1. Introduction
Table olive production has progressively increased during the past decades, expanding beyond the typical initial use in the Mediterranean countries such as Greece, Syria, Algeria, Italy and Spain. This expansion has extended to countries worldwide, including the USA, Argentina, Perú, and Australia) [
1]. This growth has been fueled by the introduction of larger fermentation/storage containers (16 tonnes) and the mechanisation of conditioning operations like pitting, stuffing, or slicing. Additionally, table olives, as a component of the Mediterranean diet, are gaining acceptance in other non-producing countries such as Canada, Brazil or Russia. Global production and consumption currently reach about 3 × 10
6 tonnes [
1].
Traditionally, sodium chloride (salt) has been the main component in brines used for fermentation/storage and packaging [
2]. Its presence in the current state-of-the-art processing technology is essential for preventing sanitary risks, characteristic taste, and appropriate pH levels. A mineral content survey of Spanish cultivars [
3] revealed that the sodium content in the most popular olive presentations falls within the following ranges, expressed in g/100 g olive pulp: for green Spanish-style, 1.44 (Hojiblanca)–1.72 (Gordal); for ripe olives, 0.58 (Gordal)–0.94 (Manzanilla); and for directly brined olives, 1.5 (Manzanilla)–1.67 (
Aloreña de Málaga). Consequently, considering that the green Spanish-style olives constitute around 50% of production, they contribute the most to sodium intake for table olive consumers. In contrast, ripe olives (accounting for 45% of consumption) contribute the least.
The link between sodium intake and cardiovascular issues is well-established. Recent reviews have delved into the relationship between sodium intake, the risk of cardiovascular diseases and their dose-response correlation [
4]. The meta-analysis encompassing 36 reports and 616,905 participants concluded that individuals with high sodium intake faced a 1.19-fold higher adjusted risk of cardiovascular disease than those with low sodium intake. Analysing 20 of these reports for a dose-response investigation, a significant linear connection between dietary sodium intake and cardiovascular risk emerged, showing a 6% increase in risk for every 1 g of dietary sodium increment. This mounting concern surrounding sodium (or salt) ingestion is reflected in recommended intakes. The EU sets the reference intake at 2.4 g Na/day [
5], and the Dietary Guidelines for Americans limit it to 2.3 g Na/day [
6]. Comparable recommendations are seen in other countries. In the EU, strategies to reduce salt levels in the diet have been outlined [
7], primarily targeting frequently consumed salt-rich foods (like bread or meat products), although not table olives due to their minor contribution to the EU diet.
In the context of table olives, the interest in reducing salt levels in table olives has parallel consumer concern. However, up to this point, most efforts have primarily focused on the fermentation/storage phases of the most relevant cultivars. Özay and Borcakli [
8] undertook research to modify the traditional fermentation process for naturally black olives. They tested brine concentrations of 14 g NaCl/100 mL (as usual) and 6 g NaCl/100 mL (reduced level) but found no sensory differences between the two conditions. However, this reduction in salt content led to a decrease in ash content from 4.7 g/100 g olive pulp to 2.41–2.81 g/100 g olive pulp. Tassou et al. [
9] also employed various brine NaCl levels to process
Conservolea naturally black olives. Brine levels of 6% and 4% in brines favoured the growth of lactic acid bacteria and prevented off-odour development.
Kanavouras et al. [
10] conducted tests involving the total or partial substitution of NaCl (16%,
w/
w) with a buffer (CH
3COOH, 0.05 M + Ca(OH)
2, 0.025 M). They found that partial substitution resulted in a product with significantly better texture, colour and high acceptability. Tassou et al. [
11] also investigated the effect of CaCl
2 (5 g/L brine) on the mechanical properties and microbial characteristics of cv.
Conservolea naturally black olives fermented at different sodium chloride concentrations (4, 6, and 8 g NaCl/100 mL). The pulp exhibited greater strength and stiffness when the calcium salt was added to the 4 g NaCl/100 mL brine. In
Aloreña de Málaga cracked olives [
12], fermentation using a mixture of Na, Ca, and K chloride salts, the behaviour of K was similar to Na, while the presence of Ca led to faster acidification, a lower pH and higher water activity. Panagou et al. [
13] researched to examine the implications of NaCl reduction on the fermentation profile of
Conservolea natural black olives, using five combinations of NaCl, KCl, and CaCl
2. It was found that olives fermented with 4% NaCl and 4% KCl had good organoleptic attributes, while the addition of CaCl
2 rendered the product bitter.
