Effect of Drying Aids on the Quality Properties of Kefir Powder

Abstract

The biological and nutritional value of kefir makes it imperative to widen its customer reach by extending its shelf life, enabling its storage and transport at ambient temperatures, and reducing its packaging cost requirements. A well-documented and widely used food processing method that can achieve the above-mentioned outcomes is spray drying through the formulation of kefir powder, with quality attributes that will closely resemble that of the original product. In the present work, a variety of drying carriers (trehalose, fructo-oligosaccharides, maltodextrin, gum arabic, and whey proteins) were studied with regard to their effect on the properties of powdered and reconstituted kefir samples. Particularly, the physicochemical characteristics (moisture content, pH, acidity, color), powder properties (bulk density, tapped density, water solubility index, insoluble matter), adsorption isotherms, particle size distribution, microbiological properties, and structural characteristics of the samples were evaluated. Gum arabic and maltodextrin produced kefir powder samples with the lowest moisture content. Spray drying affected a reduced acidity and, as a consequence, an increased pH in the reconstituted samples, especially without any carrier addition and with trehalose and fructo-oligosaccharides irrespective of their concentration. Desirable color attributes were achieved on the kefir powder samples with trehalose, fructo-oligosaccharides, and maltodextrin regardless of their concentration in the product. Fructo-oligosaccharides, added at 3%, gave the highest values of bulk density, while whey proteins, due to their nature, exhibited the lowest. All carriers tested improved the water solubility index when compared to the control sample. The sample with 10% whey proteins exhibited the lowest moisture adsorption compared to the control at the highest relative humidity environments employed for the test. In the absence of agglomeration, powder granule size and structural morphology were not affected by the carriers. The survival of lactococci in the powdered kefir samples, in comparison to the control product, was higher in the case where 4% trehalose or 5 to 10% whey proteins were added. On the contrary, yeast populations decreased significantly during drying and they were not affected by the presence of the different carriers. As shown by the findings of the present study, trehalose proved to be the most effective carrier, among the others used, for producing high-quality kefir powder products. However, further work is required with regard to the keeping quality of the product during long-term storage.

1. Introduction

Kefir is a fermented dairy product produced by the metabolic activity of bacteria and yeasts found in kefir grains, the traditional starter culture of kefir. It is considered a natural probiotic that, in addition to its nutritional value, can improve a variety of health conditions [1,2,3,4].

Extending the self-life of kefir, making it available to a broader consumer spectrum, and reducing its keeping and transportation costs can be partly met by the development of dried powder kefir-based products. Powdered products require less packaging, transportation, and storage costs due to bulk and water activity reduction, allowing such products to be more widely available. Moreover, the health benefits attributed to kefir, alongside its improved keeping performance compared to milk, render kefir dry powder an attractive new product with a substantial argument for its development.

Spray drying is a widely applicable, high-throughput, efficient, and nutritionally favorable drying method that is used in the dairy industry for producing milk and milk-derived ingredient powders. The conditions used in each of the different processing/transformation steps during spray drying, such as atomization type, inlet/outlet drying temperatures, residence time, along with the composition and treatment of the feed entering the dryer, allow a great versatility to the method with regard to the production and formulation of products with acceptable nutritional performance and varying physical characteristics, depending on the desirable outcome [5,6].

The presence of certain ingredients, mainly carbohydrates, has been proven beneficial in preserving the viability of microorganisms subjected to spray drying [7] and, in general, is considered to improve the process performance. Spray drying, in general, is utilized as a microencapsulation technique [8] to render physical protection and improve the stability of bacteria and other sensitive ingredients in food products.

Dried kefir powder production and evaluation of product properties have been reported in the literature. Nurwantoro et al. [9] studied the effect of different drying methods, cabinet drying, freeze-drying, and spray drying, on goat milk kefir powder properties and concluded that the best drying method was spray drying. Brasiel et al. [10] also stated that spray drying of kefir is a promising method for the production of dry kefir. In other literature, the viability of kefir microflora (lactic acid bacteria and yeasts) in spray-dried kefir powder without carrier addition was investigated. Atalar and Dervisoglou [11] attempted process optimization of kefir spray drying using a response surface methodology model that was based on the survival rates of microorganisms, the outlet temperature of the dryer, the moisture content, and the water activity of the produced powder. The use of different carriers on spray-dried kefir, such as maltodextrin [12,13], maltodextrin/gum arabic mixes [14], skim milk [12,15], whey permeate [12], and chemically modified starch with octenyl succinate [13], has also been reported. Other drying methods have also been utilized, such as in the case of Tontul and coworkers [16], where kefir drying was studied using a refractance window dryer utilizing conduction and radiation as heat transfer mechanisms. Based on their findings, the method produced powder of acceptable handling and microbiological quality. Rizqiati et al. [17] used a cabinet dryer to study the effect of dextrin concentration on the properties of powdered goat milk kefir.

