Introduction

Citrus australasica, also known as “finger lime” or more colloquially as “citrus caviar”, is a unique citrus species native to the eastern coast of Australia. C. australasica grows naturally in subtropical rainforests and coastal regions and has a visual appearance comparable to an adult human finger. They are often referred to as citrus caviar because they bear globular juice vesicles, also referred to as pearls, that have a popping texture like caviar, but with a more pleasant and visually striking appearance. There has been a rising global demand for C. australasica, because of its culinary potential as a luxury food ingredient (Batat [4]), which has prompted a multitude of research pathways. Their luxurious status is often attributed to their caviar-like sensory properties, labour-intensive cultivation process, and low supply.

Finger limes are now cultivated globally, which is a benefit for increasing their market value, but an obstacle for Indigenous Australians who are the targets of biopiracy (Robinson and Raven [26]). Australia predicted the finger lime industry was worth AUD 3.1 million in 2021 and farm-gate prices for whole limes can be up to AUD 80/kg (Richmond et al. [25]). The University of Florida reported the market value of the fruit at USD 30 to 50 per pound in 2018 (Singh et al. [30]). These values are very high in comparison to commonly eaten citrus such as lemon and limes which can drop to lower than USD 1 per pound when supply is high (Blare et al. [5]). Since the earliest report on finger limes in 1915 (Swingle [34]), studies on their chemical composition have been published by researchers in North America (Adhikari et al. [1]), Europe (Cozzolino et al. [9]), and Asia (Chuenwarin et al. [8]).

Further reports followed on the flavoromic, volatilomic, and metabolomic profiles (Qi et al. [23]), with the addition of some noteworthy studies exploring toxicity in animal models (Cáceres-Vélez et al. [6]) and novel packaging technologies (Nastasi et al. [20]). However, the key physical sensory attributes of finger limes such as colour and texture have not been reported, despite there being over 65 different varieties available in Australia with a wide range of phenotypes (Sheryl [29]). Instead, researchers have focused on volatile differences between species, to characterise different chemotypes (Delort et al. [11]).

Finger limes are a highly desirable Australian native bushfood because of the unique popping nature of the pearls which can deliver a powerful burst of citrus flavour and a unique sensory experience (Hay et al. [15]). Hence, the physical and mechanical properties of finger limes, and the differences between varieties requires further research. By benchmarking finger lime varieties based on their colour and the mechanical properties of their pearls, grading systems can be implemented to assist in the identification of varieties with higher economic value. Similar to the caviar industry, pearl size, colour, and texture are considered key sensory attributes before flavour and smell (Baker et al. [3]; Vilgis [36]). Hence, similar methods for the physical assessment of caviar can be applied to finger lime pearls using texture analysis instruments (Vilgis [36]).

The present study aims to profile the colour, physical, and mechanical properties of three very different phenotypes of finger lime varieties popular in the Australian market (Fig. 1). While there are over 70 Australian commercial growers, and 65 varieties available, the three varieties focused on in the present study can be regularly sourced from throughout the Australian domestic supply chain. ‘Red Champagne’ is known for its ruby-red skin and red-coloured pearls, which is attributed to its higher content of anthocyanins (Johnson et al. [17]). ‘Emerald’ has dark green/black skin with vibrant green pearls and has been reported to have a stronger citrus smell and flavour compared to other varieties (Delort et al. [11]). Lastly, ‘Chartreuse’ is well recognised for its pale green pearls and bright green skin and a taste that presents a fine balance between sweet and acidic.

Fig. 1
figure 1

Digital images of three-finger lime varieties commonly sold in the Australian fresh produce market. Images taken and provided by Keely Rose Perry (author of this study)

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It is expected that the finger lime pearls analysed in this investigation will vary in their physical properties such as wall thickness, area, volume, and diameter. Furthermore, their bursting force, and strain or per cent deformation will provide further insight into their sensory differences from a physical point of view. Using the information collected in this study, suggestions for a grading system are proposed which can benefit the wider finger lime industry. Furthermore, the physical parameters reported in this study will assist in the future benchmarking of pearls from different finger lime pearls and in determining their economic value.

