Optimization of ohmic heating parameters for rapid pasteurization of sugarcane juice

Introduction

Since colonial times, sugarcane juice (caldo de cana) has been pressed and sold fresh at Brazilian street stalls [1] . Brazil produces over 720 million tonnes of sugarcane yearly, and street vendors sell the fresh juice within minutes of pressing [2] . But its high water activity supports rapid microbial growth, exceeding 10⁶ CFU/mL within 2-3 hours at ambient temperature [3] . Conventional pasteurization (72 °C, 15 s) extends shelf life but causes Maillard browning that dulls the juice's pale green colour and reduces consumer acceptance [4, 5] . Ohmic heating offers an alternative: an alternating current passes through the juice, converting electrical resistance into heat uniformly throughout the volume, with heating rates of 1-5 °C/s [6,7] . Earlier work on orange and pomegranate juices showed 10-20% better ascorbic acid retention than conventional methods [8] . For sugarcane juice, only two reports exist, both testing a single voltage gradient [9,10] . No factorial exploration of voltage gradient, electrode spacing, and juice concentration has been reported a gap this research addressed.

Material and Methods

System design

A static ohmic heating cell was fabricated in the departmental workshop at Viçosa. The cell comprised a food-grade acrylic cylinder (inner diameter 50 mm, length adjustable from 20 to 60 mm via spacer rings) fitted with two titanium-coated stainless steel electrodes at each end. A T-type thermocouple (accuracy ±0.5 °C) was inserted through a side port at the midpoint. Electrodes were connected to a variable-voltage AC power supply (0-230 V, 50 Hz) through a digital wattmeter for real-time power monitoring. Voltage and current data were logged at 1-second intervals using a USB data acquisition module (NI USB-6009).

Material

Fresh sugarcane (variety RB867515) was procured from the university research farm at Viçosa, Minas Gerais, during April-October 2023. Juice was extracted using a motorized three-roller mill, filtered through muslin cloth, and used within 30 minutes. Initial juice properties: pH 5.2 ± 0.1, total soluble solids 16.4 ± 1.2 °Brix, electrical conductivity 0.34 ± 0.02 S/m at 25 °C. All microbiological media (plate count agar, potato dextrose agar) were from Kasvi Laboratórios, São José dos Pinhais.

Methods

A 4 × 3 × 3 factorial design tested voltage gradient (20, 30, 40, 50 V/cm), electrode gap (2, 4, 6 cm), and juice concentration (14, 16, 20 °Brix), each combination replicated three times. For each run, 150 mL juice was loaded into the cell, heated to the target temperature of 72 °C, held for 15 s, then immediately cooled in an ice bath. A water-bath control heated the same volume to 72 °C/15 s for comparison. Electrical conductivity was calculated from measured voltage and current using σ = (I × L) / (V × A), where L = electrode gap and A = electrode area [6] . Colour was measured using a Hunter Lab colorimeter (L*, a*, b*); total colour difference ΔE was calculated relative to fresh juice. Ascorbic acid was determined by the 2,6dichlorophenolindophenol titration method [11] . Total plate count and yeast-and-mould count were enumerated by pourplate technique on PCA and PDA respectively after 48 h incubation at 37 °C and 28 °C [12] . Data were analysed by three-way ANOVA; means separated by Tukey HSD at p<0.05 using R (v4.3.1). System performance index was calculated as SPI = (log reduction × ascorbic acid retention) / (ΔE × energy input), following the approach of Icier and Ilicali [13] .

Results

Across all 108 experimental runs, the time to reach 72 °C ranged from 11 seconds (50 V/cm, 2 cm gap, 20 °Brix) to 58 seconds (20 V/cm, 6 cm gap, 14 °Brix). Three-way ANOVA confirmed that voltage gradient was the dominant factor (p<0.001) for come-up time, microbial reduction, and colour change, while electrode gap and concentration showed significant but smaller effects. Voltage gradient controlled the heating rate almost linearly: doubling it from 20 to 40 V/cm halved the come-up time from 38 to 18 seconds at the 4 cm / 16 °Brix setting. This speed advantage directly preserved quality colour change at 40 V/cm (ΔE = 2.3) was 62% lower than the water-bath control (ΔE = 6.1), and ascorbic acid retention was 11 percentage points higher (87.4% vs 76.1%). At 50 V/cm, however, electrode corrosion became visible after 15 runs, and localised gas bubbles near the electrode surface disrupted uniform heating. The 40 V/cm setting therefore offered the best balance between speed, quality, and equipment durability. Electrical conductivity rose linearly with temperature (R² = 0.98), which is favourable because it means the system self-regulates: as juice heats, it conducts better, drawing more current and accelerating heating in a positive feedback loop that is nonetheless predictable and controllable [6] .

Discussion

The central result is that ohmic heating at 40 V/cm pasteurizes sugarcane juice in 18 seconds with less colour and vitamin loss than a water bath, while achieving a higher microbial kill (4.6 vs 3.7 log TPC reduction). The linear conductivity-temperature relationship (R² = 0.98) matches values reported by Icier and Ilicali [13] and Darvishi et al. [14] for fruit juices, confirming that sugarcane juice behaves as a well-behaved ohmic medium.

One unexpected finding was that 20 °Brix juice heated 8% slower than 16 °Brix at the same voltage, contrary to the prediction by Palaniappan and Sastry [6] that higher solids raise conductivity and speed heating. Higher viscosity likely suppressed natural convection in the static cell, creating stagnant electrode layers an effect that forced convection in a continuous-flow system would eliminate. For practical adoption, the energy cost is negligible: 34.2 kJ/L equals roughly 0.009 kWh. A portable ohmic unit with a timer and auto-shutoff could make rapid pasteurization accessible to street vendors without cold-chain infrastructure [16] .

Conclusion

This research aimed to find an effective ohmic heating protocol for rapid pasteurization of sugarcane juice. A voltage gradient of 40 V/cm with a 4 cm electrode gap and 16 °Brix concentration reached 72 °C in 18 seconds and delivered a 4.6 log TPC reduction, with colour change (ΔE = 2.3) and ascorbic acid retention (87.4%) both superior to conventional treatment. For vendors operating in ambient conditions, a portable ohmic unit based on these parameters could extend shelf life from 2-3 to 6-8 hours without browning. Future work should scale to continuous-flow design, test electrode durability beyond 500 cycles, and evaluate consumer acceptance.

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