The aim of the present work is to focus on the effect of different drying aids such as trehalose, fructo-oligosaccharides, gum arabic, milk proteins, and maltodextrin, on the general physical and microbial quality characteristics of kefir powder produced by spray drying. The utilization of trehalose, fructo-oligosaccharides, and whey proteins in the production of powder kefir has not been reported in the literature, so far, to the best of our knowledge.

2. Materials and Methods

2.1. Kefir Preparation and Carriers Used

Kefir was prepared using homogenized and pasteurized semi-skimmed milk and commercial starter cultures, XPL-30 and LAF-4, from CHR HANSEN (Hørsholm, Denmark). XPL-30 comprises Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis and Streptococcus thermophiles, whereas LAF-4 consisted of Kluyveromyces marxianus subsp. marxianous. Prior to fermentation, the commercial pasteurized milk (2 L) was further heat-treated at 95 °C for 5 min in order to denature the whey proteins, a practice that has been experienced by our research group to yield kefir with a smoother and denser creamy texture. Subsequently, the milk was cooled down to 30 °C, inoculated with the starter culture, and incubated at the same temperature. When the fermentation reached pH 4.4, kefir was gently mixed and cooled down at 4 °C for 24 h prior to the addition of the respective, in each case, drying carriers. Carriers, trehalose (TREH, 1, 2 and 4% w/w on fresh kefir basis; TREHA, HAYASHIBARA Co., Ltd., Okayama, Japan), fructo-oligosaccharides (FOS, 2, 3 and 6% w/w on fresh kefir basis; Orafti^®oligofructose, BENEO-Orafti S.A., Tienen, Belgium), maltodextrin (MLD, 2, 4 and 6% w/w on fresh kefir basis; Manis Chemicals, Tereos syral G190 Pharma Maltodextrin, Greece), gum arabic (GA, 2, 4 and 6% w/w on fresh kefir basis; Gomma Arabica E414, FARMALABOR, Farmacisti Associati, Canosa di Puglia, Italy), and whey proteins (WP, 2, 5 and 10% w/w on fresh kefir basis; Whey Protein Concentrate, Hellenic Protein S.A., Athens, Greece), were prepared by dissolving/suspending their appropriate, in each case, quantities in distilled water (200 g of water were used for preparing the drying aids for every 1 Kg of fresh kefir), followed by mixing with the chilled kefir and allowing the mixture to stand at 4 °C for 2 h prior to drying. A control (CONTROL) sample without carriers was also prepared.

The choice of the drying-aids concentrations was made so that they would not exceed 35% w/w of the final powdered product dry solids. This concentration was considered critical for the final products’ nutritional and functional characteristics. According to that, and bearing in mind that the average semi-skimmed milk dry solids content is at 11% w/w (moisture content of the CONTROL kefir sample was at 89.1% w/w), the concentrations of 1, 2, 3, 4, and 6% w/w on fresh kefir basis, corresponded to approximately 8, 15, 20, 25, and 35% w/w on dry matter basis, respectively. Only in the case of whey proteins, since they consist of an integral constituent of milk, a higher concentration was tested, reaching approximately 45% w/w of the final product’s dry solids. Beyond that, the concentrations tested were kept low, bearing in mind the commercial viability of the process for industrial-scale production. Εspecially for the fructo-oligosaccharides, the fact that their daily recommended dose ranges between 5 and 15 g was taken into account, considering that the usual commercial product packages range between 0.25 and 0.5 L.

