# An overview of selected “subzero” temperature storage regimes of foods: regulations and perspectives about the expected impact of the ”three degrees of change” initiative on the shelf life of frozen foods

Alain Le-Bail, Nasser Hamdami, Cyril Toublanc, Michel Havet

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## To cite this version:

Alain Le-Bail, Nasser Hamdami, Cyril Toublanc, Michel Havet. An overview of selected “subzero” temperature storage regimes of foods: regulations and perspectives about the expected impact of the ”three degrees of change” initiative on the shelf life of frozen foods. International Journal of Refrigeration, 2025, 176, pp.336-344. 10.1016/j.ijrefrig.2025.04.009. hal-05167763

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# An overview of selected “subzero” temperature storage regimes of foods: regulations and perspectives about the expected impact of the "three degrees of change" initiative on the shelf life of frozen foods

Alain Le-Bail* ID, Nasser Hamdami ID, Cyril Toublanc ID, Michel Havet ID

Oniris, Nantes Université, CNRS, GEPEA, UMR 6144, F-44000 Nantes, France

## ARTICLE INFO

### Keywords:

- Food freezing

- Quality

- HQL PSL

- Frozen storage

## ABSTRACT

Frozen storage temperature plays a crucial role in preserving the initial quality of frozen foods. The most used storage temperature of frozen food is -18°C and is based on 0°F, which corresponds to -17.8°C. This temperature does not correlate with a specific ratio of frozen water in foods ("level of frozenness"-LOF) and has been arbitrarily chosen. The fact is that at -18°C, most foods never reach a 100% LOF (usually in the range of 95% LOF). The 100% LOF refers to a state in which the maximum ice crystallization zone has been covered (usually around -30°C). New low-temperature storage modes, such as chilling, superchilling, and supercooling, gained attention in academia and in the food industry, pushing the boundaries of existing regulations. The recent "Three Degrees if Change" initiative launched by the University of Birmingham (UK) aims at exploring the interest and impact of changing the frozen food storage temperature from -18°C to -15°C. This paper proposes a review of the current regulations in connection with selected subzero temperature storages. Based on the principle of the Q₁₀ models, a Q3 model is proposed showing that frozen foods stored at -15°C may lose around 30% of their quality life from the sensorial point of view. Raising the temperature to -15°C should not pose a major microbial risk; however, this point must be under scrutiny. In particular, the permitted temperature oscillations during transport and at the retail outlet may require some adjustment.

## NOMENCLATURE (DL = Dimension Less)

<table><thead><tr><th>Value</th><th>Unit</th><th>Definition</th></tr></thead><tbody><tr><td>A</td><td>°C<sup>-1</sup></td><td>Coefficient of the PSL or HQL models</td></tr><tr><td>B</td><td>DL</td><td>Coefficient of the PSL or HQL models</td></tr><tr><td>HQL</td><td>time</td><td>High Quality Life of a food during storage (usual unit is days)</td></tr><tr><td>LOF</td><td>%</td><td>Level of frozenness representing the percentage of “free”, “freezable” water able to form ice effectively frozen at a given temperature</td></tr><tr><td>PSL</td><td>time</td><td>Practical Storage Life of a food during storage (usual unit is days)</td></tr><tr><td>Q<sub>10</sub></td><td>DL</td><td>Ratio of a rate (ie quality change) of a food at T + 10°C divided by the said rate at T°C</td></tr><tr><td>Q<sub>10</sub></td><td>DL</td><td>Ratio of a shelf life time of a food at T°C divided by the said rate at T + 10°C</td></tr><tr><td>Q<sub>3</sub></td><td>DL</td><td>Ratio of a shelf life time of a food at T°C divided by the said rate at T + 3°C</td></tr></tbody></table>

*(continued on next column)*

*(continued)*

<table><thead><tr><th>Value</th><th>Unit</th><th>Definition</th></tr></thead><tbody><tr><td>Q<sub>-3</sub></td><td>DL</td><td>Ratio of a shelf life time of a food at T + 3°C divided by the said shelf life at T°C</td></tr><tr><td>T<sub>IFP</sub></td><td>°C</td><td>Initial Freezing Point (IFP) temperature</td></tr><tr><td>T<sub>product</sub></td><td>°C</td><td>Temperature of the product = food temperature</td></tr><tr><td>T<sub>m</sub></td><td>°C</td><td>Maximum ice concentration temperature at which the LOF is 100 %</td></tr><tr><td>X<sub>ice</sub></td><td>DL</td><td>Mass fraction of frozen water per kg of freezable water. This mass fraction is sometimes expressed per mass of total water or per mass of food</td></tr><tr><td>X<sub>freezable water</sub></td><td>DL</td><td>Mass fraction of “freezable” water (water able to form ice) per kg of food</td></tr></tbody></table>

**Note:** Prof LE-BAIL is president of the Commission C2 of the International Institute of Refrigeration, Cyril TOUBLANC and Michel HAVET are members of the Commission C2.

* Corresponding author.