Concerning Spanish-style olives, Bautista Gallego et al. [
14] employed various chloride salts, including sodium (in the range of 0–4%), potassium (0–4%) and calcium (0–6%) to ferment cv.
Gordal. Adding CaCl
2 reduced the pH, delayed sugar diffusion into the brine, and decreased yeast growth. In this fermentation process, the final product’s Na exhibited a linear relationship with the content in brine, while Ca and K contents followed quadratic models. Furthermore, most sensory attributes were linearly associated with the mineral contents in the brine, except for bitterness, which followed a quadratic model [
15]. More recently, Dalloul and Erten [
16] examined the physicochemical properties of cracked green cv.
Sari Ulak olives fermented in brines where NaCl had been partially replaced with KCl, MgCl
2, and CaCl
2. The preferred olives were those prepared with NaCl, followed by NaCl + KCl and NaCl + MgCl
2, whereas those containing CaCl
2 were rejected because of their bitter taste.
Nevertheless, a more feasible avenue for salt reduction could lie in the packaging phase. This approach gains traction because the final products usually stabilise via pasteurisation, allowing for lower salt levels [
17]. Furthermore, incorporating various chloride salts can enhance the daily intake of naturally occurring minerals like Ca, K, or Mg in fruits. This makes them more appealing from a health perspective and opens up the possibility of making health claims related to vitamin E or polyphenols [
18].
However, such modifications might also have unintended repercussions on the sensory characteristics, making it crucial to thoroughly investigate the potential impacts on consumer-facing products.
This research investigated the impact of partial substitution (50%) of NaCl by KCl, CaCl
2, and MgCl
2 in the packaging brines of whole (plain) green Spanish-style Manzanilla table olives. The study focused towards examining the physiochemical and sensory properties of these olives. Throughout the experiment, the established salt level recommended by the Trade Standard Applying to Table Olives (5% NaCl in the packaging brine or juice after osmotic balance) was maintained [
17] to preserve the traditional sensory characteristics of products. This approach aimed to simultaneously decrease sodium content and enhance potassium, calcium, and magnesium contents.
4. Discussion
Most research on reducing NaCl in table olives has primarily concentrated on the fermentation phase. In the context of natural black olive fermentation, Özay and Borcakli [
8] reduced the NaCl level from 14 g/100 mL to 6 g/100 mL, primarily focusing on the growth of lactic bacteria and acid production. This reduction increased acidity, reaching up to 0.59 g/100 mL in the test with a reduced level. Tassou [
9] investigated even lower levels (4–8%.) and concluded that the best fermentation conditions were achieved at 6% salt concentration and a temperature of 25 °C. This conclusion was based on factors such as free acidity produced and lowest pH, indicating improved fermentation outcomes. Similarly, Kanavouras et al. [
10] followed the trend of Özay [
8] by substituting sodium with calcium, using readily available Ca(OH)
2. However, even with this substitution, the best product in colour, texture, and acceptability still had 12.8% salt. Additionally, calcium chloride was found to have a protective effect on the mechanical properties of natural olives [
11], which aligns with the traditional empirical use of sodium chloride in Spanish style [
2]. Subsequently, Panagou et al. [
13] explored the impact of different salt mixtures of NaCl, KCl and CaCl
2 on the fermentation profiles of natural black
Conservolea olives. They concluded that only combining KCl and NaCl resulted in olives with favourable organoleptic properties. This suggests that a specific balance of these salts is crucial for achieving desirable sensory characteristics and texture during these products’ fermentation process.
In other styles, for instance, Zinno et al. [
34] replaced 25%, 50%, and 75% NaCl with KCl during the fermentation of Nocellara del Belice olives processed as Spanish and Castelvetrano styles while maintaining a final saline concentration of 9 and 7%, respectively. Interestingly, their findings indicated that these substitutions did not significantly impact microbial dynamic, contamination risk, or proliferation of pathogens or spoilage microorganisms. Another study by Saúde et al. [
35] involved the fermentation of Maçanilha Algarvia olives with partially substituting NaCl using KCl and CaCl
2, resulting in an overall 8% salt concentration. The evaluation by the sensory panel noted that olives fermented in 8% NaCl and 4%NaCl + 4%KCl exhibited the most favourable flavour and general attributes. This formulation also yielded a remarkable 672% increase in K content and a simultaneous 19% reduction in Na. In another research, Bautista Gallego et al. [
12] applied salt substitution in cracked
Aloreña de Málaga olives that were directly brined. Their findings showed that using CaCl
2 reduced the growth of Enterobacteriaceae and lactic acid bacteria but resulted in heightened yeast activity. Furthermore, the process was accompanied by decreased pH and combined acidity, as in the current study.