2.2. Drying Process

Kefir samples of 2.2 L were spray-dried after the addition of carriers, using a Mini Spray Dryer SD-19 (MRC Laboratory-Instruments, Hagavish Holon, Israel) equipped with a 1 mm dual-fluid nozzle atomizer with a maximum feed flow rate of 5 L/h and a gas pressure nozzle set at 0.12 MPa. In all drying experiments, the inlet temperature was kept constant at 140 °C, whereas the outlet was arranged at 80 °C. The absolute humidity of the environmental air used for drying was in the range between 0.01 and 0.013 Kg H₂O/Kg dry air, and its flow rate was kept constant at 5 m³/min through a 30 cm drying chamber diameter. Powder collection was effected by means of a cyclone, and the collected dry product was bagged in multilayered laminate pouches under vacuum. The powder kefir samples were prepared in duplicate, while samples (fresh kefir, powder samples, reconstituted kefir products) analyses were performed in triplicate.

2.3. Physicochemical Properties

The gravimetric standard method was used for all moisture content determinations by drying at 102 ± 1 °C to constant weight [18]. The pH of kefir samples before drying and on the reconstituted product was determined using a laboratory pH meter (EDT Instruments, GP353 ATC-pH-METER). The titratable acidity of liquid samples (before drying and reconstituting) was measured with 0.1 N NaOH as per AOAC [18] and expressed as lactic acid concentration (% w/w).

The color was assessed using the MetaVue Paint Benchtop Spectrophotometer (X-rite, Inc., Grand Rapids, MI, USA), and the color parameters monitored were L* (brightness), a* (+red color to –green component), b* (+yellow to –blue component), and WI-ASTM (whiteness index).

2.4. Powder Properties

Bulk (ρ_bulk, Kg/m³) and tapped (ρ_tapped) densities were determined based on the volume occupied by a 20 g powder sample in a 100 mL graduated cylinder, with the only difference being that, in the case of tapped density, the cylinder was vigorously tapped by hand 30 times [19].

Water solubility index (WSI) [20] and insoluble matter [21] were determined by resuspending/reconstituting a certain quantity of dry powder with distilled water to achieve solids concentrations equal to the ones of the kefir product prior to drying. Following reconstitution and vigorous mixing, the soluble from the insoluble solids were separated via centrifugation at (4200× g for 30 min) and brought to dryness (to constant weight) in an oven at 105 °C before determining each of their weights, respectively. WSI and insoluble matter content were calculated based on the following equations:

$$\mathrm{WSI}={\displaystyle \frac{\mathrm{Soluble} \mathrm{supernatant} \mathrm{weight} \mathrm{after} \mathrm{drying}}{\mathrm{Initial} \mathrm{dry} \mathrm{sample} \mathrm{weight}}}\times 100$$

(1)

$$\mathrm{Insoluble} \mathrm{matter}={\displaystyle \frac{\mathrm{Insoluble} \mathrm{sediment} \mathrm{weight} \mathrm{after} \mathrm{drying}}{\mathrm{Initial} \mathrm{dry} \mathrm{sample} \mathrm{weight}}}\times 100$$

(2)

2.5. Adsorption Isotherms

Water vapor adsorption isotherms of the powder samples were determined gravimetrically based on the humidity adsorbed by the powder when exposed for a period of 7 days in varying relative humidity environments generated by saturated solutions of salts (LiCl 11.3%, CH₃COOK 23%, MgCl₂ 33%, K₂CO₃ 43%, Mg(NO₃)₂ 54%, NaCl 75%, KCl 85%, and ΚΝO₃ 95%) at 25 °C in hermetically sealed containers [22].

2.6. Particle Size Distribution

Particle size distributions, of the kefir solutions prior to drying, the powder samples, and the reconstituted product were monitored via laser diffraction using the Malvern Mastersizer 2000 laser diffraction analyzer (Malvern Instruments Ltd., Worcestershire, UK) in conjunction with the appropriate sample dispersion accessories: Sirocco 2000 for dry powder and Hydro 2000 for the liquid samples. The refractive index was set at 1.53.

2.7. Microbiological Analyses

Microbial populations of lactococci and yeasts were monitored in kefir samples before drying and after reconstitution by plate counting in spread plate Petri dishes. The substrates used were YGC agar (Merck KGaA, Darmstadt, Germany) for yeasts and M17 agar (Merck KGaA, Darmstadt, Germany) for lactococci incubated aerobically at 25 and 30 °C for 5 and 2 days for yeasts [14] and lactococci [11], respectively.