E-mail addresses: alain.lebail@oniris-nantes.fr, alainlebailpro@gmail.com (A. Le-Bail).

https://doi.org/10.1016/j.ijrefrig.2025.04.009

Received 18 November 2024; Received in revised form 29 March 2025; Accepted 10 April 2025

Available online 17 April 2025

0140-7007/© 2025 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
# 1. Introduction

This publication addresses two major objectives. At first, it aims to shine a light on the different subzero temperature storage methods below 0°C such as chilling, superchilling, supercooling and stiffening (which is used to facilitate the slicing of some foods), and to see how the existing regulation addresses these subzero temperature methods. After a rapid reminder about the concept of the level of frozenness (LOF) and how the LOF can be considered to state if a food is frozen or not, a discussion is proposed to benchmark the position of the regulations and the technologies used in the cold chain of foods.

A second objective concerns a recent initiative called “three degrees of change” launched in 2023 (Allouche et al., 2023), which proposes to explore the interest of changing the storage temperature of frozen foods from -18°C to -15°C for the sake of energy savings. It is worth mentioning that the same report states that “The impact on food quality (of frozen foods) of increasing by 3 degrees is not yet well understood; scientific evidence on the impact of increasing the set-point temperature is crucial”. It is thus essential to consider the potential impact on product quality and food safety. Increasing the storage temperature of frozen foods can accelerate product deterioration and heighten the risk of microbial growth as outlined in the results and analysis proposed in this study. To address this point, different models corresponding to a range of food products are exploited in this paper to assess the percentage of reduction of the shelf life in the case of a “three degrees of change” frozen regime. Frozen foods are reputed to be safe from the microbial point of view with a “non-existent” risk of microbial growth during frozen storage. However, considering a frozen storage at -15°C and considering that temperature oscillations currently permitted during frozen storage at -18°C (+3°C during transport and +6°C during retail) are adopted, the risk of microbial growth may become effective. This point is also discussed based on the few references existing in this temperature domain.

The food cold chain is clearly energy demanding. A study of the top ten food refrigeration processes (such as retail, catering, transport, storage, and primary chilling) identified the retail and catering sectors as the largest energy consumers (approximately 90%), with the greatest potential for energy savings (James and James, 2004). Simple measures, such as replacing energy-inefficient cabinets and performing regular maintenance on refrigeration systems, can significantly reduce energy consumption. In addition, any attempt enabling some energy savings such as the three degrees of change initiative is worth investigating.

We think that the material and information contained in this paper will be of interest for the different stakeholders, with the food industry particularly in mind, which may be interested in a document including both an update about the existing regulations and a connection between the regulations and the recent evolution and future trends of the cold chain.

# 2. The level of frozenness

The determination of the degree of freezing is often based on the amount of “freezable water” that is effectively frozen; this value comes as a ratio which can be based on the mass of free water (able to be converted to ice during freezing), out of the total mass of water or again on the mass of food. In order to simplify this point, we propose to adopt the “level of frozenness” (LOF) which is a more obvious concept. The Level of Frozenness (LOF) refers to the percentage of free water in a food product that is frozen when the product is exposed to temperatures below its initial freezing point (TIFP). The LOF can be calculated as follows:

$$LOF = \frac{X_{ice}}{X_{freezable water}} \qquad (1)$$

where Xice and Xfreezable water are the ice ratio and the overall freezable

water fraction in the food respectively. A LOF of 100% indicates that all the freezable water has crystallized. This is obtained when the T'm temperature is reached, which corresponds to the end of the freezing process (Roos, 1995). This temperature is determined through a calorimetric method and typically ranges between -30°C and -40°C for most foods (Le-Bail and Jha, 2019, Fikiin, 1998).

$$LOF = \left( 1 - \frac{T_{IFP}}{T_{product}} \right) / \left( 1 - \frac{T_{IFP}}{T_{m}} \right) \qquad (2)$$

where Tproduct and TIFP are the product and the product's initial freezing point, respectively. The determination of TIFP has been investigated by Le-Bail & al. (2022) for more information.

# 3. Regulations about frozen foods

According to the EU Council Directive 89/108/EEC (21 December 1988), which standardizes the laws of Member States relating to quick-frozen foodstuffs for human consumption:

* Frozen foods must be rapidly frozen, passing through the "maximum crystallization zone" quickly to reach a final storage temperature of -18°C.

* Frozen foods must be stored at a temperature of not more than -18°C in all parts of the product, possibly with short upward fluctuations of not >3°C.

The US regulation about frozen foods clearly establish that frozen foods should be stored at -18°C, which is the global standard for frozen storage all over the world. An exception was made in Japan, where the frozen food storage temperature was -25°C until 2023, but this was recently revised to -18°C to save energy. Therefore, no alternative temperature is allowed for long-term storage of food below its initial freezing point.

**The concept of “frozen food”**: The definition of a “frozen food” is still vague in the literature. The main consensus lies in reaching the temperature of -18°C, despite the fact that at this temperature most foods do not reach a LOF of 100% (usually in the range of 95%). The red Book-IIR (2006), published by the International Institute of Refrigeration, provides guidelines on the processing and handling of frozen foods. According to this reference, a food product is considered a “frozen food” when its temperature reaches -10°C or lower, or when a significant portion of the freezable water in the food has been converted into ice (typically over 80% of the water content). The percentage of 80% of LOF or the temperature of -10°C (considering the lowest of the two corresponding temperatures as proposed by the IIR) are indicative values; however, they represent a clear definition in comparison to other documents mentioning that the “maximal ice crystallisation zone must be passed” to consider that a food is “frozen”.