Materials and Methods

Fruit collection conditions and analysis time frame

Finger limes (500 g per phenotype) were harvested from Green Valley Fingerlimes (Peachester, Qld, Australia; X: -26.86, Y: 152.90) by the authors of this study. Samples were harvested on the 17th of January 2024 in what was considered the early season and wet summer months. Individual fruits were picked from the top, middle, and bottom of C. australasica plants. The meteorological parameters were as follows; Time: 10:00 am AEST, Temperature: 28 ℃, Dew Point: 23 ℃, Humidity: 74 %, Visual: minimal clouds. Samples were stored in a cardboard fruit box for two hours while transported to the University of Queensland and analysed within three hours to ensure freshness. Pearls were extracted from 10 biological replicates per variety.

Appearance and colour properties of the fruit and pearls

Finger limes and their pearls were photographed using a 48-megapixel camera in a photo box with illumination from two 63 LED light bars (12,000 – 13,000 LM). The colour properties (L*, a*, and b*) of the pearls were recorded using a FRU Precise Colour Reader (ShenZhen Wave Optoelectronics Technology Co., Ltd.) and averaged (n = 12). Approximately 300 pearls were placed in a 35 x 35 mm petri dish for colour analysis to ensure an even colour distribution, and the measurements were taken at random points. Chroma and Hue were determined according to Eqs. (1) and (2) respectively:

$$Chroma= \sqrt{{a}^{*2}+{b}^{*2}}$$
(1)
$$Hue= {tan}^{-1}\left(\frac{{b}^{*}}{{a}^{*}}\right)$$
(2)

Mechanical properties

The mechanical properties of fruit can be investigated via texture analysis instruments to determine quality parameters (Yang et al. [39]). The bursting strength and strain percentage were determined using a TA.XTplus (Stable Micro Systems, Godalming, England, United Kingdom) equipped with a TA5 Cylinder (12.7 mm D, 35 mm L) testing probe (Brookfield Engineering, Middleboro, Massachusetts, United States). The texture analyser setting was as follows; Mode: Compression, Option: Return to Start, Pre-Test Speed: 1.0 mms-1, Test Speed: 8.0 mms-1, Post-Test Speed: 10.0 mms-1, Target Mode: Strain, Strain: 50 %, Trigger Type: Auto – 3g, Tare Mode: Auto, Data Acquisition Rate: 500pps. Before analysis the instrument was calibrated using a 5kg load cell, and the height was calibrated using the base plate of the instrument. One hundred replicates per finger lime variety were tested and mechanical data were processed and exported within the Exponent Software (Stable Micro Systems, Godalming, England, United Kingdom). Bursting strength was determined as the force (g) required to completely rupture the pearl structure. The strain was determined as the percentage (%) deformation of the pearl at the point of complete rupture. The test speed setting was set at 8.0 mms-1 to mimic the speed and strain of the jaw when biting down on a bubble as recommended by the Exponent Software.

Diameter, thickness, surface area, and volume

The diameter of the pearls was determined using a TA.XTplus (Stable Micro Systems, Godalming, England, United Kingdom) equipped with a TA5 Cylinder (12.7 mm D, 35 mm L) testing probe (Brookfield Engineering, Middleboro, Massachusetts, United States). The sample diameter (mm) was measured by recording the finger lime pearl height when the testing probe detected a resistance of 3 g of force. Height (mm) was equal to the diameter of the pearl when placed on its longest edge. Measuring height via the texture analyser ensured that the pearl was not squashed before measurement which was observed when test trials were conducted using digital calipers. The thickness of the pearl wall material after bursting was measured using digital calipers (n = 10).