2.8. Morphology Analysis

Confocal Laser Scanning Microscopy (LSM 700, Carl Zeiss, Jena, Germany) was also used for observing kefir before drying, powdering, and reconstituted products. Especially for the reconstituted products, a combination of pigments—Rhodamine B, Acridine Orange, and Nile Red—was used, allowing the observation of fat globule distribution within the matrix, as well as its rehydration pattern to be visualized.

2.9. Statistical Analysis

Analysis of Variance (one-way ANOVA) was applied to the experimental data, in order to study the effect of carrier addition on the spray-dried kefir properties. The results are displayed as the mean values of measurements with 95% confidence intervals based on the pooled standard deviation of the Analysis of Variance. Statistical analysis was performed with Minitab 18 software.

3. Results and Discussion

3.1. Physicochemical Properties of Kefir Powder and the Effect of Drying Aids

The moisture content of the CONTROL kefir sample was 89.1%, while for the other kefir samples, prior to drying, it ranged between 86.9% and 89.8%, depending in the reverse order on the concentrations of the carrier used in each case. Post-drying kefir powder moisture ranged between 2.7 and 5.7% with similar results being reported by Atalar and Dervisoglu [11], Nale et al. [14], Rizqiati et al. [17], and Teijeiro et al. [12]. Higher moisture contents (6.92–8.5%, 7.75%) were reported by Setiyawan et al. [15] and Nurwantoro et al. [9]. Moisture content, especially in powdered food products, is crucial since it determines its shelf life and ensures microbial stability. Moreover, different types of yogurt powders have been reported to have moisture contents ranging from 3.7 to 8.2% [23], whereas the Greek Codex Alimentarious [24] states as a prerequisite for the moisture content of milk powders that it should not exceed 5%. Based on the aforementioned, the produced kefir powder samples are in agreement with the existing literature and the legal requirements for such products with regard to moisture.

Figure 1 depicts the effect of the different drying aids on the moisture content of the kefir powder samples. As can be observed, GA and MLD reduced the moisture content of the samples as compared to the CONTROL, irrespective of the concentration employed. WP at 10% resulted in a low moisture product, whereas at lower levels (2%, 5%), they had the opposite effect. Similarly, when compared to the CONTROL, FOS at 3 and 6% and TREH at 2% increased the water content of their respective dry powders. Bearing in mind that all drying experiments were performed under the same drying conditions, it appears that the most effective drying aids, exhibiting the lower moisture to the final products, are GA and MLD. In the case of WP [25] and TREH [26], due to their hydrophilic and hygroscopic nature, especially for the latter, the low moisture contents observed at the highest concentrations studied, were possibly the outcome of the increased solids concentration in the kefir samples prior to drying.

The initial pH value and acidity of the fresh kefir sample (CONTROL) were 4.34 and 1.02%, respectively. The addition of certain carriers to the fresh product affected the pH that ranged from 4.13 to 4.51 with a simultaneous drop in acidity (0.72–1.01%) (Figure 2). This can be attributed to alterations in the concentrations of ionized species in general, and the total number of hydroxylic groups in the system due to the addition of the different carriers used.

With regard to the shift in pH and acidity observed in the reconstituted samples compared to the fresh kefir used for their formulation, the samples CONTROL, FOS, and TREH resulted in increased pH values (Figure 2a) and reduced acidity (Figure 2b). This is possibly the combined result of spray drying since volatile short-chain fatty acids may evaporate or dissociate during the drying process, reducing their overall concentration in the final reconstituted product. In the case of WPC, the pH of the reconstituted samples remained fairly stable or increased, contrary to the acidity, due to the buffering capacity of the proteins [25]. The effect of drying on the pH and the acidity of the samples that contained GA and MLD is not clear; however, it can be inferred that the gum does not influence the pH, and MLD facilitates an acidity drop on the reconstituted kefir samples compared to the initial fresh product. In contrast to the findings of the present study, Atalar and Dervisoglu [11] reported that they did not observe any pH or acidity variation between the samples before and after drying.

Figure 3 depicts the effect of drying carries/aids on the color parameters of kefir powder. Color is an important criterion for food quality. Given that fresh kefir, like milk, is characterized by a high intensity of whiteness, the production of kefir powder with increased brightness and WI-ASTM, as well as reduced yellow attributes, is desirable.