For example, let's consider a commercially smoked salmon with a TIFP of -2.2°C (as measured by Le-Bail et al., 2022) and a T'm of -30°C (determined by calorimetry). The LOF equation plotted in Fig. 1 shows that the percentage of frozen water in smoked salmon is 84.2% at -10°C and 94.7% at -18°C. Therefore, although smoked salmon at -10°C can be considered a frozen food (LOF > 80%), it must be stored at the standard temperature of -18°C. The LOF is thus a useful tool to determine whether a food is “frozen” when leaving the freezing process; in any case, it must be stored subsequently at -18°C.

# 4. Exceptions to food storage at -18°C in the case of negative temperatures

In recent years, various low-temperature food storage techniques have been developed as proposed in Fig. 2. The concept of superchilling, first proposed by Le Danois (1920), and more recently chilling (within a
<figure><img src="image_4.png" /><figcaption>Fig. 1. Level of frozenness of smoked salmon, considering an initial freezing point of -2.2°C and a final freezing temperature of -30°C (Data from Prof. Alain Le-Bail, ONIRIS, Nantes, France).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. Scheme of different low-temperature storage regimes for foods. The standard storage temperature for frozen food is -18°C worldwide.</figcaption></figure>

few degrees of the freezing point) have gained attention. Supercooling, where food is kept below its T<sub>IFP</sub> without freezing, is an unstable condition but has been achieved with new technologies, such as oscillating magnetic fields (Jun et al., 2018; You et al., 2020a, 2020b).

## 4.1. Exceptions to conventional frozen storage of foods: mechanical slicing

A specific exception to the standard frozen storage conditions involves the "stiffening" process used for mechanical slicing of certain foods, such as salted or smoked fish. This process, which involves partial freezing, allows for easier slicing and is recognized under European regulations. According to a 2013 technical note from the French Ministry of Agriculture (DGAL, 2013) and a 2014 update (DGAL, 2014), stiffening is recognized for fish (e.g., smoked fish) to facilitate slicing, but it must not be confused with full freezing.

* The process is validated for smoked and/or salted fish, with the core temperature typically reaching -7°C to -14°C.

* Stiffened products may be stored for up to 48 h, or up to 72 h on public holidays, and up to 96 h in exceptional cases at the target temperature (usually around -10°C).

* Stiffening is not considered a conservation stage and does not allow for commercial sale or transfer.

The FAO and WHO (2020) also mention that cold tempering (partial freezing at -5°C to -12°C) is used to stabilize fish flesh for mechanical slicing, but this process is short-lived and does not constitute a full freezing process. Thus, stiffening is seen as a temporary superchilling process, used for a short period (typically <24 h).

## 4.2. Exceptions to conventional frozen storage of foods: superchilling

During frozen storage, the ice fraction or level of frozenness tends to increase over time. This is due to the gradual migration of water toward the ice crystals formed during the initial freezing process. Kaale et al. (2013) investigated the case of fresh salmon stored under superchilling conditions at -1.7°C for 28 days. The initial freezing point (IFP) of fresh salmon was around -1°C, and this storage temperature corresponds to an LOF of 20 %, according to the authors. Histological images showed significant changes in the size of ice crystals during the first few days of storage, with noticeable coarsening. This observation supports the idea that superchilling should be limited to small excursions below the IFP. More recently, Claussen (2016, 2017) presented a comprehensive report demonstrating the advantages of superchilling technology for fish transport. The use of superchilling for fish transport has now been adopted by the European Union, as outlined in the EU Regulation 2022/2258 amending and correcting Annex III to Regulation (EC) No 853/2004 of the European Parliament and of the Council on specific hygiene requirements for food of animal origin as regards fishery products, eggs and certain highly refined products, and amending Commission Delegated Regulation (EU) 2019/624 as regards certain bivalve molluscs. This regulation mentions that:

> "On 10 December 2020, EFSA adopted a Scientific Opinion on the use of the so-called 'superchilling' technique for the transport of fresh fishery products. In that Opinion, EFSA made a comparison between the temperature of superchilled fresh fishery products in boxes without ice, and products subject to the currently authorised practice in boxes with ice. EFSA concluded that, under appropriate conditions, there are no differences from a public health point of view between the traditional transport temperatures and the superchilling techniques. As regards analytical methods capable of detecting whether a previously frozen fish is commercially presented as superchilled, EFSA identified five methods that could be considered as fit for purpose. The use of the technique of superchilling should therefore be allowed under certain conditions for the transport of fresh fishery products referred to in point 1 of Chapter VIII of Section VIII of Annex III to Regulation (EC) No 853/2004.

