Smoke in Food Processing

Joseph A. Maga

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351093545
• Edition: 1
• Topic: Biowissenschaften
• Subtopic: Biologie

Smoking was one of the first forms of food processing, and through the centuries the chemistry of smoke has slowly evolved.It is now known that wood and food composition can significantly influence smoke composition, as well as the resulting textual, sensory, nutritional, antioxidative, and antimicrobial properties of the smoked food.Aside from beneficial properties, one must also consider potential health concerns associated with certain woods and their resulting smoke.

Chapter 1

WOOD COMPOSITION

I. INTRODUCTION

Before one can begin to understand the various chemical and physical interactions of wood smoke with foods, one should have an initial understanding of the basic composition of wood.

Botanically, wood used as a smoke source falls into two broad categories, namely hardwoods and softwoods, which in some cases does not correlate with the actual physical hardness of wood. For example, some hardwoods are actually quite soft (aspen and cotton-wood), while certain softwoods are quite hard (Douglas fir).

The two species can be distinguished by the fact that hardwoods produce woody, two-seed leaves (dicotyledenous) and their seeds are borne in a fruit structure. Thus, they are classified as angiosperms. In addition, the leaves of hardwoods are usually broad or bladelike and in most climates they are deciduous in that they shed their leaves at the end of each growing season.

On the other hand, the softwoods are classified as gymnosperms, since their seeds are borne naked. Softwoods have also been called conifers, since many produce seed cones, pollen cones, or a combination of both. This species has needlelike (pine) or scalelike (cedar) leaves that are commonly called evergreen, since with the exception of larches and bald-cypress, the leaves are retained for up to several years. Some typical American hardwood and softwood species are summarized in Table 1.*

Wood is essentially a plant tissue composed of various types of cells. In mature wood, most of these cells are dead and hollow. This structure is known as secondary xylem, with the hollow interiors of the cells known as lumens. The orientation of cells and lumens differ between hardwoods and softwoods. In softwoods, the cells, which make up to 90 to 95% of the wood volume, morphologically appear to be fibrous in nature, and thus are called fibers. In contrast, hardwoods are composed of much wider cells, which are called vessel elements. Also, fiber volume can be quite variable for hardwood, but averages approximately 50%.

From a chemical standpoint, wood cells are composed of several organic polymers with the structural key compound being cellulose. As seen in Table 2, it is a long-chain, linear sugar molecule or polysaccharide composed of glucose units. In most normal wood tissue, glucose, as a cellulose polymer, accounts for 40 to 45% of total dry weight.

Another major constituent of wood is called hemicellulose, which serves as the matrix substance for the cellulose superstructure. It has a lower molecular weight than cellulose and may contain short side chains. The carbohydrate composition of hemicellulose can vary and is usually composed of a combination of five-carbon sugars such as xylose and arabinose and six-carbon sugars typified by glucose, mannose, and galactose.

The third major component of wood is lignin, which serves as a cementing material that assists in the binding together of wood cells. Approximately 60% of total lignin is located in the cell walls, with the remainder spread throughout the wood. Structurally, as seen in Table 2, it is a three-dimensional, highly branched, polyphenolic molecule of complex structure and high molecular weight. It should be noted that lignin generally is not reactive with water, whereas hemicellulose and cellulose are just the opposite.

Another compositional distinction, the overall importance of which has not been evaluated from a smoke chemistry standpoint, is the presence of heartwood vs. sapwood. Heartwood represents the central portion and sapwood the outer portion of the wood. The heartwood portion is significantly higher in organic extractives, which are primarily polyphenolic in composition, and usually the cells in heartwood are dead and have a lower moisture content than sapwood. Also, most heartwoods take on distinctive colors such as brown, yellow, orange, and red. In addition, heartwood extractives from wood species such as redwood and cedar are toxic to microorganisms and insects. The potential implication of these toxic compounds, if any, to humans relative to smoke consumption is not known.

Another wood form that deserves special attention is bark. This by-product can be used as a wood smoke source and compositionally it is s...