The observed L* values ranged between 98.6 and 97.4 (97.9 was exhibited by CONTROL), noticeably higher than the ones cited in the relevant literature reports for kefir powder produced by spray drying that ranged between 84.01 and 90.6 [11]; 95.96 [14]; 90–93 [15]; and 93.29–95.53 [13]. Despite overlaps in the average values, as can be observed in Figure 3a, the intensity of brightness of the kefir powder samples, when compared to the CONTROL, increases in the presence of FOS in the mixture and at the 4% concentration for MLD, whereas it decreases at elevated concentrations of WPC and CA.

Color parameter b* varied between 6.53 and 9.08 (9.06 CONTROL sample) within the range reported in the literature (8.62–14.64 [11]; 8.18 [14]; 4.7–7.3 [15]; 3.89–8.12 [13]. According to the findings depicted in Figure 3b, all drying aids, apart from WPC, reduced marginally b* values compared to the CONTROL, but still, this change was not statistically significant. Only trehalose (TREH) at 4% concentration induced a statistically significant drop in the intensity of yellow color compared to the original without drying aids product.

The whiteness index varied from 47.24 to 58.94 (47.51 for the CONTROL) and exhibited a statistically significant increase in all samples, where fructo-oligosaccharides (FOS), trehalose (TREH), and maltodextrin (MLD) were added in comparison to the CONTROL. Arabic gum (GA) at 4% and whey protein (WP) at 5% concentrations also resulted in elevated WI-ASTM values compared to CONTROL.

The color parameter a* did not exhibit statistically significant variations between samples, even though the preparations with WP were the only ones that showed positive values for the coefficient of red color, differing from the rest, where negative values were observed (green color) mainly due to the riboflavin content of the milk used. Parameter a* ranged between −0.885 and 0.910 (−0.660 was measured for the CONTROL) being some of the highest reported in the cited literature: −2.94 to −1.09 [11]; −2.19 [14]; −2.4 to −2.25 [15]; −1.24 to −0.77 [13].

Based on the aforementioned, fructo-oligosaccharides, trehalose, and maltodextrin improve the color characteristics of the dried kefir powder, whereas for gum arabic and whey proteins, the positive effect is exhibited only at certain concentrations, 6% and 5%, correspondingly.

3.2. Physical Properties of Kefir Powder and the Effect of the Different Drying Aids

Figure 4 illustrates the effect of carriers on the bulk and tapped density of powder kefir samples. As can be seen, WP, irrespective of its concentration, and GA at 4% resulted in reduced bulk density when compared to the CONTROL. FOS at 3% and MLD at 6%, on the contrary, increased bulk density when compared to the CONTROL. With regard to sample density, the observed results become clearer when the taped density is considered, where WP is shown to negatively affect the measured values, irrespective of its concentration, and FOS increases it at concentrations of 3 and 6%. Based on a general understanding, also claimed in the relevant literature [27], low bulk densities in food powders are not desirable since they increase the packaging requirement volumes. This statement, however, should not be taken lightly into perspective, since the usability of food powders with regard to their packaging and reconstitution is a fine balance affected by hygroscopicity, flowability, and agglomeration of the fine powder particles that may severely affect the products’ process ability and ease of use by the consumer. CONTROL had 415 Kg/m³ bulk and 650 Kg/m³ tapped density, and FOS at a 3% level, 480 Kg/m³ and 720 Kg/m³, respectively. In contrast, and for comparison purposes, Nale et al. [14] reported for kefir powder a bulk density of 465 Kg/m³, whereas 400 Kg/m³ and 530 Kg/m³ were observed by Barbosa-Cánovas et al. [27] for milk and whey powders, respectively, and 538 Kg/m³ for bulk (746 Kg/m³ tapped) density by Koc et al. [28] for yogurt powder.