## 5. Impact of shifting the frozen storage temperature from -18°C to -15°C on frozen food quality

The primary motivation for changing the reference storage
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Impact of ± 3°C temperature oscillation during storage on the percentage of frozen water in a food with T<sub>IFP</sub> of –2.2°C (TOP) and T<sub>IFP</sub> of –1°C (BOTTOM) in the case of a storage at –18°C and at –15°C. The span in terms of percentage of frozen water is larger for the lower T<sub>IFP</sub>.</figcaption></figure>

temperature for frozen foods from –18°C to –15°C is the potential reduction in energy consumption according to Allouche et al. (2023). According to the same author mentioning Ritchie et al. (2023), the energy savings by a shift from –18°C to –15°C are between 5–7 % with some stages of the frozen food cold-chain which could realise savings as high as 10–12 % resulting in a worldwide energy saving of ~25 TWh/yr. The report also states that increasing the temperature of frozen foods by 3°C would save 17.7 million tons of CO₂ equivalent per year, which is the same as the carbon emissions from nearly 4 million cars annually.

Shifting the frozen storage temperature by 3°C impacts the per-
centage of frozen water in foods, especially when there is a ± 3°C
fluctuation during storage (in practice, an upward fluctuation of +3°C is
considered in existing regulations). This effect is illustrated in the Fig. 3
below, showing that the span of frozen water content will be broader for
foods with a T<sub>IFP</sub> of –2.2°C (e.g. smoked salmon) compared to foods
with a T<sub>IFP</sub> of –1°C (e.g. most foods).

The impact on food quality remains uncertain, as the report men-
tions, "The impact of moving 3 degrees on food quality (for frozen foods)

is not yet well understood. Scientific evidence regarding the effect of
increasing the set-point temperature is a determining factor.” However,
a considerable amount of data is available regarding the relationship
between time, temperature, and frozen food quality. These data were
used to assess the potential impact of a 3°C increase on the shelf life of
frozen foods in terms of consumer acceptance. Microbial risk is also an
important factor to consider in this context.

5.1. *Microbial risk in the case of –15°C frozen storage*

While microbial growth is generally considered negligible during
frozen storage at -18°C; the USDA (2025) page about frozen foods
mentions that "Freezer storage is for quality only. Frozen foods remain
safe indefinitely" and also that "Freezing to 0°F inactivates any microbes
— bacteria, yeasts and molds — present in food. Once thawed, however,
these microbes can again become active, multiplying under the right
conditions to levels that can lead to foodborne illness". This assumption
needs to be reconsidered if frozen food storage temperature is raised to
-15°C. A range of microorganisms, including psychrophilic bacteria, can contaminate food. Psychrophiles thrive at temperatures between -10°C and +10°C to 15°C, with optimal growth occurring between 0°C and 15°C. Some psychrophilic microorganisms can even grow at sub-zero temperatures.

The International Institute of Refrigeration conducted an internal study (unpublished) to investigate the origins of the choice of the -18°C threshold and the temperature at which microorganisms are completely inhibited. The current consensus is that -10°C is considered a threshold below which microbial growth is effectively halted.

Freezing can cause detrimental effects on microbial cells due to the concentration of intra- and extracellular solutes, resulting in exposure to toxic conditions and cell dehydration. Internal ice formation can also lead to mechanical damage to the cell wall. Inactivation of microorganisms can occur both during freezing and while stored frozen, as described by Schmidt-Lorenz (1976). Gram-positive microorganisms are generally more resistant to freezing than gram-negative ones, according to Erickson & Hung (1997).

Freezing reduces the water activity ($a_w$) in food, as described by Corry (1987) and Le-Bail & Jha (2019). Microbial growth is only possible if microorganisms are capable of growing in the water activity corresponding to the storage temperature, according to Scott (1962). At -15°C, the water activity of ice is 0.864, and at -18°C, it is 0.839 (interpolating between -15°C and -20°C with $a_w$ = 0.832). Most microorganisms will not grow below $a_w$ of 0.86, though exceptions exist. For instance, fungi can grow at $a_w$ levels as low as 0.7, and yeasts can grow at 0.62.

Therefore, while microorganisms do not typically grow below -10°C ($a_w$ of 0.907 for pure ice), some exceptions should be taken into account. Xeromyces bisporus, a filamentous fungus, has been reported to grow at $a_w$ of 0.61 in prunes (Pitt and Christian, 1968). Additionally, Michener and Elliott (1969) reported the minimum temperatures for microbial growth in various foods: -8°C for meat, -10°C for cured meat, -10°C for ice cream, -11°C for fish, and -12°C for fruits and vegetables. Psychrotrophic bacteria grow at temperatures ranging from 0°C to -8°C, and molds grow between -8°C and -12°C.

While foodborne pathogens do not grow at temperatures close to 0°C, enzymes produced by microorganisms during growth can still cause food deterioration even below the threshold temperature for microbial growth.

