K U L I A H U N I T P R O S E S

H A J A R S E T Y A J I

Kinetika Reaksi

Bahan Pangan dapat dipandang sebagai sistem yang memiliki reaktifitas yang tinggi. Reaksi kimia dapat terjadi antar komponen penyusun bahan pangan itu sendiri dan dengan lingkungan sekitar. Misal perubahan biokimia dalam buah, reaksi pos mortem pada daging dan serangkaian reaksi kimia biokimia selama proses pemasakan. Studi sistematis dari reaksi pada pangan merupakan materi yang dibahas peer group kimia dan biokimia, tetapi laju reaksi yang terjadi merupakan subjek yang juga harus dikuasai oleh engineer pengolah bahan pangan. Aplikasi kinetika reaksi dalam proses pangan yang penting harus diketahui a.l: Kalkulasi proses termal untuk membunuh mikroorganisme Optimasi proses termal yang berfokus pada qualitas produk Optimasi proses yang berkaitan dengan biaya Prediksi shelf life produk pangan sebagai funsi kondisi penyimpanan Kalkulasi beban pendinginan pada proses penyimpanan produk respirasi Pengembangan kontrol terintegrasi antara suhu dan waktu

Reaksi dalam proses pengolahan

Terdapat 2 macam

Reaksi yang dikehendaki,misalnya pirolisis karbohidrat selama proses penyangraian kopi, hidrolisa kolagen pada proses pemasakan daging, hidrogenasi minyak untuk memperoleh sifat padat dan lain sebagainya.

Reaksi yang tidak dikehendaki; reaksi kimia dan biokimia yang terjadi selama proses pengolahan dan penyimpanan menghasilkan dampak yang tidak dikehendaki pada kualitas produk pangan. Misal: Maillard-type browning pada juice jeruk, timbulnya ketengikan akibat oksidasi lemak, dan kerusakan produk pangan karena aktifitas mikrorganisme.

Konsep Dasar

Reaksi sederhana dan tidak sederhana

Reaksi sederhana persamaan laju reaksi sebanding dengan persamaan stoikiometri, Misal reaksi netralisasi

Reaksi tidak sederhana Persamaan laju reaksi tidak berhubungan secara langsung dengan persamaan stoikiometri, misal reaksi hidrogen dan brom, dan reaksi maillard

Laju Reaksi 5

Persamaan laju Reaksi adalah persamaan matematis yang menggambarkan laju reaksi sebagai fungsi dari konsentrasi dari spesi yang terlibat dalam reaksi

r = k[A]m[B]n

• Laju reaksi seringkali diperoleh berbanding lansung dengan konsentrasi reaktant dengan bilangan pangkat yang sederhana

• Bilangan pangkat ini disebut orde reaksi. • Jika bilangan pangkat dinyatakan sebagai laju =

k[A]m[B]n, maka reaksi ini berorde m terhadap A dan dan berorde n thd. B. Orde keseluruhannya adalah m+n

Persamaan Laju Reaksi; Bentuk Integrasi

6

Menggambarkan konsentrasi reaktant, mis A sebagai fungsi waktu;

the differential definition for the rate is equated to the rate law,

r = -dA/dt = k[A]n

[A] - [A]0 = -kt

[A] = [A]0 - kt.

Plotting [A] vs. t gives a st. line with slope = -k.

𝑑𝐴 = − 𝑘𝑑𝑡𝑡

0

𝐴

𝐴0

Reaksi Orde 1; Bentuk terintegrasi 7

n = 1 (first order)

ln [A] - ln [A]0 = - kt

ln ([A]/[A]0) =-kt , or

[A] = [A]0 e-kt

[ ]dA

k Adt

0 0[ ]

A t

A

dAkdt

A

A produk

A Straight-Line Plot for a First-Order Reaction 8

Pada reaksi orde 1, ln([A]/[A]0) = -kt

Plott ln [A] thd. t menghasilkan grs lurus

Juga plot ln ([A]/[A]0) vs. t

So does plotting the log of pressure of a reactant.