All carriers integrated into kefir increased the water solubility index measured during powder reconstitution and reduced the insoluble matter content of the samples compared to the CONTROL (Figure 5). MLD, GA, and TREH increased solubility in a concentration-related manner, whereas for FOS and WP the solubility was highest at 3 and 10% concentrations, respectively. The enhanced solubility of kefir in the presence of drying aids was mainly due to the hydrophilic character of the ingredients used for the specific purpose. Kefir is based on the principles of its preparation, as the fermentation proceeds and pH reduces with the increasing concentration of lactic acid; it creates an insoluble matter due to the agglomeration of the milk proteins at their isoelectric point. These insoluble agglomerates create a matrix that initially may not allow for whey separation, but over time this may occur. Further, the spray drying of kefir breaks down this protein matrix during atomization and dehydration, creating powder particles that, when rehydrated during reconstitution, may result in an insoluble matter, similar to the initial fresh product, but more pronounced, sometimes appearing as sediment, due to inadequate rehydration.

3.3. Adsorption Isotherms of Powder Kefir Samples

Hygroscopicity refers to the ability of a substance, such as kefir powder, to adsorb moisture from the environment, altering its properties and, in the case of food, its stability, and perishability over time. For most foodstuffs, increased hygroscopicity during storage, accelerates deteriorative reactions, such as non-enzymatic browning and oxidation [29]. In the case of powders, this makes them difficult to handle and process. Figure 6 shows the adsorption isotherms of the different kefir powder preparations, of the herein-presented study, where similarities are observed in the trends of the various curves with regard to the respective different ingredients used as drying aids. At low relative humidity environments, all carriers, each one of them to a varying extent, showed increased hygroscopicities compared to the control. As the relative humidity increased, the hygroscopicity of all samples became similar to that of the CONTROL, except for the samples where fructo-oligosaccharides were added causing, due to their hygroscopic nature, the adsorption of moisture to increase. Generally, all drying aids slightly increased hygroscopicity in a 95% relative humidity environment, apart from the WP at 10% concentration, which, in contrast to all others, exhibited a relative decrease in water intake due to the partial hydrophobic nature of proteins, always compared to the control. In contrast to the findings of the present study, maltodextrin [30] and whey proteins [31] were shown to decrease and increase moisture adsorption, respectively.

3.4. Particle Size Distribution

The powder particles produced during spray drying, in all different formulations, irrespective of drying aid concentration, resulted in differential particle size distributions with spans ranging between ~2 μm and ~50 μm with a single distinct mono-dispersed peak, indicating the most frequently observed particle size (highest concentration), at approximately ~10 μm. Fresh kefir samples, in contrast to the respective powders produced from them, showed particle size distributions of greater sizes. Fresh kefir particle size distributions, before drying, measured using the respective instrument accessory for liquid samples, exhibited dispersions with spans ranging between ~5 and ~100 μm with peaks between ~25 and ~40 μm. The particles observed in kefir when fermentation of milk is complete at pH values between 4 and 4.6, are composed of destabilized milk protein agglomerates, which, due to the occurring acidic conditions, retain their agglomerated form when dispersed in the liquid samples of the laser diffraction analyzer, resulting in the measurements reported herein. Figure 7 presents a representative particle size distribution of kefir, where trehalose at 2% (TREH 2%) concentration was added to facilitate its drying performance. As can be observed, the initial hydrated protein agglomerates, formed during the fermentative acidification of the fresh kefir, are shown to have greater particle sizes than the powder particles generated during spray drying, due to either agglomerate mechanical breakdown or shrinkage caused by dehydration. This agglomerate formation and subsequent size transitions, due to spray drying, are further confirmed during powder reconstitution, where smaller particle sizes are experienced in the rehydrated product compared to the respective fresh kefir. Furthermore, the particle distribution span increases during reconstitution, when compared to the powder, and shifts to slightly greater sizes.

3.5. Microbiological Properties

Lactococci populations (Figure 8) were determined at the initial fresh kefir (CONTROL) and the reconstituted products at the same solids concentration. As can be seen, in most different product formulations tested, a lactococci population decrease was experienced at maximum, reaching 0.6 log cfu/g (from 7.3 to 6.7) and in most cases being less than 0.4 log cfu/g. It is widely known that mechanical and thermal stress may cause significant cellular damage to living bacterial cells, DNA denaturation, and cell membrane breakdown due to dehydration, which eventually reduces their survival rates [12]. Based on the results of the present study, with regard to lactococci, this population reduction was not significant, considering that the product went through a harsh spraying and drying process. It is worth noting that the populations of lactococci for the reconstituted samples are close to the minimum limit of 10⁷ cfu/g that Codex Alimentarius [32] reports for fresh kefir products.