The volume and surface area of the pearls were calculated using Eqs. (3) and (4) respectively under the assumption that greater than 85 % of the pearls were spherical upon visual inspection.

$$V= \frac{4}{3}\pi {r}^{3}$$
(3)
$$A= 4\pi {r}^{2}$$
(4)

Statistical analysis and figure creation

Descriptive statistics (mean, standard deviation, and coefficient of variation) of the finger lime pearl data and computational analysis were performed using GraphPad Prism version 9.4.1 for Windows (GraphPad Software, San Diego, California USA, www.graphpad.com). One-way ANOVA followed by Tukey's multiple comparisons tests (α = 0.05) was used finger lime pearl properties. Microsoft Excel 2021 for Windows (Redmond, Washington, United States, www.microsoft.com) was used for data organization and curation. Graphical figures were generated using a combination of Microsoft PowerPoint 2021 for Windows (Redmond, Washington, United States, www.microsoft.com) and BioRender online software (BioRender.com).

Results

Pearl diameter

The variation in pearl size across the three finger lime varieties is displayed in Fig. 2 using histograms. The bin size of the histograms was set to 0.25 mm and the graphical images of pearls above the bins indicate the relative change in diameter of the pearls. ‘Red Champagne’ had the widest range in pearl diameter, followed by ‘Emerald’ and ‘Chartreuse’. The average pearl diameter for ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ were 2.93 ± 0.27, 2.33 ± 0.27, and 2.19 ± 0.24 mm respectively. The average size of pearls across all varieties was 2.48 ± 0.41 mm. ‘Red Champagne’ was significantly different (P < 0.01) compared to ‘Emerald’ and ‘Chartreuse’ varieties which did not differ from each other. The coefficient of variation (CV) for pearl diameter ranged from 11 - 12 % across the different varieties.

Fig. 2
figure 2

Finger lime pearl diameter distribution from ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ varieties (n = 100 per variety). Images of the pearls above the bins are scaled to represent the average diameter of the pearl for each bin

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Appearance and colour of different finger lime pearls

The colour properties of the pearls are reported in Table 1 using the CIELAB colour space coordinate system and digital images. From 0 – 100, L* is a measurement from black to white, and from a negative to a positive value, a* is a measurement from green to red, and b* is a measurement of blue to yellow (Table 1). Hue refers to the dominant colour on the colour sphere, such as red or green, while chroma, or saturation, indicates the intensity or vividness of that colour. All colour space coordinates (L*, a*, b*) and calculated parameters (Chrome and Hue) were significantly different between varieties (P < 0.01). Notably, a* for the Red Champagne pearls was positive which is reflective of a redder appearance, while a* was negative for Emerald and Chartreuse varieties indicating a greener appearance (Table 1 and). Hue and Chroma were below 90° and 40 % for all varieties indicating that the pearls have a low saturation of their respective colours.

Table 1 Colour parameter and digital images of Red Champagne, Emerald, and Chartreuse finger lime pearls
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Volume, surface area, and thickness of pearls

The graphical illustrations in Fig. 3A are scaled to the volume and diameter measurements of the finger lime pearls using a bubble graph. ‘Red Champagne’ pearls had significantly greater average volume (P < 0.01) than both ‘Emerald’ and ‘Chartreuse’ varieties, which were comparable to each other. Notably, the volume of ‘Red Champagne’ was double that of ‘Chartreuse’. The CV was the lowest for ‘Red Champagne’ (27.29 %) compared to ‘Emerald’ and ‘Chartreuse’ which were 36.29 % and 33.91 % respectively. Standard deviation (SD) was the lowest in the ‘Red Champagne’ variety compared to ‘Emerald’ and ‘Chartreuse’. The surface area and thickness differences between the varieties are displayed in Fig. 3B & C respectively. Surface area measurements followed the same trends as the volume measurements because the parameters are closely related. The thickness of the pearls was statistically different (P < 0.01) between all varieties, and ‘Emerald’ pearls had a lower thickness than ‘Chartreuse’ despite having a higher volume and surface area. Furthermore, there was a low SD for wall thickness for each variety (Red Champagne = 0.33 ± 0.03 mm, Chartreuse = 0.3 ± 0.01 mm, and Emerald = 0.25 ± 0.005 mm), indicating uniform wall thickness regardless of changes in pearl size.

