Comparative Analysis

The choice of cooling water temperature in dairy plants significantly influences process efficiency, product quality, energy consumption, and economic efficiency. This study compares the thermodynamic efficiency, cooling rate, microbiological safety, product quality, and operating costs of cooling with ice water of 0.5°C and with cold water of 2°C. The analysis shows that ice water at a temperature of 0.5 °C enables milk cooling from 32 °C to 4 °C to be 30–50% faster due to the greater temperature difference in the heat exchanger, and that the target temperature can be achieved more effectively ^[1][2]. The faster cooling significantly reduces the generation time of mesophilic and psychrotrophic bacteria, lowers total bacterial counts by 15–25%, and extends shelf life by 20–30% ^[3][4][5]. Modern falling-film ice-water systems achieve higher COP values (4.5–5.5) at evaporation temperatures of T₀ ≈ -2 °C than conventional chilled-water systems at 2 °C (COP 3.5–4.2) and reduce specific energy consumption by up to 20% ^[1][2][6]. Despite 15–25% higher investment costs, ice water systems with a temperature of 0.5°C pay for themselves economically within 4–6 years due to lower energy costs, reduced product losses, and higher throughput rates ^[2][6].

In the modern dairy processing industry, the rapid and precise cooling of raw milk from approximately 32–35 °C to a maximum of 4 °C within four hours after milking is crucial for microbiological safety, shelf life, and product quality ^[3][4][7]. Conventional chilled water systems at 2 °C are widely used, but often achieve final milk temperatures of only 5–6 °C due to insufficient temperature differences in the plate heat exchanger ^[8][9].

Ice-water systems with temperatures of around 0.5 °C enable deeper and faster cooling to 2–4 °C, offering microbiological and quality benefits ^[1][2][10]. Modern falling-film ice water chillers operate at higher evaporation temperatures (T₀ ≈ −2 °C) than traditional systems and thereby achieve better energy efficiency ^[1][2][6].

The aim of this study is a systematic comparison of both cooling concepts, taking into account heat transfer rates, cooling times, microbiological effects, product quality, energy consumption, and overall economic efficiency.

The heat flow rate Q [W] in a counterflow plate heat exchanger is described by: 

$\dot{Q}=U\cdot A\cdot \mathrm{\Delta }{T}_{\text{log} }$

Where U is the overall heat transfer coefficient [W/(m²·K)], A is the heat exchanger surface area [m²], and ΔT_log is the logarithmic mean temperature difference [K] ^[11].

The logarithmic mean temperature difference is calculated as 

$\mathrm{\Delta }{T}_{\text{log}}=\frac{\mathrm{\Delta }{T}_1-\mathrm{\Delta }{T}_2}{\mathrm{l}\mathrm{n}(\mathrm{\Delta }{T}_1/\mathrm{\Delta }{T}_2) }$

Where ΔT₁ and ΔT₂ are the temperature differences at both ends of the heat exchanger.

Table 1: Comparison of milk cooling from 32 °C to target temperature

At the same overall heat transfer coefficient U and the same surface area A, the result is:

$\frac{{\dot{Q}}_{\text{Eis}}}{{\dot{Q}}_{\text{Kalt}}}=\frac{\mathrm{\Delta }{T}_{\text{log,Eis}}}{\mathrm{\Delta }{T}_{\text{log,Kalt}}}=\frac{\mathrm{12,8}}{\mathrm{11,2}}=\mathrm{1,14}$

Ice water enables a 14% higher thermal performance with the same exchanger size or achieves the same cooling capacity with a smaller surface area ^[1][2].

2.2 Approach Temperature

The approach temperature describes the smallest achievable difference between the milk outlet temperature and the cooling water inlet temperature in the heat exchanger. In efficient plate heat exchangers, this is 1–2 K ^[9][12].

Table 2: Achievable final milk temperatures at different cooling water temperatures

Only ice water at 0.5 °C enables reliable milk cooling to 2–3 °C, which offers significant microbiological and quality benefits ^[1][2][3].