In conclusion, the risk of microbial growth at negative temperatures should remain a point of concern even though the kinetics of microbial growth at negative temperature is extremely low. Regarding the shift from -18°C to -15°C for frozen storage, this issue requires careful consideration. Although the existing literature suggests that -10°C to -12°C is a realistic threshold for microbial growth inhibition, exceptions exist. Therefore, it is important to maintain caution and integrate microbial risk assessments. Notably, EU regulations on deep-frozen foods (EU Council Directive, 1988) allow for temperature deviations of +3°C during transport and +6°C for display cabinets. If the set-point temperature is shifted to -15°C, food could enter a temperature range of -12°C to -9°C, where microbial growth could occur, with limited understanding of the actual risk. These tolerance values of +3°C and +6°C may need to be reassessed if the temperature is raised by 3°C, but reducing the tolerance range would require more precise temperature control, potentially increasing energy consumption and affecting the effective storage capacity.

## 5.2. Quality change in terms of ice crystals coarsening in the case of -15°C frozen storage

The quality of frozen food is closely linked to the coarsening of ice crystals formed during the initial freezing process. Smaller ice crystals are less stable due to higher surface tension forces, which drive the larger crystals to grow at the expense of the smaller ones during storage (Hui et al., 2004, p. 558). Ice cream is particularly sensitive to ice crystal

coarsening, as the formation of larger crystals leads to a "sandy" texture and reduced palatability. Labuza (1982) showed that the iciness perception of ice cream was highly sensitive to the storage temperature with shelf life values of around 1 week at -10°C, 2 months at -18°C and 8 months at -24°C. Le-Bail and Goff (2008) mention that consumers can detect ice crystals above 40-50 µm dimension. Ndoye and Alvarez (2015) studied the kinetics of ice crystal coarsening in ice cream stored at -18°C, -12°C, -8°C, and -5°C. They observed linear crystal growth over time, with similar kinetics across temperatures. Temperature fluctuations also had a significant impact on crystal growth. For example, at a storage temperature with ±2.5°C fluctuations, it took 66 and 123 days (about 4 months) to reach an ice crystal size of 100 µm at -12°C and -18°C, respectively. Using a semi-logarithmic model similar to the PSL models described later in Eq. (3), the time to reach the same ice crystal diameter at -15°C was estimated to be 90 days. In conclusion, ice cream appears to be very sensitive to changes in frozen storage temperature, with a shift to -15°C potentially leading to significant quality loss over time and a poor acceptance by consumers.

## 5.3. Sensory acceptance and expected shelf life in the case of -15°C frozen storage

The evolution of frozen food quality has been studied extensively, with research focusing on both physical and chemical properties as well as hedonic perception. Among the physical and chemical properties that have been examined are drip loss, ice crystal size, colour changes, vitamin content, moisture, nutrient loss, total and free fatty acids, specific volatile fatty acids (analyzed by HPLC), peroxides, and oxidative volatiles (e.g., hexanal analyzed by gas chromatography).

Hedonic scoring evaluates the overall acceptance of frozen foods based on storage time at selected temperatures. This scoring can be done through food tasting using a numerical scale (e.g., a 1-9 scale, where "1" is "extremely dislike" and "9" is "extremely like"). A threshold hedonic score must be established and serves as the cut-off for determining the end of a food's life. This threshold score should be combined with the percentage of panellists who rate the food at or below the threshold score. Another approach involves a paired comparison or triangular sensory test, where three food samples are presented to the panellists with one or two of them being reference samples, and the other sample (s) exposed to frozen storage. The reference sample could be, for example, a food stored in a glassy state at -40°C, as recommended by Labuza & Schmidl (1988) or lower.

If a certain percentage of panellists (e.g., 70-90 %) detect a difference, the end of life for the product is established. The storage duration derived from this test is referred to as the Practical Storage Life (PSL) for an untrained panel and the High Quality Life (HQL) for a trained panel. The ratio between HQL and PSL typically ranges from 2 to 3, depending on the product (Erickson and Hung, 1997, chap. 19). A minimum of 10 judges is usually recommended for this type of study.

Many studies have observed that the logarithm of storage time is a linear function of frozen storage temperature. More sophisticated models are described in Chapter 19 of Erickson & Hung (1997). The basic equation is proposed below and was used to calculate both HQL and PSL storage durations.

$$ \log_{10}(\text{Storage time}) = A \cdot T^{[C]} + B \quad (3) $$

A and B are coefficients with A in°C⁻¹ and B without dimension. A small meta-analysis of 18 different studies was conducted to fit a set of available HQL and PSL data, alongside some physical and chemical analyses, for selected food categories; the obtained results are presented in Table 1 below. The available data were fitted using Eq. (3) based on hedonic tests and some physical and chemical analyses. The equation was then used to determine changes in shelf life when storing frozen foods at -15°C. The Q₁₀ approach is often used for estimation of the temperature acceleration of shelf life (Labuza, 1982); If the temperatures are in°F, then q₁₀ is used (Q₁₀ with temperatures in°C). Q₁₀ is
<caption>**Table 1**</caption>

Results from the analysis of 18 different studies on quality changes (HQL, PSL, or other quality parameters mentioned in the first column) during frozen storage of selected food categories. The ratio of quality life (QL) at -18°C vs. -15°C (Q3), the ratio of QL at -15°C vs. -18°C (Q-3) and the percentage of QL lost are presented as key outcomes. All data were fitted with Eq. (3) with an average R² of 0.972 (standard deviation of 0.040), confirming the relevance of this model to interpolate QL values at a given temperatures.