Yeast populations of the fresh kefir were at 10⁷ cfu/g, reducing post-drying 5 log cycles to 10² cfu/g, irrespective of the drying aids and their concentrations employed. The yeast population of reconstituted samples was lower when compared to the minimum limit of 10⁴ cfu/g that Codex Alimentarius [32] reports for fresh kefir products.

According to Atalar and Dervisoglu [11], the maximum survival rate for lactococci was 2.75 × 10⁻² cfu/g, while no yeasts were observed in kefir powder samples after spray drying (yeasts counts in the kefir samples were determined as 2.1 × 10⁵ to 6.5 × 10⁶). Nurwantoro et al. [9] reported 2.25 × 10² cfu/mL total LAB and 1.33 × 10⁴ yeasts for kefir powder samples produced with spray drying, but no initial populations were reported for comparison purposes. According to Setiyawan and co-workers [15], 2.87 × 10⁸ and 2.57 × 10⁷ were the total counts for LAB and yeasts on kefir, respectively, while 6.58 × 10⁵ and 2.34 × 10⁵ was their population on kefir powder without milk powder addition, and 1.94 × 10⁶ and 2.64 × 10⁷ was their population on kefir powder with 10% milk powder addition, respectively. Teijeiro et al. [12] reported that maltodextrin and whey proteins used as drying aids did not improve the survival rates of lactic acid bacteria and yeasts during drying.

3.6. Structure

The characteristic structural morphology of fermented dairy products is depicted for the kefir samples in Figure 9. The matrix of casein micelles together with the denatured whey proteins (blue color) can be seen, together with the embedded milk fat globules (red color). The surface area of the homogenized milk fat globules consists mostly of caseins that stabilize the dispersion, attributing at the same time an active role in the formation of the protein matrix, which based on our observations is not affected by the drying aids employed in the present study.

In Figure 10, the confocal microscope microphotographs of the powdered kefir products are shown. All different powder preparations, irrespective of the drying aid/encapsulation ingredient used and its concentration, presented similarly sized, well-formed, spherical powder particles, confirming the size distribution results presented herein. The spherical particles, typical of the materials dried in spray driers, based on the drying temperatures used, may present a shrunken surface due to usually low temperatures that induce reduced water mass transfer, allowing structural alterations to occur as the drying proceeds [33].

Observing the reconstituted kefir powder samples (Figure 11), it can be seen that the powder particles retain their mostly spherical shape, jacketed by the destabilized protein matrix with its embedded lipospheres (red colored). No differentiation was observed due to the presence of the different carrier ingredients used.

4. Conclusions

The produced kefir powder samples showed moisture contents close to those reported in the related literature on milk powders, with arabic gum and maltodextrin showing the lowest levels, confirming their validity as drying aids used in the industry. Spray drying increased the pH and reduced the acidity of the reconstituted samples compared to the fresh product, especially for the CONTROL, FOS, and TREH samples, irrespective of their concentration. Sample powder brightness was increased compared to the existing literature and based on our experience the best color characteristics were the result of fructo-oligosaccharide, trehalose, or maltodextrin, irrespective of their concentration in the product. All carriers were effective in improving the water solubility index of the powders compared to the CONTROL. Water adsorption increased with the environmental relative humidity and the presence of carriers caused a small increase compared to the CONTROL. Powder particle size was controlled by the spraying procedure, and it was not affected by any other process parameter. The lactococci populations were adequately preserved by the drying process employed, and WP at a 10% concentration, along with TREH at 4%, can further improve their survival. On the contrary, yeast populations decreased significantly during drying, with a 5-log cycle observed reduction. Powder particles were round and well-shaped, as expected by the specific drying procedure. Based on the findings of the present study, the use of trehalose as a carrier is promising for the production of high-quality kefir powder products.

The commercialization of kefir powder presents significant merit, considering the functional and nutritional benefits of the specific dairy product, especially in areas with problematic logistics and distribution services, and as a stand-alone food ingredient. The development of the process within the capabilities of the milk industry, with regard to equipment and know-how, is possible without requiring further significant investment, as presented by the current study and the related literature. Further work is required with regard to the long-term storage of the product, its packaging requirements, and the keeping quality of its functional characteristics.