2.3 Specific heat capacity of milk

The specific heat capacity of whole milk (3.5% fat) is 

$c_p=\mathrm{3,93} \mathrm{ }\text{kJ/(kg}\text{·}\text{K)}$

c_p is approximately 3.97 kJ/(kg·K) for skim milk ^[13].

The amount of heat to be removed when cooling 1,000 kg whole milk from 32 °C to 4 °C is thus

$Q=m\cdot c_p\cdot \mathrm{\Delta }T=1.000 \mathrm{ }\text{kg}\cdot \mathrm{3,93} \mathrm{ }\text{kJ/(kg}\text{·}\text{K)}\cdot (32-4) \mathrm{ }\text{K}=110.040 \mathrm{ }\text{kJ}\approx \mathrm{30,6} \mathrm{ }\text{kWh}$

The growth rate of bacteria in milk is highly temperature-dependent. Mesophilic bacteria (optimum 25–37 °C) already show significantly reduced growth at 10 °C, while psychrotrophic bacteria (Pseudomonas spp.) can grow even at 4–7 °C ^{[3][4][5][14]}.

Table 3: Generation times of various bacterial groups in milk ^[3][4][14]

Critical time frame: The first 4 hours after milking are crucial, as this is when the bacterial lag phase ends and exponential growth begins ^[3][4][7]. The faster the milk is cooled to below 4 °C, the lower the total bacterial count.

3.2 Comparison of bacterial count development

Scenario: 10,000 liters of raw milk, initial bacterial count 10,000 CFU/ml (CFU = colony-forming unit)

2 °C cold water system:

• Cooling from 32 °C to 5.5 °C in 45 minutes
• Storage at 5.5 °C for 48 hours
• Final bacterial count: approx. 35,000–45,000 CFU/ml ^[4][5]

0.5 °C ice water system:

• Cooling from 32 °C to 3.0 °C in 30 minutes
• Storage at 3.0 °C for 48 hours
• Final bacterial count: approx. 25,000–32,000 CFU/ml ^[4][5]

Reduction of the bacterial count by 20–30% through ice water cooling, which directly increases shelf life and product safety ^[3][4][5].

3.3 Qualitative Effects

Lower storage temperatures reduce:

• Lipolysis (fat breakdown by bacterial lipases) → better taste ^[4][15]
• Proteolysis (protein breakdown) → longer shelf life of cheese and yogurt ^[4][15]
• Sensory defects (rancidity, bitterness) ^[4][5]

Studies show that in milk stored at 2 °C , the sensory quality is still excellent after 5 days, whereas in milk stored at 6 °C, quality losses already occur after 3 days ^[4][5][15].

The coefficient of performance (COP) of a chiller describes the ratio of cooling capacity to electrical power consumption: 

$\text{COP}=\frac{{\dot{Q}}_0}{P_{\text{el} }}$

The COP increases with higher evaporation temperatures T₀. Modern falling-film ice-water systems achieve higher COP values at T₀ ≈ −2 °C for 0.5 °C ice water than conventional systems with lower evaporation temperatures ^[1][2][6].

Table 4: Comparison of COP values for different systems

Falling film ice water systems achieve COP values up to 20–30% higher due to optimized evaporation temperatures ^[1][2][6].

4.2 Calculation example: Energy consumption for cooling of  10,000 liters milk

Input data:

• Milk volume: 10,000 liters (≈ 10,300 kg, density 1.03 kg/ l)
• Cooling from 32 °C to target temperature
• Specific heat capacity: 3.93 kJ/(kg·K)

2 °C cold water system (target temperature 5.5 °C): 

$Q=10.300\cdot \mathrm{3,93}\cdot (32-\mathrm{5,5})=1.071.000 \mathrm{ }\text{kJ}=\mathrm{297,5} \mathrm{ }\text{kWh}$

At COP = 3,8: 

$P_{\text{el}}=\frac{\mathrm{297,5}}{\mathrm{3,8}}=\mathrm{78,3} \mathrm{ }\text{kWh}$

0.5 °C chilled water system (target temperature 3.0 °C): 

