FST 140: Microwave Heating

Microwave Heating

• uses electromagnetic radiation, in microwave range

• heat is generated through dielectric heating by oscillating electric fields

• the rapid rotation creates fictional heat, hence, heating the food internally

Advantage

uniform and rapid heating

915 Mhz

used in commercial operations

2450 MHz

in home microwaves

Food Irradiation

• does not raise the temperature of the product

Ionizing Radiation

provides enough energy to knock off electrons of susceptible molecules, creating unstable ions.

Cold sterilization

Microbial inactivation is due to lethal mutations because of destruction of nucleic acids (DNA and RNAs, effectively killing them.

Radiation

• the emission and propagation of electromagnetic energy through space or a medium.

• electromagnetic radiation from I.R. to U.V.

Irradiation

• total radiation striking the surface per unit of irradiated surface

Radiosity

total radiation leaving the surface

Ionizing radiation

• commonly sourced from Co-60 and Cs-137

For food

it requires 30-40 Kgy (kilogrey) to achieve sterilization

Advantages of Food Irradiation

• can treat food without causing changes in the freshness and texture

• no heat applied

• no harmful toxic residues

• can be used to treat packed commodities

Death

inability to reproduce

Genetic Material

• key to expression of the necessary molecules needed for the cell’s normal activities

Endospore

• a metabolically limited or dormant cell that is highly resistant to acute environmental stresses such as heat, dessication, UV and gamma radiation, enzymatic digestion, organic solvents

Spore

• heat-resistant

• require more extensive processing to control and reduce its number

• requires temperature higher than 100C

Core

• resistance to heat, UV radiation, gamma radiation, hydrogen peroxide etc.

Inner membrane

impermeability to biocides

Cortex

resistance to biocides

Outer membrane

permeability barrier to biocides

Exosporium

adherence and biocide protection

Spore core

• absence of high energy compound

• High level of divalent ions (Ca²⁺) in core

• High levels of dipicolinic acid

• lower pH than cell

• Low level of hydration

presence of Ca²⁺

contributes to the integrity of the spore cortex

Dipicolinic acid

• major component of the spore cortex

• 10% of dry weight

• pyridine-2,6-carboxylic acid

Ca-DPA

this complex helps dehydrate the spore core, reducing water content of vegetative cells

Low pH

favors the activity of the active proteins (SASPs) in the spore

Low hydration

lowered water activity also means lower rates of reactions

SASPs

• small acid soluble proteins

• alpha and beta types

• DNA binding proteins, offering additional barrier to the genetic material

Spore coat and Crust Assembly

These proteins help build the protective layers of the spore

• CotX, CotYZ, CotVW

• CotE

• SafA

CotX and friends

assembly of the core crust

CotE

assembly of the outer spore coat

SafA

assembly of inner spore coat

Core physiology and Sporulation

These proteins regulate internal conditions and contribute to spore formation:

• DacB

• SpoVF

• DisA

• YwjD

DacB

control of core water content

SpoVF

Dipicolinate synthase, formation of DPA and Ca-DPA complex

DisA

DNA integrity scanning protein and sporulation

YwjD

sporulation specific UV-damage endonuclease

Germination and Cortex Degradation

• SleB

SleB

for the degradation of spore cortex during germination, allowing core expansion and rehydration

DNA repair mechanisms

• Mfd

• MutSL

• SbcDC

• PolY1, PolY2

Mfd

strand specific DNA repair

MutSL

mismatch repair proteins

SbcDc

inter-strand cross-link repair

PolY1, PolY2

translesion synthesis

LigD Ku

non-homologous end-joining (NHEJ)

ExoA, Nfo

oxidative damage repair