Effect of storage on texture: Meat

Texture is the main quality characteristic of meat, affecting price and consumer acceptance.

Tenderness and juiciness are the main textural parameters that affect a consumer’s enjoyment in general. 

Tenderness refers to the force needed to yield the meat during chewing, and the energy needed to masticate it to the point of swallowing. Juiciness refers to the type and amount of liquid released during mastication. Some specific meat products have a different textural priority, such as the chewiness exhibited by beef jerky.

A large amount of research has been dedicated to understanding the texture of meat. However, the detailed mechanism of tenderisation is still not fully understood. Tenderness is known to be influenced by properties of the raw material, such as animal breed and age, muscle contraction, amount of connective tissue, ripening and its cooking method.

The term ‘meat’ generally refers to skeletal muscles. Cells here are very long and so are referred to as fibres. Muscle fibres are composed of many small myofibrils that lie parallel to the fibre axis as well as connective tissue. This connective tissue is made mostly of collagen in branched fibre form, giving a support network to the muscle structure.

Various tests can be carried out on chilled, frozen, reformed, pre-cooked or breaded meat to gauge the quality of its mouthfeel. With the help of sophisticated texture analysis instrumentation, manufacturers can evaluate the effects of processing methods and times, highlight quality inconsistencies and adjust formulations or techniques accordingly.

Fresh

The conversion of muscle to meat occurs after death and is made up of a series of biochemical changes. A lot of research in this area has been performed on beef, but increasingly the storage-induced changes of pork and poultry are investigated as consumers attempt to reduce their red meat intake.

There are three stages of storage that affect meat texture:

1. Prerigor, when cold storage shortening causes toughening
2. Rigor mortis, when biochemical changes cause further toughening
3. Ageing, when tenderness develops as the meat relaxes

Spoilage follows these stages, which is signalled mostly by olfactory changes, and less of a textural change. Meat is generally chilled after slaughter to control this microbial spoilage. However, if the temperature is below 15°C while the meat is still in the prerigor state, muscle contraction occurs, causing toughness. The lower this temperature, the greater the toughening. It can be prevented by avoiding rapid chilling.

Rigor mortis is the stage where the muscles become stiff and inextensible – this stage’s length varies between species. After rigor, autolysis occurs (destruction of a cell through the action of its own enzymes), and tenderness is achieved upon ageing. Ageing is only important in beef – pork, lamb and poultry do not toughen significantly.

Tests that measure aspects of fibre characteristics are central to achieving simple, yet accurate, ways to evaluate meat quality. A consumer’s first assessment and perception of meat texture occurs when biting through its fibres, so a logical test approach is to measure the force required to cut the fibres. This provides an indication of the consumer’s perception and identification of undesirable textural characteristics, such as toughness.

Cutting tests suitable for measuring fresh meat texture, either cooked or uncooked, include the use of a 10mm Meullenet-Owens Razor Shear Blade to cut a specified depth into a whole fillet, a Thin V Warner-Bratzler Blade cutting blade to shear through a sample of known dimensions, or a Kramer Shear Cell 5 Bladed, which uses multiple blades to cut through a bulk sample. These measurements are all discussed in further detail below.

Frozen

Two major textural changes can occur during frozen meat storage:

1. A faulty freezing process can cause freezer burn, surface dryness developed during storage due to a condensed layer of muscle near the surface. This is produced by blast freezing and shows up as white or orange spots on the surface of frozen meat, caused by the sublimation of ice crystals into the atmosphere. This is a particular problem in poultry.
2. Properly frozen meat – changes induced by the ice crystals forming in the product. The main defect in this case is ‘drip’, the reduced capacity to hold water due to the damage caused by large ice crystal formation. These are formed by either slow freezing or storage at fluctuating temperatures. Drip can be reduced with a fast freezing rate.

Meat drip is traditionally measured without applying a large external force to the sample and measuring the liquid exudate by mass or an area of spread on filter paper. However, these methods apply to very small samples, and can be disrupted by user error or subjectivity. An Ottawa Cell measurement can be used to compress a bulk raw meat sample, measuring the deformability of the meat, while the exudate is collected in the catchment drawer.

This can be weighed once a low compressive force has been reached (and the test paused), and the test continued to a high enough force to fully deform the sample. In this way, two important measurements are made simultaneously.

Meat products

Frankfurters

Frankfurters are economically important in many countries. Their manufacturing process involves the chopping of meat and adipose tissue (destroying cellular structure and putting connective tissue, adipose tissue and other proteins in suspension and solution), stuffing (immobilising meat mixture in its casing), cooking (forming an ordered 3D protein network) and smoking. Frankfurters are generally stored in a vinegar pickle, which causes texture deterioration. The addition of xanthan gum can help stabilise the frankfurter against acid deterioration.

Frankfurters are simply tested using a shearing test with a Warner-Bratzler Blade. To test a sample, it is placed centrally to the triangular blade inset, the blade having been positioned carefully within the slotted base insert. A cutting/shearing test is then performed. This test represents a crude method with little sample preparation, which lends itself ideally to quick quality determination.

The microstructure of cooked meat emulsion, such as sausage products, may be influenced by the processing conditions, particularly cooking, and its composition, i.e. the type of meat protein, fat-protein ratio, salt level, moisture, degree of comminution and filler content. Factors that affect the results of Warner-Bratzler shear tests are uniformity of sample size, direction of muscle fibres (if present), presence of connective tissue and fat deposits, sample temperature, and speed of shearing.

Jerky

Jerky is lean trimmed meat that has been cut into strips and dehydrated to prevent spoilage. Most meats (and recently, vegetable proteins) are sold as jerky worldwide. Unlike other meat products, jerky prizes a moderately tough and chewy texture.

As jerky is cut with the front teeth when bitten into, a cutting test provides a useful, imitative measurement. This can either be performed using a sample of known dimensions and a craft blade, or a sample of known thickness and a Meullenet-Owens Razor Shear Blade. This second method is ideal, as testing is more rapid and repeatable. This minimises the experimental errors attributable to sample preparation, shortens sample preparation time and leads to a simpler testing solution.

To perform the test, samples as close as possible to uniform thickness are chosen. They are aligned with fibres perpendicular to the height calibrated 10mm Meullenet-Owens Razor Shear Blade, and the sample is cut with a compression test.

Reference

Food Storage Stability, Chapter 8 “Effect of storage on texture” – Alina S. Szczesniak