$Q=10.300\cdot \mathrm{3,93}\cdot (32-\mathrm{3,0})=1.174.000 \mathrm{ }\text{kJ}=\mathrm{326,1} \mathrm{ }\text{kWh}$

At COP = 5,0: 

$P_{\text{el}}=\frac{\mathrm{326,1}}{\mathrm{5,0}}=\mathrm{65,2} \mathrm{ }\text{kWh}$

Energy savings despite lower cooling: 

$\text{Savings}=\mathrm{78,3}-\mathrm{65,2}=\mathrm{13,1} \mathrm{ }\text{kWh} \mathrm{ }(\mathrm{16,7} \mathrm{ }\mathrm{\%})$

The savings amount to 16.7% in electricity despite a final temperature that is 2.5 K lower ^[1][2][6].

4.3 Annual operating costs

Assumptions:

• Dairy processes 50,000 liters/day = 18.25 million liters/year
• Electricity price: 0.13 EUR/kWh
• Operates 350 days/year

Table 5: Annual energy costs for milk cooling (50,000 l/day)

Annual energy cost savings: EUR 3,120 (16.8%) ^[1][2][6].

Table 6: Comparison of investment costs for 200 kW cooling capacity

Additional investment in chilled water: EUR 47,300 (36%) ^[2][6][17].

5.2 Operating Cost Comparison (Annual)

Table 7: Annual operating costs

Additional costs for chilled water: EUR 2,600/year (excluding quality benefits) ^[6][16].

5.3 Quality and throughput benefits

Reduction in product losses:

• 2 °C system: 1.5% loss due to reduced shelf life and quality defects
• 0.5 °C system: 0.8% loss
• At 18.25 million liters/year at EUR 0.72/liter raw material value:

$\text{Savings}=\mathrm{(1,5\%}-\mathrm{0,8\%)}\cdot \mathrm{18.250.000}\cdot \mathrm{0,72}=\text{EUR} \mathrm{ }\mathrm{92.000/year} \mathrm{ }$

Higher product quality enables premium prices:
For 20% of production (3.65 million L) with a 2% price premium:

$\text{Additional profit}=\mathrm{3.650.000}\cdot \mathrm{0,72}\cdot \mathrm{0,02}=\text{EUR} \mathrm{ }\mathrm{52.600/year} \mathrm{ }$

Faster cooling increases throughput:
30% faster cooling cycles → 10% higher daily throughput possible → additional contribution margin of EUR 16,500/year (conservative) ^[2][6].

5.4 Overall Economic Assessment

Table 8: Overall economic balance sheet per year

Payback period:

$\text{Payback}=\frac{\text{Aditional investment}}{\text{Net benefit/year}}=\frac{47.300}{158.500}=\mathrm{0,30} \mathrm{ }\text{Jahre}\approx \mathrm{3,6} \mathrm{ }\text{Monate}$

Even with a conservative calculation that takes quality benefits into account but excludes premium revenue and throughput benefits:

${\text{Payback}}_{\text{conservative}}=\frac{47.300}{92.000-2.600}=\mathrm{0,53} \mathrm{ }\text{Jahre}\approx \mathrm{6,4} \mathrm{ }\text{Monate}$

In practice, the payback period is typically 4–6 years for medium-sized operations without extreme quality benefits ^[2][6].

Modern falling-film ice water systems distribute water evenly over vertical stainless steel pillow plates, in which refrigerant (typically NH₃) evaporates at T₀ ≈ −2 °C ^[1][2]. 

Advantages:

• Minimal risk of icing due to even distribution
• High overall heat transfer coefficient (up to 2,000 W/ m² K ) ^[2][10]
• Low refrigerant charge (often below legal limits)
• Easy cleaning and maintenance
• Resistance to fouling

6.2 Ice water storage tanks for peak load management

A 20 m³ ice water storage tank enables:

• Nighttime production of chilled water using low-cost off-peak electricity
• Coverage of peak loads in the morning without oversizing the compressor
• Smoothing of the electrical load
• Emergency cooling in the event of compressor failure

Storage capacity at a 5 K temperature spread (0.5 → 5.5 °C):

$Q_{\text{Storage}}=V\cdot \rho \cdot c_p\cdot \mathrm{\Delta }T=20\cdot 1.000\cdot \mathrm{4,18}\cdot 5=418.000 \mathrm{ }\text{kJ}=116 \mathrm{ }\text{kWh}$

This covers approximately 3–4 hours of peak load ^[2][17].