Similarly, the application of a mixture of the same salts during the fermentation of green Spanish-style Gordal olives [
14] demonstrated that CaCl
2 impacted initial and post-fermentation pH values, delayed sugar diffusion into the brine, and produced a higher titratable acidity concentration. Concurrently, incorporating KCl during this fermentation process reduced the growth of Enterobacteriaceae and yeast, promoted the proliferation of lactic acid bacteria, and led to the lowest pH, which could be helpful for the further preservation of the packaged olives. Regarding sensory attributes, the
saltiness perception was associated with NaCl and KCl concentrations, while
bitterness,
hardness,
fibrousness and
crunchiness were linked to the presence of CaCl
2 [
15]. Most of these observations were consistent with the changes described in our packaged olives, except for microbial growth.
During packaging, this study successfully replaced NaCl with KCl, CaCl2, and MgCl2 mixtures in plain (whole) green table olives. This strategy eliminated the potential risks associated with low salt levels during processing and was exclusively applied to the ready-to-consume, typically pasteurised product. While the initial desalting process caused a noticeable rise in pulp moisture, this effect was subsequently reduced to levels similar to Control. Furthermore, substituting salt-induced modifications in the physicochemical characteristics resulted in a decrease in pH, a slight uptick in titratable acidity, and an enhancement of olive firmness, primarily attributed to the inclusion of CaCl2. The reduced combined acidity observed in the final products was mainly caused by the desalting process, compounded by the dilution effect inherent in the packaging phase. The investigation also reaffirmed that the combined acidity primarily originates from lactic salts. This deduction was supported by the close agreement between the lactic estimation, derived from both titratable acidity and combined acidity as lactate. Additionally, the data revealed that lactic acid concentrations in the pulp moisture closely paralleled those in the brine, indicating that lactic acid is nearly exclusively accumulated in the pulp moisture.
Interestingly, the introduction of salt mixtures did not cause a discernible impact on colour, as the experimental design aimed to replicate the traditional commercial presentations of the product closely. Regarding sensory descriptors, the presence of CaCl
2 significantly elevated sores for
bitterness,
hardness,
fibrousness, and
crunchiness. Conversely, MgCl
2 exerted only a slight influence on the sensory descriptors. Notably, the effect of CaCl
2 on
firmness or
hardness (as perceived by sensory evaluation) aligns with findings from previous research [
36,
37]. In contrast, the incorporation of KCl reduced the
bitterness perception. However, beyond the inherent olive
bitterness, the exact mechanism by which other elements contribute to
bitterness remains elusive.
In theory, RSM should lead to conditions that produce the best response. However, the situation becomes more complex when several variables with multiple impacts are involved, as in this case where desirability only reached 0.57. Regarding the concentration of salt mixtures for achieving overall optimum results, the selection prioritised KCl at a higher level. This choice is crucial as it can compensate for the substantial loss of potassium during the green Spanish-style olive processing [
15]. Calcium chloride also received a high priority and surely will increase the natural content of the olive fruit in Ca [
2,
3]. Finally, Mg is not abundant in table olives but its incorporation could be interesting for improving their nutritional profile [
4]. Regarding the effect of salts on the physicochemical characteristics and colour, it is worth noting that the predicted pH at the optimum point may be not the most desirable in typical packaging since other combinations could yield the lowest values. However, such a condition is not as critical under pasteurisation stabilisation. Notably, the
firmness is relatively close to maximum values and the selection could be quite appropriate for this attribute. In the case of sensory descriptors,
bitterness resulted in somewhat unfavourable treatment, due to the low proportion of potassium, which appears to mitigate this sensation. This happened despite having given
bitterness the maximum weight in the selection criteria. Notably, the selection is highly favourable for
fibrousness since the chosen point aligns with the maximum scoring region. Furthermore,
hardness and
crunchiness approach their maximum values. In summary, the selection was generally appropriate, except for
bitterness, which was challenging to minimise. To improve consumer acceptance, it might be beneficial to adjust the selected composition towards the centre of the design zone with lower
bitterness. This supports the notion that, in the case of multiple variables with various impacts, further refinement may be necessary.