<table><thead><tr><th>Quality Parameter</th><th>Product</th><th>A<br>(°C<sup>-1</sup>)</th><th>B no<br>dim.</th><th>QL(-18°C)<br>Months</th><th>QL(-15°C)<br>Months</th><th>Q<sub>3</sub> =<br>QL<sub>-18°C</sub><br>QL<sub>-15°C</sub></th><th>Q<sub>-3</sub> =<br>QL<sub>-15°C</sub><br>QL<sub>-18°C</sub></th><th>% QL Loss vs QL<br>(-18°C)</th><th>Reference</th></tr></thead><tbody><tr><td>PSL</td><td>Fat Fish</td><td>-0.0398</td><td>0.00560</td><td>5.27</td><td>4.00</td><td>1.32</td><td>0.76</td><td>24.0%</td><td>Robinson<br>(1985)</td></tr><tr><td>HQL</td><td>Fat Fish</td><td>-0.0412</td><td>-0.420</td><td>2.10</td><td>1.58</td><td>1.33</td><td>0.75</td><td>24.8%</td><td>Desrosier<br>(1978)</td></tr><tr><td>PSL</td><td>Lean Fish</td><td>-0.0544</td><td>-0.426</td><td>3.57</td><td>2.45</td><td>1.46</td><td>0.69</td><td>31.3%</td><td>Robinson<br>(1985)</td></tr><tr><td>HQL</td><td>Lean Fish</td><td>-0.0377</td><td>-0.159</td><td>3.31</td><td>2.55</td><td>1.30</td><td>0.77</td><td>22.9%</td><td>Desrosier<br>(1978)</td></tr><tr><td>Shelf life</td><td>Shrimps</td><td>-0.043</td><td>0.0041</td><td>6.00</td><td>4.46</td><td>1.35</td><td>0.74</td><td>25.7%</td><td>Sun (2012)</td></tr><tr><td>PSL</td><td>Beef cuts</td><td>-0.0398</td><td>0.464</td><td>15.14</td><td>11.50</td><td>1.32</td><td>0.76</td><td>24.0%</td><td>Robinson<br>(1985)</td></tr><tr><td>Lipids Rancidity or<br>browning</td><td>Beef</td><td>-0.0430</td><td>0.133</td><td>8.07</td><td>6.00</td><td>1.35</td><td>0.74</td><td>25.7%</td><td>Hui & al.<br>(2004)</td></tr><tr><td>Lipids Rancidity or<br>browning</td><td>Pork skinless nofat<br>cover</td><td>-0.0401</td><td>-0.117</td><td>4.03</td><td>3.05</td><td>1.32</td><td>0.76</td><td>24.2%</td><td>Hui & al.<br>(2004)</td></tr><tr><td>PSL</td><td>Lamb</td><td>-0.0251</td><td>0.787</td><td>17.30</td><td>15.13</td><td>1.14</td><td>0.87</td><td>12.6%</td><td>Hui & al.<br>(2004)</td></tr><tr><td>HQL</td><td>Chicken (packed)</td><td>-0.0533</td><td>0.429</td><td>24.46</td><td>16.93</td><td>1.45</td><td>0.69</td><td>30.8%</td><td>Desrosier<br>(1978)</td></tr><tr><td>PSL</td><td>Chicken meat</td><td>-0.0436</td><td>0.445</td><td>16.96</td><td>12.55</td><td>1.35</td><td>0.74</td><td>26.0%</td><td>Robinson<br>(1985)</td></tr><tr><td>HQL</td><td>Strawberry</td><td>-0.1294</td><td>-1.27</td><td>11.50</td><td>4.70</td><td>2.44</td><td>0.41</td><td>59.1%</td><td>Desrosier<br>(1978)</td></tr><tr><td>HQL</td><td>Peas, Green beans</td><td>-0.0911</td><td>-0.778</td><td>7.27</td><td>3.88</td><td>1.88</td><td>0.53</td><td>46.7%</td><td>Desrosier<br>(1978)</td></tr><tr><td>Colour</td><td>Peas - color</td><td>-0.0377</td><td>0.150</td><td>6.73</td><td>5.19</td><td>1.30</td><td>0.77</td><td>22.9%</td><td>Sun (2012)</td></tr><tr><td>Flavour</td><td>Peas - Flavor</td><td>-0.0883</td><td>-0.583</td><td>10.15</td><td>5.51</td><td>1.84</td><td>0.54</td><td>45.7%</td><td>Sun (2012)</td></tr><tr><td>HQL</td><td>Fruits & Veget.</td><td>-0.1332</td><td>-1.245</td><td>14.22</td><td>5.67</td><td>2.51</td><td>0.40</td><td>60.2%</td><td>Desrosier<br>(1978)</td></tr><tr><td>Sensory accept.</td><td>Chiffon cake</td><td>-0.0536</td><td>-1.17</td><td>0.62</td><td>0.43</td><td>1.45</td><td>0.69</td><td>30.9%</td><td>Desrosier<br>(1978)</td></tr><tr><td>Iciness</td><td>Ice cream</td><td>-0.0998</td><td>-0.91</td><td>1.91</td><td>0.96</td><td>2.00</td><td>0.50</td><td>49.8%</td><td>Labuza<br>(1982)</td></tr><tr><td></td><td>Average</td><td>-0.0608</td><td>-0.26</td><td></td><td></td><td>1.56</td><td>0.68</td><td>32.6%</td><td></td></tr><tr><td></td><td>Standard dev.</td><td>0.0328</td><td>0.63</td><td></td><td></td><td>0.40</td><td>0.13</td><td>14%</td><td></td></tr><tr><td></td><td>Median value</td><td></td><td></td><td></td><td></td><td>1.35</td><td>0.74</td><td>25.7%</td><td></td></tr></tbody></table>