6.3 Integration with heat recovery

The waste heat from the chiller (condensation heat) can be used for hot water (CIP cleaning, 65–85 °C) or heating ^[18]:

${\dot{Q}}_{\text{Capacitor}}={\dot{Q}}_0+P_{\text{el}}=200+40=240 \mathrm{ }\text{kW}$

At 6,000 operating hours/year: 1,440 MWh/year of heat available, equivalent value at 0.09 EUR/kWh ≈ EUR 130,000/year ^[18].

0.5 °C ice water offers three thermodynamic advantages:

• Greater temperature difference in the heat exchanger (ΔT_log 12.8 K vs. 11.2 K) → 14% higher heat output ^[1][2]
• Lower product outlet temperature (3 °C vs. 5.5 °C) due to better approach ^[1][9]
• Higher COP (5.0 vs. 3.8) due to optimized evaporation temperature in falling-film technology ^[1][2][6]

7.2 Microbiological and Quality Benefits

Reducing the storage temperature from 5.5 °C to 3.0 °C:

• Extended generation time of psychrotrophic bacteria by 30–50% ^[3][4]
• Total plate count reduced by 20–30% after 48 hours ^[4][5]
• Extended sensory shelf life by 1–2 days ^[4][15]
• Reduced enzymatic degradation (lipolysis, proteolysis) ^[4][15]

7.3 Economic and operational advantages for medium to large-scale operations

The cost-effectiveness of ice water increases with:

• Operational scale: Significant benefits starting at 20,000 l/day ^[2][6]
• Quality requirements: Premium segments justify the investment ^[4][15]
• Energy prices: The higher the prices, the faster the payback ^[6][16]
• Product mix: Fresh milk, yogurt, and cheese benefit particularly ^[4][15]

For small operations (<10,000 l/day) with simple products, 2 °C cold water may be sufficient ^[6].

7.4 Technological Trends

Modern developments increase the appeal of ice water:

• Falling film technology with low refrigerant charge ^[1][2]
• High-efficiency natural refrigerants (NH₃, CO₂) ^[1][2]
• Smart load management systems for off-peak use ^[17]
• Integration with heat recovery and combined heat and power generation ^[18]

Ice water cooling with 0.5 °C cold water significantly outperforms cold water cooling with 2 °C cold water in dairy processing plants in terms of cooling speed (30–50% faster), product quality (20–30% lower bacterial counts, 1–2 days longer shelf life) and energy efficiency (15–20% lower specific energy costs despite a lower final temperature) ^{[1][2][3][4][6]}. Modern falling-film chilled water systems achieve COP values of 4.5–5.5 at evaporation temperatures around −2 °C and enable final milk temperatures of 2– –3 °C, which conventional chilled water systems with 2 °C (COP 3.5– –4.2, final milk temperature 5– –6 °C) cannot achieve ^[1][2][9].

Despite 35 to 40% higher investment costs, ice water systems with 0.5 °C pay for themselves in medium to large dairy operations (>20,000 l/day) due to lower energy costs (EUR 3,100/year savings at 50,000 l/day), reduced product losses (EUR 90,000+/year due to improved quality), and higher throughput rates, the system pays for itself within 4–6 years ^[2][6]. For facilities with high quality requirements, premium products, or additional heat recovery, the payback period is reduced to 2–3 years.

For time-sensitive products with high quality standards (fresh milk, fermented milk products, organic milk) and sufficient processing volume, ice water cooling with 0.5 °C represents the thermodynamically, microbiologically, qualitatively, and economically superior technology. Cooling with cold water at 2°C remains a more cost-effective alternative for small businesses with low quality requirements and simple products.

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