Frozen-Storage TTT & Practical Storage Life

TTT, set out by the International Institute of Refrigeration (IIR) and used in the ASHRAE Handbook, states that the quality loss of a frozen food in storage is cumulative and depends predictably on the storage temperature and time. Each commodity has a tabulated practical storage life (PSL) — the months it keeps acceptable quality — quoted at the three reference temperatures −12, −18 and −24 °C. Colder storage extends PSL strongly and predictably.

Practical storage life is the period a frozen food keeps acceptable eating and visual quality under good storage. It is a quality limit, not a safety limit — properly frozen food held below −18 °C stays microbiologically safe almost indefinitely, but its colour, texture, flavour and nutrients slowly degrade. PSL is the point at which most people would notice that decline.

Each commodity is stored with its published PSL at −12, −18 and −24 °C. The tool fits a smooth log-linear curve through those three anchor points — the natural log of PSL is treated as linear in temperature — and reads the curve at your freezer temperature, extending the nearest segment's slope outside the anchors and clamping to the practical −6 to −30 °C band.

−18 °C (0 °F) is the universal reference and effective legal minimum for frozen retail storage worldwide. It is cold enough to give most foods a practical storage life of several months to over a year, while being a realistic target for commercial and domestic freezers. The tool marks −18 °C on the curve and reports how many months you gain or lose by storing colder or warmer.

Blanched vegetables such as peas and carrots keep longest — around 18 months at −18 °C and over two years at −24 °C. Lean beef cuts also store well. The shortest is fatty fish such as salmon and mackerel: their oils oxidise quickly, giving only about 5 months at −18 °C, so they should be stored as cold as possible and glazed with ice. Minced meat and sausage are shorter than whole cuts because mincing exposes more fat and surface area.

A lot, and non-linearly. Roughly every 6 °C colder multiplies the practical storage life by a commodity-specific factor. For green beans, moving from −12 to −18 °C lifts PSL from 5 to 15 months, and −24 °C reaches 24 months. The tool reports the exact months gained versus the −18 °C reference for your chosen commodity and temperature.

Because quality loss is cumulative and faster when warmer. Each spell at a higher temperature — from freezer cycling, door opening or a poorly performing unit — spends extra storage life and drives moisture migration that causes freezer burn and ice-crystal growth. The tool reports a fluctuation-penalty figure: the days of life lost per +1 °C held for a month, so a freezer that drifts warm ages its contents faster than its average temperature suggests.

It is the percentage of usable storage life consumed per month at the chosen temperature — simply 100 divided by the practical storage life in months. A food with a 10-month PSL loses about 10% of its quality reserve each month; one with a 5-month PSL loses 20% a month. It is a quick way to gauge how urgently a frozen item must be used.

Quality. Food frozen and kept continuously below −18 °C remains safe far beyond its practical storage life — the PSL marks when quality (texture, colour, flavour, nutrients) has declined enough to matter. Always judge frozen food on appearance and smell after thawing, and never refreeze food that has fully thawed and warmed.

Yes. Air contact drives oxidation and freezer burn, so vacuum packing, tight wrapping and ice-glazing of fish extend life beyond the table values, while loosely wrapped or partially opened packs fall short. The PSL figures here assume good commercial packaging; treat them as planning values and store everything well-sealed.

No. The calculation runs entirely in your browser using the built-in TTT practical-storage-life tables and the log-linear curve model. Nothing you enter is sent anywhere.