Fig. 3
figure 3

Volume (A), surface area (B), and wall thickness (C) of ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ finger lime pearls (n = 100 for surface area and volume and n = 10 for wall thickness per variety). Finger lime pearls are scaled in the bubble graph (A) for their volumetric and diameter differences. * Indicates statistical differences (P < 0.01) using the postdoc Tukey test. ⌀ = diameter and ns = not significant. The error bars are reported as standard deviation from the mean

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Mechanical properties

The force-strain traces from individual pearls with parameters closest to the average values for bursting force (g) and strain (%) are displayed in Fig. 4A. In addition, the spread in the mechanical data across the 100 replicates for each variety is presented in Fig. 4B & C using histograms. The bursting force was similar between ‘Chartreuse’ and ‘Red Champagne’, but ‘Emerald’ was significantly different from the other varieties (P < 0.01). The strain was significantly different between all varieties (P < 0.01). Notably, ‘Chartreuse’ had the highest bursting strength and strain overall, but also the greatest range in bursting strength. Also, the bursting strength of ‘Red Champagne’ was very similar to ‘Chartreuse’, except for a significantly lower strain percentage.

Fig. 4
figure 4

Force-strain traces (A), bursting force (B), and strain (C) of ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ finger lime pearls (n = 100 per variety). The traces of finger lime pearls replicates closest to the average are displayed (a). * Indicates statistical differences (P < 0.01) using the postdoc Tukey test and ns = not significant

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Discussion

Finger lime pearl diameter and its comparison with similar foods

The globule vesicles of finger limes have often been referred to as “citrus caviar” by both scientific and culinary institutions because of their similarity in size, spherical nature, and popping texture (Delort and Yuan [12]). Caviar eggs from six different sturgeon species were reported to have ranged from 2.0 - 4.5 mm in diameter (Debus et al. [10]), which is similar to the finger lime varieties investigated in the present study (1.62 - 3.60 mm). Finger lime pearls are regularly used as garnishes for seafood-based meals and desserts as opposed to being used for cooking as an ingredient (Sultanbawa and Sultanbawa [32]). This practice is similar to sea grapes (Caulerpa lentillifera) which are also paired with seafood-based meals because of their popping texture and taste which is reminiscent of salt and the ocean (Paul et al. [21]). Likewise, the diameter of sea grape species ranges from 1.0 - 5.0 mm which is also similar to finger lime pearls (Estrada et al. [13]). Therefore, it can be discerned that spherical garnishes with small diameters and a strong popping nature are high-value products for the seafood industry because they offer a contrasting texture accompanied with a unique blend of a sweet and citrusy flavour (Adhikari et al. [1]).

From a commercial perspective, the size of the pearls is an important parameter to monitor because this can indicate differences between varieties as well as operate as a key quality attribute. Regarding sturgeon caviar, the size of the eggs is often used as a part of a grading system to discern higher-value stock (Ghelichi et al. [14]). This is because the larger the egg, theoretically the greater the bursting sensation and thus the flavour (Baker et al. [3]). Similarly, this viewpoint can be applied to the finger lime industry for quality grading matrices, which is why parameters such as pearl diameter should be regularly measured and reported for different varieties. To the best of our knowledge, the present study is the first time the diameter of finger lime pearls from three different varieties has been reported. Taking into consideration the data collected in our study, and the richer profile of bioactive compounds in red finger limes (Qi et al. [23]), the ‘Red Champagne’ variety has the potential for a higher economic value compared to ‘Emerald’ and ‘Chartreuse’ varieties because it has a significantly large diameter. Moreover, the CV was low across the varieties, indicating the finger limes produce a consistent pearl size of approximately 2.25 - 2.75 mm (Fig. 2), which can be used as a benchmark for other varieties and future studies.

