Committing offsets in SinkActor

We are using an alpakka pipeline that uses Source(Consumer.committableSource) as KafkaConsumer and Sink.ActorRefWithAck as the sink for BackPressure.

The problem statement - We need to commit Kafka offset only after reading and processing the value in the sinkActor. How can we do it? How can I commit the offset in the SinkActor? We need to process the consumed element in the Sink actor as the Init method keeps one element in the buffer.