Such a plot can be used to determine k.

Half-Life of a First-Order Reaction

9 The time required for a reactant to reach half its

original concentration is called the half-life of a reactant and is designated by the symbol t1/2.

t1/2 = half-life of the reaction, k = rate constant

For a first-order reaction, the half-life does not depend on concentration.

ln[A]

[A]kt when t = t then [A] =

[A]

2

ln[A]

[A] / 2kt ln(2) = kt

t =ln(2)

kt =

0.693

k

o

1 / 2

o

o

o

1 / 2 1 / 2

1 / 2 1 / 2

Second-Order Rate Law 10 For aA products in a second-order reaction,

Integrated rate law is

A plot of 1/[A] versus t will produce a straight line with a slope equal to k

The above equation shows how [A] depends on time and can be used to calculate [A] at any time t, provided k and [A]o are known

Rate =

AA

tk

2

1

A +

1

A o

kt

Half-Life of a Second-Order Reaction

11

When one half-life of the second order reaction has elapsed (t = t1/2), by definition, [A] = [A]o/2 then the integrated rate law becomes

t1/2 = half-life of the reaction, k = rate constant,

Ao = initial concentration of A

The half-life is dependent upon the initial concentration.

1

[A] / 2kt +

1

[A]

2

[A] A]kt

1

[A]kt

solving for t gives the expression

t =1

k[A]

o

1 / 2

o o o

1 / 2

o

1 / 2

1 / 2

1 / 2

o

1

[

Zero-Order Rate Laws 12

The rate law for a zero-order reaction is

Rate = k[A]o = k(1) = k

For a zero-order reaction, the rate is constant. It does not change with concentration as it does for first-order or second-order reactions.

The integrated rate law for a zero-order reaction is

[A] = -kt + [A]o

13

continued…

[A] = -kt + [A]o

In this case a plot of [A] versus t gives a straight

line of slope –k.

[A] = [A]o/2, when t = t1/2

[A]o/2 =-kt1/2 + [A]o

Solving for t1/2 gives,

t1/2 = [A]o/2k

Effect of temperature on reaction kinetics

All chemical reactions are accelerated when the temperature is increased. The relationship between the reaction rate constant k and the temperature is described in Eq.

R universal gas constant 8.314 kJ./K.kmol A a constant, named the ‘ pre-exponential factor ’ . T absolute temperature, K E activation energy, kJ/kmol

Persamaan tersebut dikenal dengan prsamaan Arrhenius

Hubungan antara lnk dengan 1/T

The activation energy actually represents the sensitivity of the reaction rate to changes in temperature. If k 1 and k 2 are the rate constants at temperatures T 1 and T 2 respectively, then:

Eq. 2

Another way of representing the sensitivity of a reaction to a change in temperature is the factor known as Q10 or the temperature quotient . Q 10 is the ratio of the rate constant of a reaction to that of the same reaction at a temperature lower by 10°C. Applying this definition to Eq. 2, we obtain the relationship between the activation energy and the temperature quotient:

Contoh: Laju reaksi enzimatis meningkat dengan faktor 3.2 jika suhu ditingkatkan menjadi 45 C dari suhu awal 37°C. Hitunglah energi aktivasi E dan Q10.

Kinetics of Biological Processes

Enzyme-catalyzed reactions The rate of enzymatic reactions depends on a number of conditions, such as the concentration of the enzyme and the substrate, the temperature, pH, ionic strength etc. If the substrate is present in large excess, the rate of an enzymatic reaction is proportional to the concentration of the enzyme. This is the basis for the definition and determination of enzyme activity . The effect of substrate concentration is more complex. The velocity of an enzymatic reaction, in terms of the rate of product formation is given by the well-known Michaelis–Menten equation :

where: v velocity of the reaction (rate of product formation) v max a maximum value to which v tends asymptotically as the substrate concentration

increases

K m a parameter of the reaction, known as the Michaelis–Menten constant .

