, likely due to the degradation of bioactive compounds when exposed for longer duration at higher temperatures, as observed in pomegranate by [12] and [13] in pomegranate juice concentrate.

5.4 Effect of vacuum concentration process parameters on TAA

The Ferric Reducing Antioxidant Potential (FRAP) assay evaluates sample antioxidant activity and can indicate phenolic content. Results of ANOVA indicate a well-fitted model for TAA (p-value: 0.042), with strong predictive ability (R2: 0.90, adjusted R2: 0.76). The RSM equation describing the effects of process variables on total antioxidant activity (Y) of vacuum concentrated karonda juice in terms of actual levels of variables is given as:

$$ {{Y}}_{{({{TAA}})}} \, = \,{{486}}.{{61}}\, + \,{{49}}0.{{96X}}_{{{1}}} {-}{{98}}.0{{1X}}_{{{2}}} {-}{{64}}.{{52X}}_{{{1}}} {{X}}_{{{2}}} \, + \,{{275}}.{{3}}0{{X}}_{{{1}}} ^{{{2}}} {-}{{2}}0{{3}}.{{97X}}_{{{2}}} ^{{{2}}} $$

Figure 1e illustrates the impact of concentration, temperature, and time on TAA. Linear and quadratic effects of concentration and temperature positively influence activity, contributing to the observed peak activity at 55 °C for 150 min. Conversely, negative effects, such as those from prolonged evaporation time or temperature–time interaction, result in decreased activity, as evidenced by the decline to 196.61 mg AAE 100 g−1 at 210 min, likely due to phenolic compounds degradation. Understanding these positive and negative effects is crucial for optimizing conditions to maximize total antioxidant activity in vacuum-concentrated juice, in line with the findings of [14] in vacuum-concentrated pomelo juice.

5.5 Effect of vacuum concentration process parameters on TAC

Results of ANOVA (Table 3) indicate that the RSM model effect is not significant, with a p-value of 0.5265, suggesting adequate explanation of variability in the experiment, supported by a maximum R2 value of 0.97. The RSM equation describing the effects of process variables on anthocyanin content (Y) of vacuum concentrated karonda juice in terms of actual levels of variables is given as:

$$ {{Y}}_{{({{TAC}})}} \, = \,{{41}}.{{23}}{-}{{165}}.{{83X}}_{{{1}}} {-}{{4}}.{{86X}}_{{{2}}} \, + \,{{11}}.{{8}}0{{X}}_{{{1}}} {{X}}_{{{2}}} {-}{{158}}.{{7}}0{{X}}_{{{1}}} ^{{{2}}} {-}{{69}}.{{46X}}_{{{2}}} ^{{{2}}} $$

Figure 1f illustrates the 3D relationship between concentration, temperature, and time on anthocyanin content, revealing negative linear and quadratic effects of these factors, yet a significant positive interactive effect between temperature and time. The anthocyanin content of vacuum-concentrated karonda juice was ranging from 11.36 mg 100 g−1 at temperature of 50℃ for 90 min to 129.34 mg 100 g−1, peaking at 55 °C for 150 min, likely due to low temperature and rapid mass transfer via the vacuum process. These findings align with [15] work on concentrated pomegranate juice, though a significant decrease in anthocyanin content occurred after exposure to 55 °C for 150 min, possibly due to pigment polymerization, consistent with [16] findings in beets.

5.6 Effect of vacuum concentration process parameters on iron content

From the results of ANOVA presented in Table 3, the RSM model effect is significant (p < 0.005) i.e., 0.025, which implies that the response surface model has explained adequately the variability present in the experiment conducted and max R2 value is 0.99, which ensures RSM fit is adequate. The RSM equation describing the effects of process variables on iron content (Y) of vacuum concentrated karonda juice in terms of actual levels of variables is given as:

$$ {{Y}}_{{({{iron content}})}} \, = \,{{217}}.{{2}}0{-}{{2}}0{{9}}.0{{7X}}_{{{1}}} {-}{{37}}.{{14X}}_{{{2}}} {-}{{17}}.{{99X}}_{{{1}}} {{X}}_{{{2}}} \, + \,{{119}}.{{18X}}_{{{1}}} ^{{{2}}} {-}{{99}}.{{78X}}_{{{2}}} ^{{{2}}}$$

Iron content was negatively affected by the concentration time and positively affected by concentration temperature in the linear model, while temperature was positively affected and time was negatively affected in the quadratic model. For interactive effects concentration temperature and time had negatively affected the iron content. Figure 1g shows the 3D graph for the effect of concentration temperature and time on iron content based on generated model.

The values of treatment combination of temperature for vacuum concentration was 45, 50 and 55 ℃, time was 90, 150 and 210 min and the corresponding iron content range happens to be 45.65 to 150.54 mg 100 g−1 (Table 2). As concentration temperature was increased to 55℃ and duration to 150 min, the iron content increased to 150.54 mg 100 g−1, the moisture content decreased thereby increasing the concentration of iron content in the concentrated samples.

5.7 GC–MS analysis of fresh karonda juice and vacuum concentrated karonda juice

The GC–MS chromatograms comparing fresh juice with juice concentrated at 55 °C for 150 min are depicted in Fig. 2a and b, respectively. From the total compounds, only the five major volatile compounds present in vacuum-concentrated karonda juice are listed in Table 4. Among these, the predominant volatile compound identified in both fresh and vacuum-concentrated juice was hydrocarbon tetrahydrofuran, consistent with findings by Pino et al. [17] on Karanda fruit. Tetrahydrofuran is recognized for its antioxidant properties against oxidative stress-related ailments, as noted in studies by Fujimoto et al. [18] and Vo et al. [19]. Vacuum concentration at 55 °C for 90 min revealed oxirane (15.42%) as a major volatile compound, known for its reported anticancer, anti-inflammatory, immunosuppressive, and antitumor activities by [20]. Additionally, the second highest volatile compound identified was 3-Hydroxy-3-methylvaleric acid (9.04%), which aids in muscle growth promotion and reduces muscle breakdown. Conversely, vacuum concentration at 45 °C for 150 min yielded acetoacetate (24.32%) as the major volatile compound, known for its neuronal cell protection from oxidative glutamate according to [21]. The second highest volatile compound identified was sotolone (11.67%), recognized for its anti-virulence properties, corroborating findings by [22].

Fig. 2
figure 2

GC–MS Chromatograms of a fresh karonda juice and b vacuum concentrated karonda juice at 55 °C for 150 min

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Table 4 Major volatile compounds profile (GC–MS) in fresh and vacuum concentrated karonda juice
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6 Conclusion

Present study investigated the effects of concentration temperature and evaporation time on TSS, moisture content, ascorbic acid, TPC, total antioxidant activity, TAC, iron content and GC–MS profile of volatile compounds of karonda juice concentrate. Temperature and time showed significant changes in quality parameters of concentrated samples. In conclusion, the vacuum concentration technique applied to karonda juice emerges as a promising and efficient method for enhancing its overall quality. Through meticulous experimentation and analysis, this research has demonstrated the significant impact of vacuum concentration at 55 ℃ for 150 min on the concentration of valuable bioactive compounds. The concentrated karonda extract showcased enhanced antioxidant properties, indicative of its potential for diverse applications in the food and beverage industry.