Visual appearance of finger lime pearls

The colour properties and pictures displayed in Table 1 demonstrate stark differences between the varieties. Previously, only descriptive colour measurements have been reported for ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ finger lime varieties, which are ineffective for benchmarking requirements (Johnson et al. [17]). Numerical colour values are often used to benchmark citrus varieties via the CIELAB system because they can used for potential class discrimination and prediction models (Walsh et al. [37]). Therefore, the colour parameters recorded in this study may be used to benchmark against future studies focused on finger lime quality and sensory attributes. It has been reported that consumers have a preference towards any colour other than green when purchasing fruit because it indicates an acceptable level of ripeness and superior flavour (Velasco et al. [35]). This preference is well established within fruits such as apples (Iglesias et al. [16]), however, there are citrus varieties that appear green at peak ripeness such as Tahitian lime (Citrus latifolia) (Pristijono et al. [22]). Therefore, finger lime pearls with red/yellow/orange/pink colour could be considered to have higher marketability and consumer preference, based solely on their colour properties without considering flavour or texture. Analysis of the secondary metabolites of finger lime pearls and other citrus have reported that anthocyanins (Qi et al. [23]), and chlorophylls are the chief pigmentation compounds present, and responsible for hues of red and green (Xie et al. [38]). Moreover, it is well established that a redder colour in citrus is also correlated with a higher total phenolic, anthocyanin, and flavonoid content (Alam et al. [2]). Consequently, finger lime varieties with higher a* values can be of higher economic value from a sensory and health point of view when compared to their green counterparts (Scuderi and Zarbà [28]). This is because there is a plethora of secondary metabolites, such as anthocyanins, present in red citrus that assist in metabolic process within the body to improve overall well-being (Li et al. [19]; Rapisarda et al. [24]; Scuderi and Zarbà [28]). Therefore, pink, and red varieties of finger limes could be marketed as a more luxurious variety, accompanied by a higher price point (Richmond et al. [25]). However, sensory based studies would need to be conducted to confirm these relationships across finger limes.

Relationship between mechanical properties of finger lime pearls and their physical sensory experience

When a finger lime pearl is bitten, the internal pressure of the vesicle increases and the pearl structure is strained until a point of maximum yield, at which the structure is irreversibly damaged. Similar to caviar, these mechanical parameters are chiefly important in the sensory profile of finger limes because the majority of the flavour is released only after bursting has occurred (Callahan [7]). The maximum yield stress can be defined as the bursting strength and the maximum distance of compression can be defined as strain. Fig. 5 is a graphical illustration of how these mechanical parameters are perceived when biting down on a finger lime pearl and Fig. 4 displays the respective force-strain curves and mechanical data for each variety tested using a texture analyser. It was determined that ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ pearls can be deformed by an average of 28.59 ± 2.89, 33.57 ± 4.03, and 36.81 ± 3.53 % respectively before a burst of citrus flavour is released. In addition, the bursting strength of ‘Red Champagne’, ‘Emerald’, and ‘Chartreuse’ pearls is 355.79 ± 80.90, 260.94 ± 78.44, and 373.98 ± 99.57 g of force respectively. A high SD in the bursting strength of the pearls can be attributed to their differences in diameter (Fig. 2) and volumetric properties (Fig. 3). A low SD in the strain percentage indicates that regardless of pearl size, strain properties are relatively consistent (Fig. 4C). Therefore, the strain percent of the pearls can be used as a reliable measurement to track differences between varieties compared to bursting force. Analytical tests that produce low variance during data collection are good metrics that can be used for benchmarking varieties and establishing a quality grading system (Lawless and Heymann [18]).