The effect of substrate concentration on the rate of a hypothetical enzymatic reaction, according to the Michaelis–Menten model is shown above

The rate of enzymatic reactions is strongly affected by the temperature. As the temperature is increased, the reaction rate increases up to a maximum value (at the optimal temperature) and then decreases ( Figure 4.5 ). This behavior is not in contradiction of the Arrhenius model. The bell-shaped rate-temperature curve is the consequence of the simultaneous occurrence of two contradicting processes, namely the enzymatic reaction itself and the thermal inactivation of the enzyme, both enhanced by higher temperature. The effect of pH on the rate of enzymatic reactions also follows a bell-shaped curve with maximum activity at the optimal pH. Control of enzymatic activity by adjusting the pH is a frequently applied practice in food processing.

Growth of microorganisms

The term ‘ growth of microorganisms ’ may be interpreted either as the increase in the number of living cells or as the increase in biomass. In this discussion, we shall study the kinetics of microbial growth with reference to the number of cells and not to their mass.

The classical curve of microbial growth (number of living cells N versus time, Figure 4.6 ) shows four phases

● Lag phase : initially, there is no growth. The cells may use food to increase in mass but not in number ● Log (logarithmic, exponential) phase : the number of living cells increases with time exponentially ● Stationary phase : the number of living cells remains nearly constant in time. This phase is usually explained as one during which the rate of new cell generation is equal to the rate of death ● Decline : the number of living cells declines with time, usually in exponential manner.

Reactors in food processing

In chemical process engineering, the term ‘ reactor ’ often means a specialized device (generally a vessel with accessories) used to carry out a controlled reaction. In this section, however, any portion of the physical process system where reactions occur will be considered a reactor. By this defi nition, a fermentor, an oven, an extruder, a drying tunnel, an oak barrel used for ageing wine or a box of cookies are reactors.

Macam macam Reaktor

Batch Reactor adalah tempat terjadinya suatu reaksi kimia tunggal, yaitu reaksi yang berlangsung dengan hanya satu persamaan laju reaksi yang berpasangan dengan persamaan kesetimbangan dan stoikiometri. plug fl ow reactor (PFR) : in this type of reactor, the material flows as a block

(plug). Each part of the fl uid has the same velocity. There is no mixing within the fluid. Consequently, the residence time is equal for every part of the fluid.

The laminar fl ow reactor (LFR) : the fl uid moves through the reactor (usually a tubular reactor) in laminar fl ow, i.e. in parallel layers. The layers do not move at the same velocity but there is no mixing between the layers

The continuous stirred tank reactor (CSTR) : physically, this type corresponds to a perfectly agitated vessel with continuous feeding and discharge. As a result of perfect mixing, the composition and all other conditions at a given moment are uniform at all points within the reactor.

Penggunaan Batch Reactor Reaktor jenis ini biasanya sangat cocok digunakan

untuk produksi berkapasitas kecil misalnya dalam proses pelarutan padatan, pencampuran produk, reaksi kimia, Batch distillation, kristalisasi, ekstraksi cair-cair, polimerisasi, farmasi dan fermentasi.

Plug Flow Reactor

Model matematikanya dapat diterapkan untuk semua jenis fluida: gas, cairan, dan slurries (setengah cairan yang merupakan campuran dari air dan semen, tanah liat, atau pupuk)

Di dalam PFR, fluida mengalir dengan perlakuan yang

sama sehingga waktu tinggal (τ) sama untuk semua elemen fluida. Fluida sejenis yang mengalir melalui reactor ideal disebut plug. Saat plug mengalir sepanjang PFR, fluida bercampur sempurna dalam arah radial bukan dalam arah axial (dari arah depan atau belakang). Setiap plug dengan volume berbeda dinyatakan sebagai kesatuan yang terpisah-pisah (hampir seperti batch reactor) saat dia mengalir turun melalui pipa PFR

CSTR

Thank You