defined as:

$$Q_{10} = \frac{\text{rate} @ T + 10^{\circ}\text{C}}{\text{rate} T} \qquad (4)$$

Or

$$Q_{10} = \frac{\text{shelflife} @ T}{\text{shelflife} @ T + 10^{\circ}\text{C}} \qquad (5)$$

In the case of a + 3°C change in frozen storage temperature, we proposed to introduce “Q₃” and “Q-3” values obtained from the equations (6) below considering a reference T value of -18°C:

$$Q_3 = \frac{\text{shelflife} @ T}{\text{shelflife} @ T + 3^{\circ}\text{C}}$$

or

$$Q_{-3} = \frac{\text{shelflife} @ T + 3^{\circ}\text{C}}{\text{shelflife} @ T} \qquad (6)$$

A focus on lines 1 and 3 of Table 1 (data from Desrosier and Desrosier, 1978) shows that fatty fish underwent faster quality deterioration compared to lean fish; this is shown by the A value which express the kinetics of quality change during frozen storage. The higher the A value, the faster the change in quality will be. This observation is valid for the data of Desrosier and Desrosier (1978), whereas the opposite trend was observed by Robinson (1985) in lines 2 and 4. A general trend is to expect that fatty meat products should overall be more sensitive to lipid

oxidation and appearance of rancidity than lean products. Chicken and fruits/vegetables exhibited the longest frozen shelf life across all food categories. The last column proposes a percentage of quality life (QL) loss; on average, the shelf life or quality life of frozen foods decreased by approximately 32.6 % when shifting from -18°C to -15°C, with a standard deviation of 14 %. The R² of the fit using Eq. (3) was 0.972, with a standard deviation of 0.40, confirming the quality life model's applicability for predicting quality changes in relation to storage duration. The sweet products (chiffon cake and ice creams) exhibited the shortest quality life duration, combined with a high sensitivity to a change in frozen storage temperature with Q₃ values of 1.45 and 2 respectively, compared to other products (meat, fish, fruits and vegetables) for which the average Q₃ was of 1.54.

The quality of foods is also attributed to nutritional properties. The kinetics of loss in vitamin C has been investigated by different authors. As an example, Giannakourou and Taoukis (2003) investigated the impact of selected frozen storage temperature (-5, -10, -15 and -20°C) on the decrease in vitamin C for green peas, spinach, green beans and okra. The results presented in Fig. 4 show that spinach is much more sensitive than green peas in terms of quality loss. The kinetics are very close (rate of the linear fit as indicated in the frames of Fig. 4); however, the time to reach a 50 % reduction in vitamin C are quite different with 225 and 100 days at -18°C and 134 and 55 days at -15°C for green peas and spinach respectively. These values have not been inserted into Table 1 as the criteria for the end of life differs from one author to another.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Shelf life of green peas and spinach (based on 50% loss in vitamin C) in function of storage temperature. The spinach appears as more sensitive than green peas (Giannakourou and Taoukis, 2003).</figcaption></figure>

A similar investigation is presented in Allouche et al. (2023) presenting an interesting graph (Fig. 5 below) in which the loss in vitamin C of spinach has been measured in function of time for different storage conditions including a chilled storage at 2–3°C. The benefit of frozen storage is obvious with a rapid drop of the concentration in vitamin C for the chilled spinach. A critical value of 6 mg/100 g was proposed to determine the shelf life. The observed values are in the same range as those of Giannakourou and Taoukis (2003).

Recently, the Nomad Foods study (2024) proposed a Landmark study, which confirms the potential to increase freezer temperatures and reduce carbon emissions. This study, carried out in 2023–2024,

experimented four temperatures (ranging from –18°C up to –9°C) with a 6 months storage, and eight key areas including food safety, texture, nutrition, energy use and packaging impact were tested. Results showed no significant change to the products at –15°C, although for –12°C and –9°C there was a drop in Vitamin C for vegetable products and some changes in sensory performance. It can be observed from the analysis presented in this manuscript that the quality life of most frozen foods considered in Table 1 showed a quality life at –15°C storage in the range of around 6 months. Based on this study, adopting a storage at –15°C appears as a realistic solution; however, the quality life of such frozen storage conditions will expose foods to a faster quality loss.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Decrease in vitamin C of spinach for different storage conditions (Allouche et al., 2023).</figcaption></figure>
It is most likely that the results presented in Table 1 would be expressed as probabilities of a frozen food reaching a consumer with an exhausted shelf life. It is difficult to anticipate the reaction of the frozen food industry and retailers if a 3°C more shift is proposed; such a measure may require reconsidering frozen food formulations to make them more resistant. As an example, as mentioned in Allouche et al. (2023), Unilever (2003) has announced it will grant a free non-exclusive licence to the ice cream industry for a variety of patents to help keep ice cream products at a warmer freezer temperature of -12°C, rather than the current industry standard of -18°C".