Fig. 5
figure 5

Graphical illustration of the impact of force and deformation on a finger lime pearl as it is squished by the teeth. F1 = Initial force, F2 = Force at burst (rupture), D1 = Diameter of the pearl, and D2 = Diameter of the pearl at burst. D1 – D2 = the percent deformation (strain) at burst

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The bursting strength of ‘Red Champagne’ and ‘Chartreuse’ varieties is significantly higher than ‘Emerald’, which suggests that the flavour explosion and subsequent sensory experience may be greater, due to faster dispersion of flavour compounds from the vesicle (Subhasri et al. [31]). The force-strain curves in Fig. 4A demonstrate this relationship effectively from a visual standpoint which aids in assessing the physical sensory properties of finger lime pearls by consumers (Yu et al. [40]). Future quality grading and benchmarking matrices may benefit greatly from force-strain graphs as they can be used to assist in training panelists for the development of texture-based sensory lexicons for fresh citrus (Rosales and Suwonsichon [27]).

Effect of wall thickness, volume, and surface area on the bursting and strain properties of finger lime pearls

Regarding the mechanical properties of edible spherical objects, volume, surface area, and wall thickness are important parameters to measure because they can modulate bursting strength (Sunarharum et al. [33]). For example, the bursting force between ‘Red Champagne’ and ‘Chartreuse’ is comparable (Fig. 4B), however, they statistically differ in their strain properties (Fig. 4C). The difference in their strain properties can be understood by evaluating their physical and volumetric properties displayed in Fig. 3. ‘Red Champagne’ pearls have a higher wall thickness, surface area, and volume compared to ‘Chartreuse’. Because ‘Chartreuse’ pearls have a smaller surface area, there is a higher increase in internal pressure during biting, resulting in a higher strain. Furthermore, because ‘Chartreuse’ pearls have a smaller diameter and volume, the smaller pearl can withstand the higher pressure before reaching its material strength limit or yield point, and this produces a higher strain value during testing (Fig. 4A). However, ‘Red Champagne’ pearls have a higher wall thickness, therefore, the pearl structure can withstand a higher internal pressure. In summary, a larger pearl with a higher wall thickness can also achieve a higher bursting strength with less strain due to the increased amount of material available (surface area) to absorb and distribute the stress during biting. Understanding this relationship between the physical and mechanical properties of finger lime pearls allows the development of grading tools where different varieties can be used for different applications. For example, smaller or larger pearls may be selected because of their varying bursting strength which is modulated by wall thickness, volume, and surface area. Taking into consideration the varieties tested in this study, we have shown that ‘Red Champagne’ are unique in that they are a larger pearl with the bursting strength of smaller pearls. These physical parameters can be extrapolated to their sensory experience in the following manner: ‘Red Champagne’ pearls may deliver even more flavour compared to smaller pearls because they have more volume (more flavour compounds), without compromising the bursting experience.

Conclusion

The colour, physical, and mechanical properties of three finger lime varieties were investigated to highlight their visual and physical differences. The data collected from this investigation can be used to further understand the functional differences between finger lime pearl varieties and the implication the physical properties may have on their economic value. Varieties with redder colours can be associated with increased health benefits and visual appearance as they are not commonly observed in citrus fruit. In addition, varieties with smaller pearls have a high bursting strength and therefore a higher flavour release compared to varieties with larger pearls. However, it was determined that when the wall thickness of the pearl is higher, varieties with larger pearls can also exhibit a similar bursting strength to smaller pearls. Therefore, investigating the mechanical and physical properties of finger pearls via texture analysis instruments is highly valuable for understanding their different sensory properties. Future research on the sensory properties of finger lime pearls will benefit from coupling mechanical data with flavour and aroma to discover how these parameters work alongside each other to dictate consumer preference. Moreover, the physical analysis performed in this study can be used as a workflow to evaluate the changes in finger lime pearls during storage to ensure that the bursting properties are not degraded.