Bansal and Rani [
38] have attributed the limited adoption of potassium chloride as a substitute for NaCl in lemon pickles to its tendency to impart a bitter taste. This bitter perception of KCl has been observed in meat products like fermented sausages and other applications. In the context of NaCl substitution with KCl, it has been noted that a maximum of 16% substitution is feasible to prevent the development of
bitterness perception, apparently caused by the K cation [
39]. Youssef et al. [
40] studied the production of low-sodium pickles suitable for hypertensive patients, exploring binary and ternary mixtures. The research found that a blend of 4%NaCl + 4%KCl resulted in negligible changes in taste and overall acceptability compared to 8%NaCl formulations. While the
firmness of cucumbers was reduced, carrots exhibited a minor increase. However, no
bitterness increase was reported.
Regarding table olives, Ambra et al. [
41] investigated the partial replacement of NaCl with KCl (at levels of 50% or 75%) in brine used during the fermentation of Spanish and Castelvetrano styles. This substitution led to a product with significantly reduced sodium content without substantially affecting the presence of the bioactive compounds. However, introducing KCl was associated with a heightened
bitterness in both debittering methods. Despite this effect, the
bitterness perception, persistence and aftertaste in the Castelvetrano system olives remained below that of the classic Spanish style, as the latter retained better the characteristic “sweet olives”. Erdogan et al. [
42] studied five combinations involving NaCl, CaCl
2, and KCl to partially replace sodium in table olive production via the traditional Gemlik method. The sensory analysis revealed that mixtures containing only CaCl
2 and KCl produced bitter olive products. In contrast, combinations containing 5% NaCl and 5% KCl yielded low sodium olives with satisfactory organoleptic characteristics, aligning with the outcomes of our current research. In recent experiments conducted with the Turkish
Sari Ulak cv. cracked table olives [
16], the preference was given to olives processed with a combination of NaCl and KCl over those processed using CaCl
2 alone or in combination with NaCl. Consequently, the precise impact of CaCl
2 and KCl salts on sensory characteristics after NaCl replacement remains unclear. Nevertheless, including KCl in brines can yield acceptable low-sodium table olives.
Our study further highlighted a robust relationship among kinesthetic attributes. Nonetheless, despite detailed explanations, discerning between
firmness,
fibrousness, and
crunchiness remained intricate for testers. This challenge, however, is general and has been observed in previous research [
15]. Despite its complexity,
crunchiness is valuable and distinguishes certain presentations, such as
Aloreña de Málaga [
12].
The positive correlation between
acid,
salty, and
firmness suggests a parallel trend. However, it might also indicate confusion in distinguishing between
acid and
salty. The outcomes in
Figure 1B and
Figure 2B support earlier findings concerning
Gordal olives [
14]. Interestingly, our results suggest that the most pronounced bitter scores materialise without KCl, implying that KCl might not significantly contribute to
bitterness. Conversely, an increase in MgCl
2 seems to intensify the perception of
bitterness.
In contrast, CaCl2 was associated with firmness and crunchiness scores, with their peak values observed at minimal proportions of KCl and MgCl2. This correlation between CaCl2 and bitterness and kinaesthetic attributes also finds support in the research conducted by several authors, utilising an enlarged centroid mixture design. These researchers discovered that elevating CaCl2 concentrations enhanced a range of properties, encompassing attributes like Ic, L and b * values, firmness, bitterness, hardness, fibrousness, crunchiness, and salty (negatively), as assessed through PLS-R analysis. Most treatments exhibited healthier properties (highly favourable mineral nutrients), resembling commercial products. Our study similarly determined that bitterness was associated with all kinaesthetic attributes. This correlation is logical, given that elevated scores in these attributes may indicate less mature olive or mild lye treatment, leading to more pronounced bitterness in the fruits. Nonetheless, a comprehensive understanding of the changes in bitterness is still in progress, possibly due to the intricate interplay of the various salts involved in the experiments. Then, this aspect remains a subject still requiring further research.
The identification that bitterness had a noteworthy impact on the overall scoring and buying predisposition is outstanding. Conversely, higher ratings in smell and crunchiness corresponded to improved overall scoring and a greater inclination to purchase. The categorisation of runs into distinct sensory profiles through clustering based on sensory attributes can, in turn, guide their choices by processors based on consumer preferences or specific market demands.