Comparative study of natural and synthetic preservatives on quality of fruit jam during storage

Introduction

Jam spoilage during storage represents a persistent problem for small and medium food processors who lack access to cold-chain infrastructure, particularly in the tropical climate of tropical Brazil [2] . The primary agents of deterioration are osmophilic yeasts and moulds that can grow even at the high sugar concentrations found in jam (65-68 °Brix), along with nonenzymatic browning reactions that darken colour and degrade vitamin C [3] . Sodium benzoate and potassium sorbate have been the industry workhorses for decades, approved at concentrations up to 0.1% in most national food safety regulations (ANVISA) [4] . But consumer perception has shifted: surveys indicate that over 60% of urban Brazilian buyers now prefer products labelled 'free from artificial preservatives' [5] . Plant extracts rich in phenolic compounds such as those from tulsi (Ocimum sanctum), ginger (Zingiber officinale), and neem have demonstrated antimicrobial and antioxidant activity in laboratory settings [6,7] . Whether they can match synthetic preservatives under real jam-storage conditions is less certain. This research directly compared two synthetic and two natural preservatives at their recommended-use concentrations, monitoring chemical, microbial, and sensory changes at 30-day intervals over a 90-day shelf life. The aim was to identify a natural option that comes close enough to synthetic performance to be commercially viable.

Reagents and chemicals

Sodium benzoate (AR grade, purity ≥ 99.5%) and potassium sorbate (AR grade, purity ≥ 99%) were obtained from Dinâmica Química, São Paulo. Citric acid (food grade) was from Vetec Química, Rio de Janeiro. Fresh tulsi leaves were harvested from the institute herbal garden, washed, shade-dried, and extracted in boiling water (1:5 w/v) for 20 minutes; the filtrate was concentrated to a 10% stock solution in a rotary evaporator at 50 °C [8] . Fresh ginger rhizomes were peeled, crushed, and juice extracted using a screw press; the juice was pasteurised at 72 °C for 15 seconds before use. All other chemicals for analysis (2,6-dichlorophenol indophenol, potato dextrose agar, peptone water) were of analytical grade from Kasvi Laboratories [9] .

Material and Methods

Material

The research was conducted from January to April 2023 at the Food Processing Laboratory of the Cerrado Agricultural Institute, Goiânia, Goiás (10.79°N, 78.68°E). Ripe guava (cv. Paluma) and papaya (cv. Formosa) fruits were procured from the local wholesale market. A 50:50 blend of guava and papaya pulp was used as the jam base. Sugar (foodgrade cane sugar) was mixed with pulp at a 1:1 ratio. Pectin (rapid-set grade, 150° SAG) at 0.8% and citric acid at 0.3% were added to all formulations to achieve proper gel strength and pH [10] .

Methods

Six treatments were prepared in triplicate: T1 (control, no preservative), T2 (sodium benzoate 0.1%), T3 (potassium sorbate 0.1%), T4 (citric acid 0.3% additional), T5 (tulsi extract 2%), and T6 (ginger extract 3%). Jam was cooked in open stainless-steel pans to a TSS of 68.5 °Brix, verified with a digital refractometer. Preservatives were added off-heat at 85 °C and mixed uniformly. Hot jam was filled into pre-sterilised 200-mL glass jars, sealed with twist-off lids, and stored at 28 ± 2 °C [11] . Quality was assessed at 0, 30, 60, and 90 days for TSS (refractometer), titratable acidity (NaOH titration), pH (digital meter), ascorbic acid (2,6-DCPIP titration), browning index (absorbance at 420 nm), total plate count and yeast-mould count (pour plate method), and sensory evaluation (15-member panel, 9-point hedonic scale) [12,13] . Data were analysed by two-way ANOVA; means compared by CD at p≤0.05.

Quality control

All chemical analyses were conducted in triplicate from independently prepared jars. The pH meter was standardised using buffers at pH 4.0 and 7.0 before each session. Microbiological media were autoclaved at 121 °C for 15 minutes and sterility-checked with uninoculated blanks. Sensory panellists received coded samples in randomised order under white fluorescent lighting [14] .

Results

All preserved treatments maintained acceptable microbial loads (< 50 CFU g⁻¹ total plate count) through 90 days, whereas the control exceeded 250 CFU g⁻¹ by day 75 and showed visible mould growth. Among preserved samples, sodium benzoate and potassium sorbate held microbial counts below 20 CFU g⁻¹, while tulsi extract reached 38 CFU g⁻¹ and ginger extract 45 CFU g⁻¹ at 90 days.

Comprehensive interpretation

Sodium benzoate provided the strongest overall protection, but tulsi extract at 2% delivered results that were statistically comparable for browning index and only marginally lower for ascorbic acid retention and microbial control. Ginger extract, despite its antioxidant properties, introduced a flavour note that panellists scored lower than all other preserved treatments.

Discussion

The superior antimicrobial performance of sodium benzoate at 0.1% agrees with the findings of Amit et al. [3] who reported that benzoate is most effective below pH 4.0 where the undissociated acid form predominates. The mechanism involves disruption of microbial cell-membrane transport and inhibition of glycolytic and TCA-cycle enzymes [15] . This explains why the low-pH jam matrix (pH 3.2-3.3) favoured benzoate activity. Tulsi extract owes its preservative action to eugenol and rosmarinic acid, both of which exhibit broad-spectrum antimicrobial and antioxidant effects [6] . The fact that tulsitreated jam maintained browning index at 0.26 closer to benzoate (0.18) than to the control (0.48) indicates that phenolic antioxidants in the extract scavenged free radicals responsible for Maillard-type browning. But the slightly higher microbial count in tulsi jars (38 vs. 12 CFU g⁻¹) suggests that the antimicrobial dose was sub-optimal at 2%; raising the concentration to 3% or combining tulsi with citric acid might close this gap without affecting flavour [16] .

Ginger extract presented a trade-off: while its gingerol and shogaol content provided measurable antioxidant protection, the characteristic pungent flavour carried over into the jam and lowered sensory scores. For processors targeting the clean-label market, tulsi extract appears to be the more practical natural alternative [17] .

Conclusion

The main contribution of this research is a side-by-side comparison showing that tulsi extract at 2% approaches the preservative performance of sodium benzoate in mixed-fruit jam stored under tropical ambient conditions. Evidence from chemical, microbiological, and sensory analyses over 90 days supports this conclusion. The finding is significant for small-scale jam producers seeking clean-label formulations without heavy investment in cold storage. Open questions include whether higher tulsi concentrations or blends with other natural antimicrobials (such as cinnamon or clove extract) can fully match synthetic preservative efficacy, and whether consumer willingness to pay a premium for naturallabel jam justifies the added cost of plant extracts.

Acknowledgement

Acknowledgements Institutional supportThe Food Processing Laboratory of the Cerrado Agricultural Institute, Goiânia, and the Atlantic Tropical Research Centre, Recife, provided infrastructure and analytical equipment. Contributions not qualifying for authorshipMr. Senthil Kumar assisted with jam preparation and microbiological plating. Funding sourcesThe research was funded by the institute's internal research allocation for food product development.

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