Lastly, it is worth mentioning that in addition to the TTT (Time-
Temperature-Tolerance) issue addressed in this manuscript, the PPP
(Product-Process-Packaging) and overall the initial quality of the raw
material as well as the methods of pre-treatment, freezing and packaging
of any frozen food have an important impact on the end quality of a food
product after frozen storage. More information is available in
Böhgh-Sörensen L. (1985).

6. Conclusion

This manuscript gathers different information related to selected
non-conventional below 0°C cold chain regimes (chilling, superchilling,
supercooling, stiffening for slicing) and to the status and possible evo-
lution of the associated regulations. It also discusses the definition of the
level of frozenness and the definition of a “frozen food”, considering that
for most foods the LOF does not reach 100 % at -18°C. In addition, the
impact of the “three degrees of change” initiative has been converted in
terms of expected loss in shelf life of frozen foods based on existing data
from the literature.

Based on the information gathered, the following observations and
conclusions can be drawn:

* The temperature of -18°C is currently considered the international standard for frozen foods. Despite this, the level of frozenness for most foods is typically in the range of 95 % to 98 %. According to the International Institute of Refrigeration, food can be considered as “frozen” when its temperature is below -10°C or when 80 % of the freezable water is frozen, considering the lower corresponding temperature level. However, any frozen food must be stored at -18°C.

* Chilling (storage between 0°C and T<sub>IFP</sub>) is an accepted method in some European countries, such as France, and is adopted by several industries to extend the shelf life of non-frozen foods. This type of cold storage requires specific storage chambers with enhanced temperature control and is used mostly at an industrial level to gain a few days of shelf life (chilled storage).

* Superchilling (storage at a few degrees below T<sub>IFP</sub>) was adopted by the EU in 2022 for fish transport only; other applications of superchilling are not allowed in the European Union up to now. This represents a significant step forward in non-conventional frozen food storage.

Another deviation is the stiffening of fish for mechanical slicing,
which is recognized by EU regulation and the CODEX as an accepted
technology, provided the temperature is in the range of -10°C for up to
96 h (CODEX CXC 52-2003 mentions -5°C to -12°C for short periods).
According to existing regulations, food stored between its T<sub>IFP</sub> and
-10°C cannot be stored for long periods.

In the case of the adoption of a “three degrees of change” frozen
storage, and considering a reference temperature of -18°C, the
following information has been obtained:

* A shift from -18°C to -15°C for frozen food storage is expected to reduce energy consumption by approximately 10%, according to Allouche et al. (2023).

* A shift from -18°C to -15°C for frozen food storage is expected to reduce the shelf life of frozen foods with respect to quality changes (hedonic perception) by around 30 %. This value is based on 18 different studies, with a standard deviation of 14 % and a median value of around 25 %.

* A shift from -18°C to -15°C for frozen food storage may result in restricted acceptance in terms of temperature fluctuation during frozen storage. This may yield enhanced packaging solutions, which may be thicker with the use of phase change material for example resulting in a lower occupation ratio of food products in frozen storage and a rise in packaging weight. Also, a better control of temperature oscillation in frozen storage may be considered.

* A risk related to the shift from -18°C to -15°C for frozen food storage is that it may result in a new source of food wastage if the food industry reduces the shelf life presented on the packaging and if the consumer discards the food once the shelf life date has been passed.

Specific measures may be considered to mitigate these risks. A differentiated approach could be considered as mentioned in previous publications, with for example:

* Low-sensitivity products: A 3°C increase in storage temperature with upward temperature fluctuation of 3°C could be considered.

* Medium-sensitivity products: A 3°C increase in storage temperature with upward temperature fluctuation of 1°C could be considered.

* High-sensitivity products: Current storage temperatures should be maintained to preserve quality and safety.

* The level of sensitivity could be determined based on the Q3 value and on the storage life at -18°C for example. By implementing these strategies, energy savings can be achieved while minimizing the negative impact on product quality and food safety.

* The temperature range of -10°C to -12°C is considered a threshold below which the risk of microbial growth during storage is negligible. However, some exceptions exist, and it would be worthwhile to track the risk of microbial growth in the range between -18°C and -10°C, especially when temperature fluctuations are allowed during transport (e.g., +3°C is accepted in Europe). Virus may also be considered.

CRediT authorship contribution statement

**Alain Le-Bail:** Writing – original draft, Validation, Supervision, Formal analysis, Data curation, Conceptualization. **Nasser Hamdami:** Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization. **Cyril Toublanc:** Writing – review & editing, Validation, Conceptualization. **Michel Havet:** Writing – review & editing, Data curation, Conceptualization.

Declaration of competing interest

The authors declare no conflict of interest.

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